# Optical Forums > Ophthalmic Optics >  Take a Master Exam

## Darryl Meister

Okay, while we're on the subject of the ABO's Master in Ophthalmic Optics Exam, I have provided a practice test, below. These sample questions are similar to many of the questions from the actual Master Exam I took. (No one really offered practice questions back then though, which made preparation especially difficult.) For those interested in trying some of these questions out, I'll post the answers in a few days so you can see how well you did.

*Practice Master Exam*

1. Prism-thinning is most often done to reduce thickness in what type of lens?
a. Minus-powered progressive lenses
b. Plus-powered progressive lenses
c. Flat-top bifocal lenses
d. Minus-powered single vision lenses

2. Which _spectacle_ wearer has to accommodate more for a giving working/reading distance?
a. Myope
b. Hyperope
c. Emmetrope
d. They accommodate the same

3. Given a spectacle prescription for a _right_ lens that requires 2D base in, 1.5D at 30, and 0.75D at 60, what would be the resultant prism?
a. 3.93D at 21°
b. 3.93D at 159°
c. 2.75D at 30°
d. 2.75D at 45°

4. If your lens measure reads +6.00 D while measuring the front curve of a 1.66 high-index plastic lens, what is the actual refractive power of the lens surface?
a. +4.82 D
b. +6.00 D
c. +7.47 D
d. +6.25 D

5. What kind of prism can be incorporated into single vision aspheric lenses without significantly affecting lens performance?
a. All prism
b. Prescribed prism
c. Prism for decentration
d. No prism

6. Which _spectacle_ wearer enjoys the widest field of view through his or her lenses?
a. Myope
b. Hyperope
c. Emmetrope
d. They are the same

7. Which organization or agency requires a minimum thickness for _dress_ (non-safety) spectacle lenses?
a. FDA
b. ANSI
c. OSHA
d. None of the above

8. An object 50 cm in front of a +5.00 D lens will come to a focus at what distance from the lens?
a. 25.0 cm in front of the lens
b. 33.3 cm behind the lens
c. 33.3 cm in front of the lens
d. 20.0 cm behind the lens

9. What is the _sagitta_or depthof a 6.00 D curve at 50 mm (assuming a refractive index of 1.530)?
a. 2.8 mm
b. 3.0 mm
c. 3.6 mm
d. 4.5 mm

10. Given a prescription of +10.00 D prescribed at a vertex distance of 13.5 mm, what lens power would be ordered for a vertex distance of 8.5 mm?
a. +10.50 D
b. +9.50 D
c. +10.25 D
d. +9.25 D

11. What type of prescription could be used to make a prosthetic eye appear higher in a depressed socket?
a. Minus cylinder at axis 180
b. Plus cylinder at axis 180
c. Base down prism
d. Base up prism

12. Corrected curve lenses generally reduce what type of lens aberration?
a. Diffraction
b. Spherical aberration
c. Chromatic aberration
d. Oblique astigmatism

13. The reflected images seen from the cornea and crystalline lens are referred to as?
a. Purkinje images
b. After images
c. Aerial images
d. Ghost images

14. The chromatic aberration in the periphery of a spectacle lens is?
a. Directly proportional to the power of the lens
b. Directly proportional to the distance from the optical center
c. Inversely proportional to the Abbe value of the material
d. All of the above

15. Given the following FT-35 bifocal prescription:
+2.00 DS with a +2.50 D add, O.U. (both eyes)
PDs = 66/62 mm
How could you induce a 1D _base in_ prismatic effect, per eye, at _near_ only?
a. Use a 56 mm distance PD
b. Use a 60 mm near PD
c. Use a 54 mm near PD
d. Cant be done

16. The transmittance of a Transitions photochromic plastic lens will _not_ depend upon which factor?
a. Lens thickness
b. Temperature
c. Sunlight exposure
d. Age

17. Given the following single vision prescription:
-5.00 DS with 1.5D _base in_ prism, O.U. (both eyes)
PD = 64 mm
Frame eyesize = 54 mm, bridge size= 16 mm, and effective diameter = 64 mm
What would be the minimum blank size required for a finished, stock lens (assuming a 1 mm allowance)?
a. 71 mm
b. 65 mm
c. 68 mm
d. 77 mm

18. A normal eye is most sensitive to which of the following colors during _photopic_ (daylight) vision?
a. Red
b. Blue
c. Green
d. Equally sensitive

19. What is the focal length of a +5.00 D lens?
a. 5.0 m
b. 0.2 cm
c. 25 cm
d. 20 cm

20. If a near PD of 64 is specified for a pair of bifocal lenses with plano distance powers, and the segments are actually fabricated to a near PD of 60, what kind of prism is induced while reading?
a. Base in prism
b. Base out prism
c. Vertical prism
d. No prism

21. What is the ANSI tolerance on the near PD for multifocal segments?
a. 1.5 mm
b. 2.0 mm
c. 2.5 mm
d. 3.0 mm

22. When a lens is tilted, what prescription changes occur?
a. Sphere power is increased; cylinder power equal in sign to sphere power is induced
b. Only cylinder power is induced
c. Sphere power is decreased; cylinder power equal in sign to sphere power is induced
d. Sphere power is increased; cylinder power opposite in sign to sphere power is induced

23. Given an Rx for 4D Base In and 4D Base Up in the right eye (O.D.), what would generally be an acceptable way to split this prism in order to improve cosmetics and comfort?
a. O.D. 4D Base In and 4D Up; O.S. No prism
b. O.D. 2D Base In and 2D Up; O.S. 2D Base Out and 2D Base Down
c. O.D. 2D Base In and 2D Up; O.S. 2D Base In and 2D Base Up
d. O.D. 2D Base In and 2D Up; O.S. 2D Base In and 2D Base Down

24. Which of the following frames will return to its original shape when reheated?
a. Optyl
b. Cellulose Acetate
c. Carbon Fiber
d. Polyamide

25. What is the approximate overall refractive power of a typical human eye?
a. 17 D
b. 43 D
c. 60 D
d. 65 D

26. When the eyes focus on a near object, which of the following processes occur?
a. Eyes converge
b. Lenses accommodate
c. Pupils constrict
d. All of the above

27. Given a distance correction of O.D. +2.00 DS and O.S. +4.00 DS, which eye/lens would have the _reverse_ slab-off applied for a vertical imbalance correction?
a. Right (O.D.) eye/lens
b. Left (O.S.) eye/lens
c. Either lens could use it
d. Neither lens could use it

28. A hard resin lens (CR-39, n = 1.500) loses approximately what percentage of light at _each surface_ due to reflection?
a. 3.5%
b. 4.0%
c. 4.5%
d. 8.0%

29. What is the spherical equivalent of a lens with a power of +1.00 DS -2.00 DC × 180?
a. +1.00 D
b. +0.50 D
c. 0.00 D (plano)
d. -1.00 D

30. A Hi-Drop lens would most likely be used for what type of vision condition?
a. Partial sight (low vision)
b. Aphakia
c. Aniseikonia
d. Nystagmus

31. How far should the optical centers of a high-powered lens be placed above or below the pupil for every 2 degrees of pantoscopic tilt?
a. 1 mm above
b. ½ mm below
c. 1 mm below
d. 2 mm below

32. What is the cylinder power of a lens with a front curve of +4.00 D and back curves of -6.50 D and -8.00 D?
a. -1.50 D
b. -4.00 D
c. -2.50 D
d. +2.50 D

33. What is the ANSI Z80.1 tolerance on center thickness when specified?
a. +/-0.25 mm
b. +/-0.30 mm
c. +/-0.50 mm
d. +/-1.00 mm

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## rolandclaur

damn it where are the answers!!!!

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## Darryl Meister

Okay, but don't look until you've tried to answer the questions:
.
.
.
.
.
.
.
.
1. b
2. b
3. a
4. c
5. b
6. a
7. d
8. b
9. c
10. a
11. c
12. d
13. a
14. d
15. c
16. a
17. b
18. c
19. d
20. a
21. c
22. a
23. d
24. a
25. c
26. d
27. b
28. b
29. c
30. b
31. c
32. a
33. b

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## slaboff

Darryl, this was absolutely great... would you do another?


I really made me realize how much more i need to study :Nerd:

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## fvc2020

Darryl 


You so rock:)   Thanks for a great study guide.  You actually stumped my doctor with some of the questions:hammer: 


Christina

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## Darryl Meister

> Thanks for a great study guide


I'm glad you guys had fun with it.




> Darryl, this was absolutely great... would you do another?


Well, maybe down the road. As you can imagine, it takes a bit to put something like this together. ;) I actually developed most of this particular test over ten years ago for a Master Exam Review Course I gave. Maybe I'll add a few more questions to round it to an even 50 over the weekend.




> I really made me realize how much more i need to study


The Master Exam has apparently been watered down a lot in terms of its technical content. So, if you can get through a test like this, you should be able to sail through the ophthalmic optics portion of the examination. Don't let this discourage you.

Personally, I had no idea what to expect when I took the Master Exam, and really prepared more than I needed. I went through all of the ABO's suggested reading materials thoroughly, which cost me a great deal of time and money. Several were even out-of-print. However, I now have a formidable optical library because of it:

Where Darryl's Early Paychecks Went

I got several of these signed by the authors, which I think is neat. In any event, I guess it's better to be too prepared than ill prepared. Frankly, Brooks & Borish's _System for Ophthalmic Dispensing, 2nd Ed_ would have been the only book needed to answer the majority of the questions from the old Master Exam.

By the way, each question on this practice test is worth about 3 percentage points, so you would need to get at least 23 of the questions right to pass a 100-question exam at 70%.

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## mullo

Great work Darryl......nice to have some material to refresh ourselves. Many thanks!!

Isn't the answer (b) for question #27?


27. Given a distance correction of O.D. +2.00 DS and O.S. +4.00 DS, which eye/lens would have the _reverse_ slab-off applied for a vertical imbalance correction?
a. Right (O.D.) eye/lens
b. Left (O.S.) eye/lens
c. Either lens could use it
d. Neither lens could use it

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## Darryl Meister

> Isn't the answer (b) for question #27?


Oops, Yes, Sorry. Unfortunately, I prepared my answer key rather haphazardly on a separate sheet of paper. I'll go through and double-check the rest right now to ensure I haven't transposed any other answers.

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## Bev Heishman

Gees Darryl,

It certainly does look like the old exam we did. There were also several questions regarding one problem that required answering several questions and if you make an error on one you would miss several. 

I wrote several items for the NAO Masters Review book too.This brought back some of those memories. One I can remember was converting radius to diopters. There was no use of preprogramed calculators either.

I still have many Optical Index magazines that asked questions like this and taught you how to solve problems. Nothing is like it out there today.

Bev

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## Darryl Meister

I double-checked all of the problems, and the answer key is correct. I fixed the answer key for 27, too; it turns out I had added "reverse" to the original question when posting it here, but forgot to change the original answer in my key.




> I still have many Optical Index magazines that asked questions like this and taught you how to solve problems. Nothing is like it out there today.


In the US, anyway. The British opticianry exams make this stuff look like a warm-up exercise. (To give you an idea, I pulled Question 3 from a British test.) I'll add a few British questions over the weekend to challenge even current Master Opticians a little.




> I wrote several items for the NAO Masters Review book too


Perhaps you should post a few here...? Actually, e-mail them to me and I'll make a test form people can fill out to find out their answers instantly.

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## mullo

It's awesome to see this type of thread. Keep them coming!!!

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## rolandclaur

I was scanning this practice test exam and one of the problems has me stumped.  Question 3.  The vertical and horizontal components are not given so you can't solve the problem simply using Pythagoras theorem.  So how would you go about solving this problem?

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## OPTIDONN

When I was studying for the advanced test I went through Darryl practice questions as well and was stumped by that very question. It's a little easier than you may expect. Sad thing is I can't remember it right now!! :Eek:  My Nyquil induced drowsiness isn't helping me! But this is an important problem to understand, it came up countless times in my test! Darryl was kind enough to explain it and maybe Darryl, Harry or Lensgrinder can shed a little more light on this than I can at this time. Now my pillow is calling me!:D

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## HarryChiling

> 3. Given a spectacle prescription for a _right_ lens that requires 2D base in, 1.5D at 30, and 0.75D at 60, what would be the resultant prism?
> a. 3.93D at 21°
> b. 3.93D at 159°
> c. 2.75D at 30°
> d. 2.75D at 45°


You could do it graphically drawing vectors, or the more simple solution would be to use the resolving prism formula

Vertical Prism = Prism * Sin (a)
Horizontal Prism = Prism * Cos (a)

so for the the first prism:

H=2 BI

for the second prism:

V=1.5*sin(30)=0.75BU (for right lens 30 degrees is nasal and above datum)
H=1.5*cos(30)=1.30BI

For the third prism:

V=0.75*sin(60)=0.65BU (for right lens 60 degreesis nasal and above datum)
H=0.75*cos(60)=0.38BI

Now we add them all together

V=0.75+0.65=1.40BU
H=2.00+1.30+0.38=3.68BI

Now we an use pythagoron theorem:

P2=V2+H2
P2=1.96+13.54
P=3.94

tan(a)=V/H
tan(a)=1.4/3.68
tan(a)=0.38
a=tan-1(0.38)
a=21

so 3.94@21 (so I rounded sue me):D  Hey Donn lay off the nyquil cocktails.:p

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## rolandclaur

Ahhh now I get it.  Each prism is its own separate vector!!  I was reading too much into the question and I confused myself.  Once again, thanks Harry for helping me out with the question.

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## jofelk

[quote=HarryChiling;208377]


... for the second prism:

V=1.5*sin(30)=0.75BU (for right lens 30 degrees is nasal and above datum)
H=1.5*cos(30)=1.30BI

For the third prism:

V=0.75*sin(60)=0.65BU (for right lens 60 degreesis nasal and above datum)
H=0.75*cos(60)=0.38BI

Maybe this is a browser problem ?

Internet Explorer:
[font=Times New Roman]3. Given a spectacle prescription for a _right_ lens that requires 2D base in, 1.5D at 30, and 0.75D at 60, what would be the resultant prism?
a. 3.93D at 21&#176;
b. 3.93D at 159&#176;
c. 2.75D at 30&#176;
d. 2.75D at 45&#176;

Firefox:
3. Given a spectacle prescription for a _right_ lens that requires 2D base in, 1.5D at 30, and 0.75D at 60, what would be the resultant prism?
a. 3.93D at 21&#176;
b. 3.93D at 159&#176;
c. 2.75D at 30&#176;
d. 2.75D at 45&#176;

The symbol for the prism in Firefox is translated as a "D". When I first read the Quote by Harry I read the prism as DOWN. I thought there was a mistake, then I checked IE. This could have caused some confusion.

I just realized this post does not make sense unless it is viewed in Internet Explorer.

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## rolandclaur

Well I just took the test.  I cannot believe I got the same exact version of the one I took in May in San Diego.

I was preparing for this exam anticipating taking a more technical version of the test.  After all I thought that there were three versions, so I studied up on image formations, more of the equations, but lo and behold it was the same exact versions.  Over 30 of the questions had to deal with management questions.  Oh well I don't think that I passed so I'll take it again next May!!

Did anybody else experience the same thing?

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## RA

Yes Roland 
I took the test a couple years ago and had a similar experience to yours. There was one question about some kind of accounting or business method and I haven't been able to find anything anything about that in the literature. I understand trying to make the test hard but using outdated references is questionable. 

:)

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## mwophthalmics

This will be quite helpful.

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## optin

Hi Daryl,

I am new to Optiboard and currently studying to take the Advanced ABO exam in November. I am trying to figure out what to study to be able to answer queston 4, 8 and 19. Forgive me for asking something that is probably so obvious. I tried to figure out it out on my own...no luck.




> *Practice Master Exam...*
> 4. If your lens measure reads +6.00 D while measuring the front curve of a 1.66 high-index plastic lens, what is the actual refractive power of the lens surface?
> 8. An object 50 cm in front of a +5.00 D lens will come to a focus at what distance from the lens?
> 19. What is the focal length of a +5.00 D lens?

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## Darryl Meister

> I am trying to figure out what to study to be able to answer queston 4, 8 and 19.


Optin, Any decent textbook on ophthalmic optics will cover these formulas, although for the original ABO Master Exam, I generally recommended System for Ophthalmic Dispensing. This is a comprehensive reference that every optician should own, in my opinion.

That said, this practice test was based upon the original ABO Master Exam, which focused much more on ophthalmic and geometrical optics than the current ABO Advanced Certification exam. The current exam covers more topics, like business management, but the actual optics questions are generally less difficult.

For the new ABO Advanced Exam, I usually recommend reviewing the NAO Advanced Opticians Tutorial, which provides content that applies directly to the questions on the ABO Advanced Exam. This book is fairly expensive, so you may try to locate a used copy, if the cost is an issue.

Best regards,
Darryl

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## optin

> Hi Daryl,
> 
> I am new to Optiboard and currently studying to take the Advanced ABO exam in November. I am trying to figure out what to study to be able to answer queston 4, 8 and 19. Forgive me for asking something that is probably so obvious. I tried to figure out it out on my own...no luck.
> 
> [/FONT]


Hi again thank you for the feedback; can you narrow it down for me a little. For example; to get the distance that the image will be if the object is 50cm in front of a +5.00 diopter lens; would I use the 1/f = 1/p + 1/q     where f = focal length, p = object and q = image

By the way I have read all of your Opti campus courses and some that I found on line that you did for Zeiss. Actually I should say that I have read them over and over because there is alot to comprehend.  I woke up one day and decided that if I am an Optician then I should understand the visual system and how a lens affects vision.  It is way to easy to just fill an order. I really want to understand what is happening in the eye and when I put a lens in front of it. So I will get this all down; probably never to the extent of someone like yourself. I appreciate anything that can shorten the learning curve especially since I committed myself to this Advanced Test.

I figured out #19 forgot to convert to cm, please tell me the formula to determne refractive surface power if the know variable are D+6.00 and n=1.66
Best Regards and Thank You

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## Darryl Meister

> Hi again thank you for the feedback; can you narrow it down for me a little. For example; to get the distance that the image will be if the object is 50cm in front of a +5.00 diopter lens


Using the common optical sign convention, you can solve this problem using the following equation, known as the Conjugate Foci formula:



Or, substituting the reciprocal object (_l_) and image (_l'_) distances for the vergences,





where _l_ is equal to -0.50 m, since the object is 50 cm in front of the wearer, and _l'_ is equal to +0.333 m, which is 33.3 cm behind the lens. Note that your equation is also the Conjugate Foci formula, but based upon a different sign convention.




> If your lens measure reads +6.00 D while measuring the front curve of a 1.66 high-index plastic lens, what is the actual refractive power of the lens surface


Since the lens measure or lens clock is typically calibrated to a refractive index of 1.530, known as the "tooling" index, you can solve this problem using the following equation to convert surface power from the tooling index to the actual refractive index of the lens material, assuming that the actual physical curvature hasn't changed:





where _FREADING_ is the reading of 6.00 from the lens measure and the refractive index _n_ is 1.66.

Best regards,
Darryl

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## optin

I can't thank you enough. You saved me hours and maybe even days trying to figure it out on my own. I don't think I will forget this after so much struggle to figure it out.

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## optin

> Using the common optical sign convention, you can solve this problem using the following equation, known as the Conjugate Foci formula:
> 
> 
> 
> Or, substituting the reciprocal object (_l_) and image (_l'_) distances for the vergences,
> 
> 
> 
> 
> ...


I took this formula from the NAO Math Review book and no-where is it refered to as the conjugate foci formula nor is there any reference to it in the NAO Advanced Opticians Tutorial. Your Ray Tracing C.E. course at the Opti Campus site makes it very clear.  
http://www.opticampus.com/cecourse.php?url=ray_tracing/
I have ordered Ellen Stoners Optical Formulas Tutorial hopefully it is referenced there and with the common sign convention.  I just want to understand how to evaluate and solve the requirement or problems and which formula gets the job done. Otherwise I will be thrown when I take the test and the same terminology or sign convention is not used.  I am putting these comments on because I assume that others who plan to sit for the Advanced Exam will find value here. The Advanced Opticians Tutorial is a fine review source for everything but geometric optics and lens design. If the optical formulas tutorial by Stoner is not sufficient then I will order the Opthalmic Guide to Dispensing since it receives high reviews. I have been avoiding spending the money and yet I have wasted almost as much on these other sources.

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## Wes

You could try "Geometric, Physical, and Visual Optics, 2e" if you really want to get into it.  
Link:  http://www.amazon.com/gp/aw/d/0750672625

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## optin

> You could try "Geometric, Physical, and Visual Optics, 2e" if you really want to get into it.  
> Link:  http://www.amazon.com/gp/aw/d/0750672625


 Thanks Wes I found the 1986 version on http://product.half.ebay.com/_W0QQprZ150365anks do you think it will be sufficient or do I need to spend the $100 plus.

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## Wes

I haven't seen the first edition, so I can't say for sure.  My feeling on it is that the applicable math, physics, and anatomy hasn't changed in the last 30 years.  It makes me wonder about the decision to release a 2nd edition.

Keep in mind these books are way overkill for taking the Advanced NOCE, but will give you a very advanced understanding of the optics of vision and corrective devices.   

Stoner, Perkins and Ferguson's book is probably more appropriate for the exam.  

Both are part of my still-growing library. 

(Fezz, do you have any more boxes of books for sale?)

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## Darryl Meister

> I took this formula from the NAO Math Review book and no-where is it refered to as the conjugate foci formula nor is there any reference to it in the NAO Advanced Opticians Tutorial.


Some writers may refer to it as the "Fundamental Paraxial Equation," although "Conjugate Foci" formula is actually more descriptive, since the formula describes the the relationship between conjugate object and image distances produced by a lens or surface.

Just keep in mind that, at the end of the day, it is the _relationship_ that is important, not the _name_ of the relationship.

And _any_ textbook on optics will rely on this formula, since it represents one of the most fundamental principles of geometrical optics. You will also find this formula and its applications described in the textbook that I recommended earlier, _System for Ophthalmic Dispensing_.




> I have ordered Ellen Stoners Optical Formulas Tutorial hopefully it is referenced there and with the common sign convention.


As Wes indicated, this should be a good reference for the ABO Advanced Exam.




> The Advanced Opticians Tutorial is a fine review source for everything but geometric optics and lens design.


Oddly enough, this topic was the emphasis of the original ABO Master Exam, which is why the certification was described as the "Master in Ophthalmic Optics" certification. Personally, I don't agree with the content of the new ABO Advanced Exam.

Best regards,
Darryl

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## agr8194

Darryl, would you please elaborate on why you don't agree with the content of the new ABO Advanced Exam.

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## Wes

> Darryl, would you please elaborate on why you don't agree with the content of the new ABO Advanced Exam.


I'm not Darryl, but I agree with his point.  These are my thoughts:
The ABOM certification reads: _Master in Ophthalmic Optics_, not _Master Optician_.  Yet the exam content was changed from a focus on ophthalmic optics to a focus on advanced opticianry, while the title remains.  It is not as accurate.

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## Darryl Meister

> Darryl, would you please elaborate on why you don't agree with the content of the new ABO Advanced Exam


I think Wes pretty much summed it up. I would describe the issues with the new "Master" certification as follows:

1. The exam content no longer emphasizes ophthalmic optics and dispensing, as the description of the certification implies. Instead, the exam has become a catchall for virtually any and every topic possibly associated with opticianry. In particular, there are now many questions associated with business management, which I believe are inappropriate for a certification exam on either basic or advanced _ophthalmic optics and dispensing_. In fact, fewer and fewer opticians actually run their own business anymore. If the ABO organization really feels the need to test opticians on business management topics, they should do so with a separate certification exam dedicated specifically to that very broad subject.

2. I have been told that the difficulty level of the Advanced Exam has been reduced significantly compared to the original Master Exam, with less difficult optical questions and fewer computational problems. I assume that this was done because of the low pass rate of the original Master Exam, which in my experience was due more often than not to a lack of sufficent preparation because the ABO organization did not adequately characterize the nature of the exam to potential test takers. So, in my opinion, the Advanced Exam no longer truly demonstrates _mastery_ of the principles and practical applications of ophthlamic optics and dispensing.

Best regards,
Darryl

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## optin

According to the domain information in the NOA Advanced Optician Tutorial there appx 47 questions that cover anatomy, physiology, refractive errors, pathology, medical conditions and surgeries as well as visual assessment. There are approximately 57 questions related to Identifying patient wants, Opticas, Lens materials, tints, AR and coatings, fitting and adjusting and low vision devices. There are roughly 25 questions on HIPAA and Finance/Business management. Finally there are roughly 22 questions on Surfacing methods, lensometer/Verfication, FED, STATE and local guidelines, regulations and standards for fabrication. I have talked to people who did not pass who had gone so far as to take review courses and who felt very prepared. So my fear is that we prepare for a very technical exam and fail because there is very little technical in the exam. I am going to cover all bases. I am enjoy learning the computations and increasing my understanding of optics and will continue after the exam pass or fail. Maybe there should be two levels Advanced and Master what do you think?

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## optin

I am thrilled with Optical Formulas Tutorial by Stoner, Perkins and Feruson. It is thorough, clear and has alot of great review questions. It is a true text book with a thorough 'Table of Contents' and Index so everything is easy to find. I am so happy I have been struggling with very poorly compiled Tutorials that even have errors which blows my confidence in them. I will reiterate the Advanced Opticians Tutorial is fine if you want to get a very general idea of the types of topics to study but for the technical topics definitely get the Optical Formulas Tutorial its awesome!!!! Is anyone else taking the Advanced Exam November 4th in Virginia?

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## agr8194

> I am thrilled with Optical Formulas Tutorial by Stoner, Perkins and Feruson. It is thorough, clear and has alot of great review questions. It is a true text book with a thorough 'Table of Contents' and Index so everything is easy to find. I am so happy I have been struggling with very poorly compiled Tutorials that even have errors which blows my confidence in them. I will reiterate the Advanced Opticians Tutorial is fine if you want to get a very general idea of the types of topics to study but for the technical topics definitely get the Optical Formulas Tutorial its awesome!!!! Is anyone else taking the Advanced Exam November 4th in Virginia?


I will be at the November Advanced exam along with a classmate of mine from the Opticianry program. Are you takin a review class?

----------


## optin

No review course but I am studying intensely and praying a lot. Are you taking review course? None of it scares me except making sure I enter the correct sequence of keys in the calculator for the formulas. It is so much faster than following the order of operations the old fashion way. Many of the formulas in the above referenced tutorial are not referenced in the NAO Advanced Exam review tutorial but I am making sure I have them memorized. Any that are referenced in NAO review will be on the back pages of the test. They will provide us with scientific calculators. Have you talked to anyone who took it in the last couple of years? There are two different formulas referenced for Martins tilt one that requires index of refraction and the other has 3 as the denominator. My understanding is that the later is a close approximation for glass and cr39.  I know that I have to really read the questions carefully and I have to be careful to make sure I use the correct sign. Good to talk to someone else that is dealing with preparing for the exam. Best of luck.

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## Darryl Meister

> There are two different formulas referenced for Martins tilt one that requires index of refraction and the other has 3 as the denominator. My understanding is that the later is a close approximation for glass and cr39


Yes, a term like "2n" in an equation would be very nearly equal to 3 for hard resin (n = 1.499) or glass (n = 1.523).




> I know that I have to really read the questions carefully and I have to be careful to make sure I use the correct sign


Formulas are fine, but be sure to understand the _principles_ well enough to know what sign the answer _should_ have. Then you can simply modify your answer as needed.

For instance, increasing the vertex distance of a plus lens brings the focus forward, thereby _increasing_ the effective power of the lens. So, when calculating vertex distance compensation for this lens, your answer should have _less_ plus power.

Best regards,
Darryl

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## agr8194

No review course for me either. I'm going over my old tests from school for the formulas and reading, reading, reading. I think that after I'm through using the ABO Tutorial for my guide of what areas to study I will donate it to my instructor where I received my Opticianry Degree. It has no use for a reference whatsoever, and he may be able to incorporate a few more things into the program for those who wish to take the Advanced Exam. Good luck to you.

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## Barry Santini

> According to the domain information in the NOA Advanced Optician Tutorial there appx 47 questions that cover anatomy, physiology, refractive errors, pathology, medical conditions and surgeries as well as visual assessment. There are approximately 57 questions related to Identifying patient wants, Opticas, Lens materials, tints, AR and coatings, fitting and adjusting and low vision devices. There are roughly 25 questions on HIPAA and Finance/Business management. Finally there are roughly 22 questions on Surfacing methods, lensometer/Verfication, FED, STATE and local guidelines, regulations and standards for fabrication. I have talked to people who did not pass who had gone so far as to take review courses and who felt very prepared. So my fear is that we prepare for a very technical exam and fail because there is very little technical in the exam. I am going to cover all bases. I am enjoy learning the computations and increasing my understanding of optics and will continue after the exam pass or fail. Maybe there should be two levels Advanced and Master what do you think?


I think "no child left behind" thinking should be left behind in an ABOM exam.

B

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## Darryl Meister

> I think "no child left behind" thinking should be left behind in an ABOM exam


Well put.

Best regards,
Darryl

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## gmc

I had a conversation with Curt Duff at a meeting a year or so ago and he told me there is only a pool of 150 questions for the advanced exam vs about 1500 for the basic exam.

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## optin

Absolutely! I was refering to the actual calculator entries related to various formulas. I could get a completely different answer if I don't enter the correct signs in the correct order using the correct keys. I am very comfortable with the principal behind vertex changes and their effect on the power of the lens as well as the effect of tilt both pantoscopic and face form on spherical and cylinder power. There are advanced concepts that I am still wrapping my brain around; curved mirror being one. Thanks to your review questions I am very comfortable with Phythogorean Theorem and resolving prism. Math is not my strength and using a scientific calculator is new to me so the learning curve in these areas is slowing me down especially since I don't have an instructor to ask for help. Thank you for your help.

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## optin

Good luck to you as well and thank you! I agree that the tutorial is primarily useful for a guide of what topics will be covered and is OK for topics other than optical optics. I have found errors but there are errors in the Optical Formulas tutorial as well. My biggest complaint about the Advanced tutorial is that it is not indexed so I cannot refer to a chapter if I am stumped on a question. I also find that their sample review questons are exspecially easy and don't reflect much advanced knowledge. This is why I went looking for more advanced review material. The Optical Formulas review fits the bill and online resources including the review test that Daryl put in an Optiboard thread has been helpful. I am a little jealous that you have recently finished school since this will all be more fresh in your brain. I love a challeng but I am petrified of failing this exam.

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## agr8194

I do feel a little advantage having just finished school. We were tested on all but the thick lens power calc. & the lens thickness equation. All prism calculations including vert imbalance were tested without the formulas supplied for the test as well as layout, decentration, vertex, etc. It's just all this rambling about the management questions that has me worried. Plus I'm taking the Advanced Contact Lens test that same day.

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## Wes

> ... Plus I'm taking the Advanced Contact Lens test that same day.


  You're braver than I was.  I waited a year between the two.

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## Barry Santini

A number of decades ago, I and another very talented optician worked for one if the NYS Board members. He adked us to write about 2/3's new questions for the Ophthalmic Dispensing part of what was a 7 (!) part written exam, taken over 3 days.

The total questions for each part was 100, so we wrote about 65 that year. The passing grade was 65.
Unfortunately, questions the three of us thought were really challenging caused the average grade for that section to drop to 35!

After some deliberation, I understand the board simply threw out the ophthalmic dispensing part that year.
My boss defended the questions to no avail.

I guess some will always see more the challenge and fun in our field than others.

B

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## Wes

> A number of decades ago, I and another very talented optician worked for one if the NYS Board members. He adked us to write about 2/3's new questions for the Ophthalmic Dispensing part of what was a 7 (!) part written exam, taken over 3 days.
> 
> The total questions for each part was 100, so we wrote about 65 that year. The passing grade was 65.
> Unfortunately, questions the three of us thought were really challenging caused the average grade for that section to drop to 35!
> 
> After some deliberation, I understand the board simply threw out the ophthalmic dispensing part that year.
> My boss defended the questions to no avail.
> 
> I guess some will always see more the challenge and fun in our field than others.
> ...


I remember this story.  It seems like this happened all over the country.  Opticianry in America threw out the ophthalmics years ago.

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## agr8194

> You're braver than I was.  I waited a year between the two.


I took both of my certification exams on the same day as well. I received a lot of "You're crazy!" comments from others taking the exams along with the test officials. There were 9 of us from our program taking both tests that day. One of us failed the CL test by 1 pt. & passed the Spec., everyone else passed both exams. Hoping to keep the streak rolling.

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## TheJpawn

> *Practice Master Exam*
> 
> 5. What kind of prism can be incorporated into single vision aspheric lenses without significantly affecting lens performance?
> a. All prism
> b. Prescribed prism
> c. Prism for decentration
> d. No prism


Maybe I do not understand the question properly but I chose "d. No prism".

I chose that answer because I agree with this statement.
No prism can be incorporated into single vision aspheric lenses without significantly affecting lens performance. 

If an OD prescribed prism and it was incorporated into a single vision aspheric lense (or any lens) of course it would significantly affect lens performance. Hopefully it would improve the lens performance. i.e. The patient would be more satisfied. Correct?

Thanks very much everyone who contributed to this thread especially Darryl Meister. I enjoyed taking the practice test thoroughly and appreciate it as excellent preparation for the ABO Advanced Exam.

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## Wes

> Maybe I do not understand the question properly but I chose "d. No prism".
> 
> I chose that answer because I agree with this statement.
> No prism can be incorporated into single vision aspheric lenses without significantly affecting lens performance. 
> 
> If an OD prescribed prism and it was incorporated into a single vision aspheric lense (or any lens) of course it would significantly affect lens performance. Hopefully it would improve the lens performance. i.e. The patient would be more satisfied. Correct?
> 
> Thanks very much everyone who contributed to this thread especially Darryl Meister. I enjoyed taking the practice test thoroughly and appreciate it as excellent preparation for the ABO Advanced Exam.


It is actually prescribed prism.  It implies surfacing.  
You can't decenter an aspheric for prism without screwing up the optics. 
Since one is true and one is false, then the other two questions denoting all and none are wrong.

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## jonathan barber

30/33 correct! *Pats myself on the back* ... Bring on the advanced exam!

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## gmc

> 30/33 correct! *Pats myself on the back* ... Bring on the advanced exam!


Good job! The new computerized advanced exam will be given for the first time at OptiCon in Nashville on Sunday, September 7.

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## optin

Congratulations Jonathan,

I will send you my emial if you want to talk about the advanced test. I took it in 2013.  As you would expect there are things you need to know for the advanced that are not on the standard exam.

Deb

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## tx11

Funny...many of the test questions posted look very similar to the questions that were on my basic ABO certification test back in'89 :Wink:

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## tx11

Oh and by the way ..a belated congrats to you Jonathan! You know your stuff!

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## BradB

Looking a this makes me realize how stupid I really am. I feel like I'm a fairly good Optician, but wish I was more intelligent in the depth of optics as you guys. You'd think, being in this business 26 years I'd know more... guess not. I've looked on ABO-NCLE.org, I still don't see any study materials for Advance ABO Testing.

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## wmcdonald

That is the overarching problem with Opticians in this country. They often get offended, and I hope you do not, but this group of people really has no clue what they do not know. Like you, they all feel they are pretty good at what they do, because they have done it for a long time. We must get better somehow if we are to remain relevant at all in the field.

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## CCGREEN

wmcdonald...........there comes a time when the rewards are not always worth all the knowledge that one could ever have about optics. Not everyone has a desire to walk around with a head stuffed full of optical knowledge that they may use bits a pieces of every other couple years or so. My suggestion is if you want to dispense know all their can be known to be the best damn dispenser possible. If you want to work finish then know all there is to know about finishing and the equipment so that no one will be better then you. And the same goes for surfacing. If you want to get into lens design and material frame design and materials then be the top expert in that area. 
But it sounds like your suggesting to be super knowledgeable about all areas. Realistically not practical. Unless of course you want to own your own business. And most of us know how many hats that will entail. The list could go on and on.

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## optin

The NAO has a textbook on the Advanced Exam. http://www.nao.org/?page_id=2625#toggle-id-12  I also used _Optical Formulas Tutorial_  by Stoner, Perkins and Ferguson. Do not get too bogged down, this book goes way deeper than the exam will,  concentrate on the formulas that are referenced in the NAO textbook. I can honestly say that studying for the exam expanded my knowledge of optics and helped me understand the concepts behind lens design better.  You will enjoy the topic it is broad and fascinating. It had been way to many years since I had dealt with trigonometry and I had to learn how to use an algebraic calculator, but I enjoyed it all the same. It is great that you are interested in growing your knowledge of optics to enhance your already impressive dispensing skills. You are far from stupid, don't be discouraged, I gaurantee you that many of us who are now certified Master Opticians had to study hard for this exam. For me it had been too long since I had taken trigonometry and I had to learn to use an algebraic calculator. Am I giving my age away?  If you don't let yourself get intimidated most of the answers can be worked out without even using the calculator. This requires that you understand it rather than just memorizing formulas. I hope you go for it. By the way the formulas are provided as part of the exam material. So, do not waste  energy stressing over memorizing them...instead put your energy into terminology and knowing what the formulas solve. Of course you have to understand the order of operatons for inputting the formula in the algeabraic claculator.  This is in the _Optical Formulas Tutorial_. Much of the material is not all that advanced but because it is not something we deal with on a daily basis you would have to memorize the information.

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## wmcdonald

> wmcdonald...........there comes a time when the rewards are not always worth all the knowledge that one could ever have about optics. Not everyone has a desire to walk around with a head stuffed full of optical knowledge that they may use bits a pieces of every other couple years or so. My suggestion is if you want to dispense know all their can be known to be the best damn dispenser possible. If you want to work finish then know all there is to know about finishing and the equipment so that no one will be better then you. And the same goes for surfacing. If you want to get into lens design and material frame design and materials then be the top expert in that area. 
> But it sounds like your suggesting to be super knowledgeable about all areas. Realistically not practical. Unless of course you want to own your own business. And most of us know how many hats that will entail. The list could go on and on.


I understand your perspective, and appreciate it. But the lack of knowledge in most areas today is quite disturbing. Until we improve our basic level of knowledge, we will go nowhere as a field. We are not called finisher, and dispensers (I hate the term......I get shaving cream from a dispenser), etc. We are called Opticians, and we need a requisite sphere of knowledge that all who use that title bring to the table.

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## King.Matthew

> 15. Given the following FT-35 bifocal prescription:
> +2.00 DS with a +2.50 D add, O.U. (both eyes)
> PDs = 66/62 mm
> How could you induce a 1D _base in_ prismatic effect, per eye, at _near_ only?
> a. Use a 56 mm distance PD
> b. Use a 60 mm near PD
> c. Use a 54 mm near PD
> d. Cant be done


Not  sure if I agree with C being the right answer? Just trying to get some  clarification as I am taking the advanced test next month. My  calculation would be more of a near PD of 57.5? Correct me if I'm wrong  but wouldn't you add the distance Rx along with the add power as that is  how the Pt would be viewing it? Mathematically that seems to make more  sense to me, but I guess I can cut it tomorrow and find out. Also I  would be interested in any advice I can get on the Advanced Test. Thank  you for any and all help.

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## ml43

> Not  sure if I agree with C being the right answer? Just trying to get some  clarification as I am taking the advanced test next month. My  calculation would be more of a near PD of 57.5? Correct me if I'm wrong  but wouldn't you add the distance Rx along with the add power as that is  how the Pt would be viewing it? Mathematically that seems to make more  sense to me, but I guess I can cut it tomorrow and find out. Also I  would be interested in any advice I can get on the Advanced Test. Thank  you for any and all help.


"Hi Mike,

Brooks makes no mention of neutralizing the prismatic effect at distance  because it is not necessary. Essentially, you will still be placing the  distance optical centers in front of the pupils (assuming that the  distance Rx doesn't call for prescribed prism). You are simply  decentering the bifocal segments from their intended locations, which  changes near prism -- not distance prism. Consequently, the power of the  distance Rx will not play any role either for the calculations.

Basically, the bifocal segment acts like a little plus lens. If you have  a PD of 60/57, you would normally place the distance OCs at 60 mm and  the segment centers at 57 mm. To determine how much to decenter the  segments from that point (57 mm), you need to use Prentice's rule with  the segment. The add power of the segment is used.

Prism = Dec (cm) * Power
3.00 PD = Dec (cm) / +2.50 D
Dec (cm) = 3.00 PD / +2.50 D
Dec (cm) = 1.2 cm = 12 mm

Since you need base _in_ prism, each bifocal segment would have to be decentered _in_  an extra 12 mm. This means that the segment near PD would have to be 57  mm - (2 * 12 mm) =  33 mm to induce a total of 6.00 PD base in. The  distance PD should still be set to 60 mm.

Unfortunately, obtaining this much prism at near by decentration  requires a great deal of inset. Oversized (e.g., FT-45) or  Franklin-style segs will be necessary to get a sufficient field of view  through the segments, unless you use prism segs or Press-On optics or  something. You can cut down on the amount of decentration necessary by  either increasing the add power or by cutting back on the prism at near.  Good luck.

Best regards,
Darryl"

per an old post from Darryl
http://www.optiboard.com/forums/show...g-decentration

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## Robert Martellaro

> Not  sure if I agree with C being the right answer? Just trying to get some  clarification as I am taking the advanced test next month. My  calculation would be more of a near PD of 57.5? Correct me if I'm wrong  but wouldn't you add the distance Rx along with the add power as that is  how the Pt would be viewing it? Mathematically that seems to make more  sense to me, but I guess I can cut it tomorrow and find out. Also I  would be interested in any advice I can get on the Advanced Test. Thank  you for any and all help.


If we ignore the prismatic effect of the eyes converging behind the distance Rx, a near PD of 62mm - 8mm = 54mm (.8cm x 2.50 D = 2∆ BI total) is the correct answer. 

If we include the effect of the distance Rx which is 2.00 D x .4cm = .8∆ BO total, that can be cancelled by decentering the segs an additional 3.2mm (.32cm x 2.50 D = .8∆ BI total). A near PD 50.8 achieves a resultant prismatic effect of 2∆ BI total during the near gaze. 

I'm not going to consider the fact that the eye will rotate towards the prism apex .3mm per prism diopter upsetting the distance calculation in a way that's somewhat like going back in time and killing your grandparents.

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## MakeOptics

> Not  sure if I agree with C being the right answer? Just trying to get some  clarification as I am taking the advanced test next month. My  calculation would be more of a near PD of 57.5? Correct me if I'm wrong  but wouldn't you add the distance Rx along with the add power as that is  how the Pt would be viewing it? Mathematically that seems to make more  sense to me, but I guess I can cut it tomorrow and find out. Also I  would be interested in any advice I can get on the Advanced Test. Thank  you for any and all help.


A follow up question will help shed a little light on this, what near PD would you use to induce 0Δ at near?

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## King.Matthew

Ok, so I ran a pair of glasses running the scenario from the test and get 4 base in OU, 2 diopters of prism per eye? The only way the scenario would work in real life application from what I can see is if there was no Rx for distance?!! Also while playing with lenses I'm looking at stock lenses with no Rx verses lenses with + Rx' and the ones with + Rx' require less movement to get the same amount of prism induced as the stock lenses with no Rx and the same add. I am by no means at the level of optical knowledge most of you that have responded have, I'm sure but I would highly appreciate further looking into this and getting to the bottom of it, because the theory behind what I've read here and the actual putting to the test is not matching up.
And I guess to add to this theory, wouldn't the way we calculate for the amount of prism needed for Bicentric Grinding come into play to a degree, theoretically of course.

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## lensgrinder

> Not  sure if I agree with C being the right answer?



_Given the following FT-35 bifocal prescription:_
_+2.00 DS with a +2.50 D add, O.U. (both eyes)_
_PDs = 66/62 mm_
_How could you induce a 1__D__ base in prismatic effect, per eye, at near only?
_
1. Find the inset


Seg Inset is 2 mm

2. Find the additional inset needed





* 0.4 cm or 4 mm 
* New Seg Inset = 2 + 4 = 6 mm(double this for total)
* 66 - 12 = 54 NPD
66/54

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## optical24/7

I'm in King's camp. There are two OC's on a segmented bifocal. If I'm moving the near only OC to create prism in the seg only, I have to use the total power at the near OC to compute the total decentration to achieve "X" prism. 

I also took a look at a lens I have here that happens to be a FT35 +3. dist/2.00 add. The power at the near OC is a +5.00 I move it 4mm in my lensometer and I get 2D prism (in any direction.) Thus, if I want to induce 1D prism with my test lens I only need to move the OC 2mm.

If me and King are wrong, I'll need a better explanation than what I understand (and demonstrated to myself).

----------


## Wes

> Ok, so I ran a pair of glasses running the scenario from the test and get 4 base in OU, 2 diopters of prism per eye? The only way the scenario would work in real life application from what I can see is if there was no Rx for distance?!! Also while playing with lenses I'm looking at stock lenses with no Rx verses lenses with + Rx' and the ones with + Rx' require less movement to get the same amount of prism induced as the stock lenses with no Rx and the same add. I am by no means at the level of optical knowledge most of you that have responded have, I'm sure but I would highly appreciate further looking into this and getting to the bottom of it, because the theory behind what I've read here and the actual putting to the test is not matching up.
> And I guess to add to this theory, wouldn't the way we calculate for the amount of prism needed for Bicentric Grinding come into play to a degree, theoretically of course.


Measured at 62 mm?  Consider the seg as it's own lens.  You are only moving the near seg.  The distance OC stays in the same place, at 66mm.  Any small amount of prism in the distance portion of the lens at 62mm will stay the same as the seg is slid in.  How far do you have to move a +2.50 to induce 1 diopter of prism?  4 mm right?  Now do the other eye.  Same answer, for a total of 8mm.  Therefore, 62mm - 8mm = 54mm.  Measure the result at 62mm, where the patient's near gaze falls.

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## optical24/7

So, for arguments sake, If I have a -2.00 dist/+2.00 add, I can inset the seg 5mm and induce 1D of prism (in a plano powered part of the lens?)

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## Wes

> So, for arguments sake, If I have a -2.00 dist/+2.00 add, I can inset the seg 5mm and induce 1D of prism (in a plano powered part of the lens?)


 How much prism is at the Distance OC of that lens?  None.  Assume a seg inset of 1.5mm.  How much prism is at the near OC?  .3 diopters base in due to the -2 carrier.  We aren't really considering this, though, because it will be there regardless of where the seg is actually moved. If you move the seg in 5 mm, now how much prism is there, 1.5mm in from the distance OC as the eye drops into the seg?  Actually 1.3, but we disregard that .3 for the thought experiment.  If you wanted to be nitpicky, you'd actually slide the seg  3.5mm, for a total inset of 5mm vs 1.5mm to get exactly 1 diopter of base in prism at near only. 
 If you had a +2 seg and a +2 carrier, you'd have .3 base out in the carrier as the eye converged and dropped into the OC of the seg.  Therefore, you'd technically move the seg 6.5mm in instead of 5mm to get that 1 diopter of prism base in at near only.  But again, we normally disregard the .3 for the thought experiment.
You have to think about the seg as its own lens to begin to understand it.

----------


## King.Matthew

Just for the record my last name is King, LOL. Wes, I completely understand where your coming from, my problem is the theory and the actual results in lab are not matching. I will run the test with a -2.00 in the distance with a +2.00 add tomorrow to find out what will happen. But like I stated earlier I plugged in the number's 66/54 +2.00 with a +2.50 add and got four base in prism total measured at 62mm. So this tells me that the calculations are wrong?
And again why would it be different than the calculation for vertical calculations where one finds out how much prism is in the distance and then adds the add power to the distance power to figure out how much prism is in the near part?

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## Wes

The calculations aren't wrong.  Something went wrong in your process.  Spot up the distance PD and see if it's actually 66.

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## King.Matthew

I will recheck it tomorrow in the lab, have you done this in actual glasses or just in math? A Dr. I used to work for would request base in prism in the near about 2 times a week on our older patients and I would always calculate it out using the distance Rx in connection with the Add power? I guess I could have been doing it wrong but the results using my method always portrayed the end result I wanted. I guess I would say don't take my word for it, try it in your lab tomorrow and see what you get? I would be interested to know what your lab equipment does with the Rx of +2.00 with an Add of +2.50 and the PD 66/54?

----------


## ml43

> I'm in King's camp. There are two OC's on a segmented bifocal. If I'm moving the near only OC to create prism in the seg only, I have to use the total power at the near OC to compute the total decentration to achieve "X" prism. 
> 
> I also took a look at a lens I have here that happens to be a FT35 +3. dist/2.00 add. The power at the near OC is a +5.00 I move it 4mm in my lensometer and I get 2D prism (in any direction.) Thus, if I want to induce 1D prism with my test lens I only need to move the OC 2mm.
> 
> If me and King are wrong, I'll need a better explanation than what I understand (and demonstrated to myself).


It's easier to treat the bifocal part as a separate lens.

Think about it this way, if you had a SV +2.00 DS OU with a PD of say 62mm

if you wanted to make a +2.50 DS OU clip on that induced 1D of BI prism per eye, 
what would the PD of the clip on need to be?

assuming of course that the front curve of the +2.00 lens, and the rear curve of the +2.50 clip matched exactly, and there is exactly zero space between the two.


edit:
I believe the problem/miscalculation is in the fact that we are trying to achieve near BI prism relative to the distance Rx, not relative to no Rx.

Of course if you take a +2.00 DS ou +2.50 add and place the bifocal segment OC in front of the pupil and then decenter(by finish blocking), you will use the full reading power(+4.50) to calculate induced prism.


But that's not what the problem is asking. 
The problem is basically asking how much surface decentering of the segment is required to achieve BI prism at the near Rx only.  
When you surface decenter/block a bifocal/trifocal, you are moving the segment away from the OC.
It's part of the reason why you don't surface block a bifocal with the segment at the generator blocks geometric center.
It's also the reason when ordering a Bifocal from a lab, you are supposed to supply OC height/seg drop, and both distance and near PD, to prevent unwanted induced prism.

----------


## optical24/7

So, does this rule only work with flat tops? Wonder if I was doing a Franklin bifocal with a -2.00 dist/+2.00 add? I would be using a -2.00 for dist and a Plano for near. How am I going to decenter the plano's OC enough to get 1d of prism?

I'm really sorry guys. I hear what you are saying to think of the seg as it's own little lens, but in my minds lensometer, it's not seeing it!

----------


## ml43

> So, does this rule only work with flat tops? Wonder if I was doing a Franklin bifocal with a -2.00 dist/+2.00 add? I would be using a -2.00 for dist and a Plano for near. How am I going to decenter the plano's OC enough to get 1d of prism?
> 
> I'm really sorry guys. I hear what you are saying to think of the seg as it's own little lens, but in my minds lensometer, it's not seeing it!


it works for all bifocals, even round segs.

remember, bifocals are basically a smaller lens fused into a larger lens.

In a -2.00 DS OU, +2.00 add

yes the total power in the segment is Plano, but you are not placing the segment oc directly in front of the eye.

Also,
Just because the total power is Plano in the segment, does not mean you cannot acheive prism.

A perfect example is the clip example I used above.

or how about this,
if you put on -2.00 sph contacts, then put on a pair of +2.00 readers.
would you need to take a PD measurement? 
Or does PD not matter?

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## optical24/7

Forgive an old man's brain. Well, your +2.00 reader has power. You displace the OC from the normal PD and you can create a prismatic error. How do I move a plano's OC to create 1D of prism without grinding it in?

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## King.Matthew

I think the issue is we are not talking about two separate lenses in reality? Of course if you take say a 1 base in prism in front of contacts the image will enter the first lens and exit then enter the contacts with said end result and travel theoretically without prism due to the contact moving with the eye while converging. But this is not the case with eyeglass lenses. We are talking about a image that is entering and exiting "one" lens. 
We or most I hope agree with what a separate +2.50add will do if moved said mm. But that's really not what were talking about. We could talk math all day long and not put our math to use, our math is only as good as the end result of said math so lets put our math to the test on Monday? If your interested to know if "your" math works cut or order problem #15 with "C" being the answer and see what you get and then post your result here for the world to see. I am by no means at the level of most optically, just a guy that has the hands of someone who is twenty years my senior.




> 15. Given the following FT-35 bifocal prescription:
> +2.00 DS with a +2.50 D add, O.U. (both eyes)
> PD’s = 66/62 mm
> How could you induce a 1D _base in_ prismatic effect, per eye, at _near_ only?
> a. Use a 56 mm distance PD
> b. Use a 60 mm near PD
> c. Use a 54 mm near PD
> d. Can’t be done


Ok so everybody is clear we've got a PT with a PD of 66/62 were going to grind or order depending on our situation 66/54. Cut, edge and mount said lens and neutralize the end result. Display your finding here and we'll go from there?
Game anybody?

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## Wes

> Forgive an old man's brain. Well, your +2.00 reader has power.


So does a +2.00 seg.




> You displace the OC from the normal PD and you can create a prismatic error. How do I move a plano's OC to create 1D of prism without grinding it in?


It's not plano.  The seg has power.

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## Wes

> So, does this rule only work with flat tops? Wonder if I was doing a Franklin bifocal with a -2.00 dist/+2.00 add? I would be using a -2.00 for dist and a Plano for near. How am I going to decenter the plano's OC enough to get 1d of prism?
> 
> I'm really sorry guys. I hear what you are saying to think of the seg as it's own little lens, but in my minds lensometer, it's not seeing it!


An old style Franklin actually was 2 lenses.  However, in this example, you couldn't decenter the lower half of the lens, because it actually would be plano, but most times that wouldn't be the case, and you could use decentration..  But, in your example, you'd have to grind the prism into it, and glue it to the top.

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## Wes

> I will recheck it tomorrow in the lab, have you done this in actual glasses or just in math? A Dr. I used to work for would request base in prism in the near about 2 times a week on our older patients and I would always calculate it out using the distance Rx in connection with the Add power? I guess I could have been doing it wrong but the results using my method always portrayed the end result I wanted. I guess I would say don't take my word for it, try it in your lab tomorrow and see what you get? I would be interested to know what your lab equipment does with the Rx of +2.00 with an Add of +2.50 and the PD 66/54?


I've done it many times in the lab.  I now work for an LMS software company and help others do it.  The math is solid. I'll type up an example of a rather difficult one I helped one of our lab customers with a little later.  Gotta run.

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## King.Matthew

> An old style Franklin actually was 2 lenses.  However, in this example, you couldn't decenter the lower half of the lens, because it actually would be plano, but most times that wouldn't be the case, and you could use decentration..  But, in your example, you'd have to grind the prism into it, and glue it to the top.


But I thought the "little add" has no dependance on the "distance" Rx? The thread has stated that regardless of the distance you only calculate the power of the add for decentration?

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## optical24/7

> An old style Franklin actually was 2 lenses.  However, in this example, you couldn't decenter the lower half of the lens, because it actually would be plano, but most times that wouldn't be the case, and you could use decentration..  But, in your example, you'd have to grind the prism into it, and glue it to the top.


This is exactly what I am talking about. This formula does not work for all scenario's, or at least all types of bifocals. If the formula doesn't work for a Franklin, how does it work in a ft35 if the segmented area is nutrilized down to Plano? In a lensometer, can you see the OC move when you view it across the segments area (Plano)?

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## King.Matthew

> I've done it many times in the lab.  I now work for an LMS software company and help others do it.  The math is solid. I'll type up an example of a rather difficult one I helped one of our lab customers with a little later.  Gotta run.


How difficult can it get when your only considering the add?

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## Wes

Matt said:We or most I hope agree with what a separate +2.50add will do if moved said mm. But that's really not what were talking about. 

That is exactly what we are talking about.

Matt said:
We could talk math all day long and not put our math to use, our math is only as good as the end result of said math so lets put our math to the test on Monday? If your interested to know if "your" math works cut or order problem #15 with "C" being the answer and see what you get and then post your result here for the world to see. 

My math?  The math has been around far longer than I.  I am just a user of the technique.

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## Wes

> But I thought the "little add" has no dependance on the "distance" Rx? The thread has stated that regardless of the distance you only calculate the power of the add for decentration?


Apples and oranges.

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## Wes

> This is exactly what I am talking about. This formula does not work for all scenario's, or at least all types of bifocals. If the formula doesn't work for a Franklin, how does it work in a ft35 if the segmented area is nutrilized down to Plano? In a lensometer, can you see the OC move when you view it across the segments area (Plano)?


I never said it would work in every scenario.  Someone else said that.  With a ft-28, the seg is superimposed over the carrier.  With a Franklin, the seg is its own lens.  Apples and oranges.

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## King.Matthew

> Apples and oranges.


Apples and orange trees continue to produce apples and oranges? If I get a grape fruit from one of the trees I would have to rename the tree?

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## optical24/7

Wes, I never doubt you're knowledge. You're one of the smartest guys around on technical optical stuff. I'm sure you and others are right. I'm just having a hard time understanding how the total power of the segmented area has no bearing on how much you would decenter, only the amount of add.

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## King.Matthew

> I never said it would work in every scenario.  Someone else said that.  With a ft-28, the seg is superimposed over the carrier.  With a Franklin, the seg is its own lens.  Apples and oranges.


I am only talking about problem #15 of the review test here? I had issue with "C" being the correct answer and wanted some clarification as I did not agree with it? I cut it and did not get what "C" said I should get, end of story. I am asking you as I'm sure working for a LMS software company would have a test lab and could generate said scenario, and show that "C" is correct, not by math, because what would it matter if the math said this is the result but the glasses measured said no its not?

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## Wes

> How difficult can it get when your only considering the add?


Imagine a pt DPD of 73, NPD 70.  +2.25 add.  Distance Rx irrelevant.  Doc needs 3 diopters base in OU at near.  Use ft-45.  Each seg needs to be decentered 13.3 mm from 70, for a total inset of 14.8 mm.  Lab software only allowed 5 each eye, for a total of 10.  We amended the sf lens data from seg inset 5 in to 4.8 out the get the extra 9.8mm required to meet Rx.  Worked like a charm.

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## Wes

> I am only talking about problem #15 of the review test here? I had issue with "C" being the correct answer and wanted some clarification as I did not agree with it? I cut it and did not get what "C" said I should get, end of story. I am asking you as I'm sure working for a LMS software company would have a test lab and could generate said scenario, and show that "C" is correct, not by math, because what would it matter if the math said this is the result but the glasses measured said no its not?


I have already done this, many times, myself.  Please see above example as the most recent occurrence of helping someone else do it.  I'm not going to ask someone to do it again for no reason other than to satisfy you.  I'm sorry, but you made some kind of mistake in your procedure or in your reading.

The reason math works, is because it represents reality.  If it didn't, it would be useless, and there would be no technology, and no civilization past the hunter-gatherer level.  

To add:  this practice exam was put together by the brilliant Darryl Meister, ABOM(RIP) and has been vetted for years.  It's not likely to be proven incorrect today.

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## optical24/7

Once again Wes, I'm sure you are correct. But as I asked earlier, if the segmented area is Plano due to Rx, would you see the near OC move as you moved it horizontally in a lensometer? Once again, forgive me my friend, I'm just having a dickens of a time wrapping my brain around this!

----------


## Wes

> Once again Wes, I'm sure you are correct. But as I asked earlier, if the segmented area is Plano due to Rx, would you see the near OC move as you moved it horizontally in a lensometer? Once again, forgive me my friend, I'm just having a dickens of a time wrapping my brain around this!


Assuming your earlier Rx of -2, +2 add.  Assume near pd of 62. If you decentered the seg 5mm and read the near power at 62 mm, you would see plano with 1 diopter prism, base in.  Remember, you aren't moving the distance OC.  The seg has an OC and power of its own.  5 mm away from the OC of a +2 lens, how much prism do you see?  Conversely, in your Franklin example, using -2 dist, +2 add, the lower portion would indeed be plano.  That's not the case with a "carried" seg like a ft.

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## optical24/7

Ok, I'm starting to visualize that. I'm still gonna order me a ft like above so I can see it myself!� :Cool:

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## King.Matthew

> I have already done this, many times, myself.  Please see above example as the most recent occurrence of helping someone else do it.  I'm not going to ask someone to do it again for no reason other than to satisfy you.  I'm sorry, but you made some kind of mistake in your procedure or in your reading.
> 
> The reason math works, is because it represents reality.  If it didn't, it would be useless, and there would be no technology, and no civilization past the hunter-gatherer level.  
> 
> To add:  this practice exam was put together by the brilliant Darryl Meister, ABOM(RIP) and has been vetted for years.  It's not likely to be proven incorrect today.


I am by no means trying to prove somebody that is obviously ten times more knowledgeable than I am wrong. But I will add if we continue down a path that our predecessors had it right and we do not will only limit us in furthering our knowledge. They are a foundation to build upon?!
I have an electrical engineering degree and can assure you that math and reality are two separate things. I can build a circuit board using 3D software that allows me to plug and play if you will with resistors, gates, capacitors, you name it and have it run perfectly in a perfect world, and then build a live model and promise you it will not work "correctly". There will need to be tweaking done after the fact because lets face it, we do not live in a perfect world. 
Could I have done something wrong in my mock test of problem 15? Of course, will I run another test and do it again from scratch to see what I get of course I will. I only ask that somebody else do it as well because if they get what I get then "C" is wrong!
Do you run test's after software is created to see if the end result is what was expected? Of course you do, why?
I by no means am trying to argue a point, I just need 2+2 to equal 4. 
At the end of the day if said answer will be "C" on the ABO Advanced Test then that will be what I will pick. Easy enough.

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## Wes

We don't live in a perfect world, and we don't manufacture to exact specs.  We manufacture to tolerances. In your circuit board example, sometimes the parts weren't manufactured to tolerances, or perhaps, tolerances add up.  Also, quantum physics and relativity must come into play with very small distances found in microprocessors and the time dilation that affects GPS, respectively, for example.
But that's not really the case in our situation.  The question at hand is old hat in this industry, proven thousands of times over.

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## King.Matthew

> We don't live in a perfect world, and we don't manufacture to exact specs.  We manufacture to tolerances. In your circuit board example, sometimes the parts weren't manufactured to tolerances, or perhaps, tolerances add up.  Also, quantum physics and relativity must come into play with very small distances found in microprocessors and the time dilation that affects GPS, respectively, for example.
> But that's not really the case in our situation.  The question at hand is old hat in this industry, proven thousands of times over.


I guess the only question I would have is a Ft-35 add power super imposed as stated over the carrier or is it one lens? if one lens then personally I would think the image path only encounters two surfaces and thus the thickness "apex to base" of said lens has to be considered ultimately resulting from distance and add Rx,  if in fact the lens is "super imposed" then there would be four surfaces the image would be traveling through front and rear of add lens then front and rear of distance lens.
As I learn more I'm sure I or somebody else will prove me wrong, I am ever learning and ultimately do not want to be right, just want to have the right knowledge.

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## Wes

There are obviously only two surfaces in question on a molded plastic ft35.  The word superimposed was used to try to explain the thinking that solves the question, primarily because nothing else had worked to explain it to that point.

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## ml43

Sorry for the confusion, when you said franklin bofocal, I assumed you meant executive bifocal(old habits die hard).


you don't need to surface anything to prove the math.

Easiest way is to take two FSV lenses with opposing powers(i.e. +1.00 ds and -1.00 ds)

Put them back to back, total power of plano.  
If you moved them together in a lensmeter there should be no prism induced.
But if you move one separately from another, it will behave like a +/- 1.00 DS in terms of prism

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## King.Matthew

> Sorry for the confusion, when you said franklin bofocal, I assumed you meant executive bifocal(old habits die hard).
> 
> 
> you don't need to surface anything to prove the math.
> 
> Easiest way is to take two FSV lenses with opposing powers(i.e. +1.00 ds and -1.00 ds)
> 
> Put them back to back, total power of plano.  
> If you moved them together in a lensmeter there should be no prism induced.
> But if you move one separately from another, it will behave like a +/- 1.00 DS in terms of prism


Ok that makes sense, I will have to play with that on Monday, so for arguments sake I will grab a -2.00, plano, +2.00, and for kicks and giggles a +4.00 and use these for the distance Rx. I will also grab a +2.50 for my super imposed add and in order to get 2 diopters of base in prism I will need to move the +2.50 lens 8mm for each example?

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## optical24/7

Ok Wes, I had an ah ha moment. I think this explains my -2/+2 question better to understand the mechanics of what's going on:

To decenter the D OC out from the seg, you'd have to grind B in prism. The resulting B in would show up in the seg. Do I have it now?

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## Wes

> Ok Wes, I had an ah ha moment. I think this explains my -2/+2 question better to understand the mechanics of what's going on:
> 
> To decenter the D OC out from the seg, you'd have to grind B in prism. The resulting B in would show up in the seg. Do I have it now?


Sorry, no.  The distance OC would have zero prism in this case, yet prism is ground into the carrier to move THE OC away from the bifocal.  But from our perspective, the prism is created by sliding/decentering the seg over, (while keeping the distance OC in the same location) so that rather than looking through the seg's OC as the eye dropped down, you would be looking through the seg 5 mm away from the seg OC, thus creating prism at near only.
Remember, the eye isn't looking through the middle of the seg where the OC is, in this case.  It's looking through the seg 5mm out from the OC of the decentered seg.

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## optical24/7

Yes, no prism at the D OC, but when grinding the above Rx for a say 66/54 pd wouldn't you grind B in to move the D OC out so you could decenter the seg in to 54?

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## Wes

> Yes, no prism at the D OC, but when grinding the above Rx for a say 66/54 pd wouldn't you grind B in to move the D OC out so you could decenter the seg in to 54?


I suppose you may have to, depending on the blank, from a manufacturing perspective.  But considering that most ft blanks have the seg inset a few mm (5 is common) and dropped a few mm (again, 5 is common), perhaps not.  It depends on your layout methodology.  Do you block on center?  Do you block on seg?  I think this train of thought is as likely to add confusion to the issue as it is to add clarification.  Do we want to discuss the question at hand, or lens surfacing in general?

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## ml43

> Yes, no prism at the D OC, but when grinding the above Rx for a say 66/54 pd wouldn't you grind B in to move the D OC out so you could decenter the seg in to 54?


you're decentering the seg to avoid prism at the distance while achieving decentered prism at the near/seg.

easiest way to model this is like I stated about with the two lenses with apposing powers.

if you want to get really graphic.

take a -2.00 DS FSV lens, 
then edge a +2.00 DS to say 35mm.

place the +2.00 lens on the -2.00 then read the power, now move the +2.00 lens around while keeping the -2.00 stationary.

you'll notice a prism effect equal to +2.00, even though your lensometer reading is technically Plano, and you aren't grinding any prism

----------


## optical24/7

> I suppose you may have to, depending on the blank, from a manufacturing perspective.  But considering that most ft blanks have the seg inset a few mm (5 is common) and dropped a few mm (again, 5 is common), perhaps not.  It depends on your layout methodology.  Do you block on center?  Do you block on seg?  I think this train of thought is as likely to add confusion to the issue as it is to add clarification.  Do we want to discuss the question at hand, or lens surfacing in general?


Yes, in my lab days, you surfaced on geo center, you used prism rings to move the D OC to a position you wanted in relation to the seg OC. On minus lenses, usually base out to move the OC in from geo center. With a D OC going out instead of in from geo center, I'd grind base in to move the OC out. Am I making sense?

----------


## ml43

> Yes, in my lab days, you surfaced on geo center, you used prism rings to move the D OC to a position you wanted in relation to the seg OC. On minus lenses, usually base out to move the OC in from geo center. With a D OC going out instead of in from geo center, I'd grind base in to move the OC out. Am I making sense?


makes perfect sense,

now translate that "prism" into decentration and the problem should make sense

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## MakeOptics

Tests deal with theory.  All the equations used in this thread so far have been first order approximations without taking any additional measurements into account.  The question even specifies near only.  Ifvany tester puts this much effort into any single question they would fail by running out of time.

Good luck.

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## optical24/7

When I thought of it from a manufacturing angle (surfacing it). It made much more sense.

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## MakeOptics

There are a lot of shortcuts in manufacturing.  Software in this day and age takes the complexity out of the process and adds additional layers of accuracy.  As a former lab person, a lot of what is learned is rule of thumb, some in the lab will start looking into the why's that make the answers what they are.  This quest for knowledge is a good thing, with schooling and theory it works the other way around you learn the fundamentals as building blocks and build on them until the picture becomes more clear.  Sure we can add accuracy to this equation and even get it to the point of near flawless, but the question still remains simple and the answer is only graded one way.  The discussion is nice, but the question and answer remain brief with only 4 choices.  I liked the test Darryl put together and enjoyed taking it.  I am glad to see others have as well, I think this thread will live on and constantly have additional questions and explanations as to how the questions can be made better, stronger.  I enjoy reading.

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## King.Matthew

I am enjoying it as well, as I do not want to be "right" but to know "right"?! I understand completely the idea of two separate lenses and the image being bent/displaced by the add of the lens in the front and then passing the second lens through the optical center so no deviation. But looking at it from a FT-35 standpoint that is not really the case because I am not pulling the bifocal off the lens and replacing it. Would it be safe to say the optical center of the distance RX is being pushed further out in order to be able to decenter the lens said amount in order to gain said near prism? And if that be the case then I'm not sure how one wouldn't need to take into consideration the total power? Anyway I hope it's safe to say "C" will be the answer on the test even if I do not agree with it nor have been able to recreate it in an actual pair of glasses. Though I have a pair being made from an outside lab to take myself out of the equation. I am as hard headed as they come and sometimes need a sludge hammer to get it to sink in so thank you all in advance for your patience with me. :)

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## MakeOptics

> I am enjoying it as well, as I do not want to be "right" but to know "right"?! I understand completely the idea of two separate lenses and the image being bent/displaced by the add of the lens in the front and then passing the second lens through the optical center so no deviation. But looking at it from a FT-35 standpoint that is not really the case because I am not pulling the bifocal off the lens and replacing it. Would it be safe to say the optical center of the distance RX is being pushed further out in order to be able to decenter the lens said amount in order to gain said near prism? And if that be the case then I'm not sure how one wouldn't need to take into consideration the total power? Anyway I hope it's safe to say "C" will be the answer on the test even if I do not agree with it nor have been able to recreate it in an actual pair of glasses. Though I have a pair being made from an outside lab to take myself out of the equation. I am as hard headed as they come and sometimes need a sludge hammer to get it to sink in so thank you all in advance for your patience with me. :)


You're making the assumption that the near PD specified in the question doesn't take the power into account.  Since the question supplies a near PD the assumption should be the other way around, we should assume that this near PD is accurate for the same scenario given 0 prism, so now the question is a bit simplified.  With your assumption we are throwing out the near PD and working the whole scenario from scratch.  Again theory vs reality are two separate beasts.  In reality most opticians I have seen even the ones that say they don't just subtract 3mm from the binocular distance PD and supply that as the near PD.

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## King.Matthew

> You're making the assumption that the near PD specified in the question doesn't take the power into account.  Since the question supplies a near PD the assumption should be the other way around, we should assume that this near PD is accurate for the same scenario given 0 prism, so now the question is a bit simplified.  With your assumption we are throwing out the near PD and working the whole scenario from scratch.  Again theory vs reality are two separate beasts.  In reality most opticians I have seen even the ones that say they don't just subtract 3mm from the binocular distance PD and supply that as the near PD.


Now were talking about a whole other ball game, taking the question as it is none of the answer's seem correct as it can be done so D is not right but 54 will induce too much prism to be right as well. But like I said I will post what comes tomorrow.

----------


## ml43

> Would it be safe to say the optical center of the distance RX is being pushed further out in order to be able to decenter the lens said amount in order to gain said near prism? And if that be the case then I'm not sure how one wouldn't need to take into consideration the total power?


You have a degree in engineering, right?

Let's do this with mostly somewhat basic mathematics.

Given:
- Base curve of the distance of the bifocal is spherical 
- Base curve of the seg is spherical 
- Rear curve of the the lens is spherical 

Let's define what a bifocal is:
A smaller lens fused to a larger lens where the (distance between the two lenses) = n, where n is the limit as x approaches zero(0). 
if and only if the front curve of the base lens(carrier) matches the rear curve of the top lens(seg)


Since we are only dealing with horizontal prism, we can take half of a horizontal cross section by dividing the entire lens into little slices(n), as n approaches infinity. 

So we are left with two very thin prisms that for all intensive purposes are the exact height of a ray of light and symmetrical at both ends in regards to the vertical. 
Further more, we can model(test) these prisms using any vertical height we want, because we are only concerned with horizontal prism and one ray of light.

Now, as the apex of the rear prism(OC of the distance) moves towards or away from the base of the front prism(geometric center of the seg), 
it displaces(causes a prismatic effect) a ray of light at a rate that is equal to the curve difference between the front curve of the Seg, and the base curve of the carrier.
Which is equal to the power of the Seg. 

In other words, moving the OC of the rear curve, or moving the inset of the Seg has the same effect.
Further more, this has the exact same effect as moving the Seg itself on the carrier lens. 


The only reason you would use total power(front curve of the seg, minus the rear curve of the carrier),
is if you are moving both the seg and the distance oc at the same rate in opposite directions, and you are measuring from the origin to one spot.  
Not the actual difference between the two.

i.e. if you take a +2.00 DS, mounted on a +2.00DS.  If you only move one lens, you only need to calculated prism for +2.00D, not +4.00D.
You would only use +4.00D if you are moving both lenses together 


edit:

would it make more sense to say you are decentering 2D BI of prism in the near, then "grinding" 1D of base out prism?

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## optical24/7

> Now were talking about a whole other ball game, taking the question as it is none of the answer's seem correct as it can be done so D is not right but 54 will induce too much prism to be right as well. But like I said I will post what comes tomorrow.


Matt, I think your making the same mistake I did...Not accounting for the canceling prismatic effect in the near due to the prismatic effect of decentering the distance. Lets surface the test question using prism rings instead of blocking to move the D OC.

+2.00/+2.50 add; PD 66/54 intended final OC placements. If you block the lens with the usual 2mm inset, you'd need to move the D OC out another 4mm. To move the OC out 4mm from that point, you'd use a .80 prism ring B out. When you inset the seg 4mm more than the N PD you have 1.8 B in (+4.50 x .4 cm), BUT you HAVE to account for the .80 out you ground to move the D oc out those 4mm... 1.8 in + .80 out = 1 B in. at near in the seg

When I thought about it this way, it makes perfect sense. The same way it works with my -2.00/+2.00add scenario. If I wanted the same 1 D base in near only I would be grinding 1 B in at the D oc to move it out, the residual prism I used in the distance would show up in the seg (plano power) as 1 base in.

So once again, the prism you created to move the D oc shows up in the near and cancels the prism induced by moving the seg in extra mm's.

----------


## ml43

exactly,

when you finish decenter, you are moving both the front and rear curves of the lens.  Thus, you calculate using the total power.

when you surface decenter you are only changing the rear curve relative to the front(or vise versa).  in a spherical SV, this just moves the OC(creates prism relative to the GC).

in a bifocal, this is the same as moving just the seg, given you are placing the new OC in front of the pupil.  therefore you calculate induced prism using only the seg power because that's technically all that moved relative to the rear curve.

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## King.Matthew

Eating Crow I am!!!!! First please let me say thank you for your patience!! I had my light bulb moment after receiving said job and noticed I was wrong. So I was able to continue down the train of thought past where I was stopping and realize that ultimately we are just moving one of the two lenses, I could not get past that as I did not see them as two lenses, I do now, we ultimately are moving the back lens if you will by pushing the OC out and thus theoretically are decentering the seg by itself by doing so and thus only need to take the seg power into consideration. 
Again thank you all for your input and patience!!

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## MakeOptics

Knowledge is best approached like a dog on a bone.

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## King.Matthew

Pretty tasty bone I might add?!!

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## Wes

I'm glad that's all worked out.  Welcome to Optiboard, Matthew King.

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## King.Matthew

> I'm glad that's all worked out.  Welcome to Optiboard, Matthew King.


Thank you, glad to be be here Wes.

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## MakeOptics

> Pretty tasty bone I might add?!!


Yum ; )  Welcome and great questions to cut your teeth on.  I know many don't like older posts rekindled but your questions are good ones that sparked a lot of thought.

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## wmcdonald

I love this forum, but often get frustrated with the lack of substantive posts. It is always filled with folks wondering why their ABO or NCLE grade has not come in yet, of I xyz frames come in blue. Both important topics, but this is the kind of dialogue I enjoy. Best thread we have experienced here in some time, and I was pleased to read it. Welcome, Matthew. There are some very bright folks here, as you have seen already.

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## onthegrind

Ans for Ques 17 is a) 71mm. Reason: MBS = (frame PD/geometric center distance - person PD) + ED.  mbs = ((54 +16) - 64) + 64. mbs = (70-64) + 64. mbs = 6 + 64 = 70. 71mm is within 1mm tolerance

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## Wes

> Ans for Ques 17 is a) 71mm. Reason: MBS = (frame PD/geometric center distance - person PD) + ED.  mbs = ((54 +16) - 64) + 64. mbs = (70-64) + 64. mbs = 6 + 64 = 70. 71mm is within 1mm tolerance



Nope.  You didn't account for the 1.5 diopters of base in prism in the Rx.  You'd decenter each eye OUT from the PRP/Patient PD to induce the prism, making the blank requirement 6mm LESS than what you calculated.

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## onthegrind

yeah, i see it. one of those questions where the answer is more than just the formula.

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## superjo

Wow, what a thread this is, refreshing my memory. I really need to study

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## Lori

Here are the 2 Level 3 courses I wrote to obtain my ABO Master certification. You can follow the links and take the courses for CE credits. ABO approved my ABOM certification last week so it's official! 

https://www.2020mag.com/ce/achromatopsia

https://www.2020mag.com/ce/anisometropia

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## Wes

Congratulations!

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## optical24/7

Congrats on the “M” Lori. 2 excellently presented papers.

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