# Optical Forums > Ophthalmic Optics >  PD on wrap frames

## Bob Price

I hear a lot of talk about Rx calculator's for wrap frames, but not much about verifying PD. The common practice of verifying a PD is to take a ruler and measure the distance from the front of the frame. After tracing these frames we have found that the frame PD comes out narrow, so we will take a frame PD prior to blocking and inset the lens more depending on the difference. The PD then verifies correct.

I am questioning whether this is correct and how does it affect the patient? If you take a PD from the minus side you will get a narrower reading. By doing what we are doing is this creating too narrow of a PD for the patient? So, what is correct? Am I hallucinating here or is this a valid point?

Bob

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## Barry Santini

> I hear a lot of talk about Rx calculator's for wrap frames, but not much about verifying PD. The common practice of verifying a PD is to take a ruler and measure the distance from the front of the frame. After tracing these frames we have found that the frame PD comes out narrow, so we will take a frame PD prior to blocking and inset the lens more depending on the difference. The PD then verifies correct.
> 
> I am questioning whether this is correct and how does it affect the patient? If you take a PD from the minus side you will get a narrower reading. By doing what we are doing is this creating too narrow of a PD for the patient? So, what is correct? Am I hallucinating here or is this a valid point?
> 
> Bob


Depending on the degree of wrap angle, the *effective* frame PD will narrow.  You can use simple trigonometry to calculate it exactly.  Just visualize that the wrapping of a frame would effectively tighten it against a client's temples, and you'll undertand conceptually how the finished frame PD becomes more narrow

Barry

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## bt5050

I have found that when i get jobs in from zeiss - and they use their spazio software - the pds - does seem to come out exact - with the base in prism  they induce - as well as the adjustements made to the rx- 

do have a few ?'s - 

1- when you are finaling a rx - that uses a adjusted pd - and rx - 
do you apply the adjusted PD + rx to apply the tolerances - or do you the actual perscribed rx-  and pd ??

2- is it correct to measure these pd's from the back side - do u get a more actuarate measurment - since you are not compensating  or reading the ruler at such an angle - ?

3- when laying out this work - to be edged - should you also - take the frame pd - from the backside - and use that for your decentration- or should you - take the a + the dbl - and then calulate the decentration on these wrap frames ? 

look forward to your answers - 

b

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## lensgrinder

*The shortest distance between two points is a straight line.*
When taking a Frame PD the key is not to bend your PD stick, if this requires you to take the measurement from the back (this method seems to be problematic due to the temples, sometimes), take DBL + A, use Frames Book measurements, then do so. 
The patient's PD is the same, do not bend your PD stick, take it from the back if needed.

If you are using a compensated Rx, then you would apply ANSI to the compensated Rx, because this is what they are made to, not the original Rx.

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## Samuel Jong

> Depending on the degree of wrap angle, the *effective* frame PD will narrow. You can use simple trigonometry to calculate it exactly. Just visualize that the wrapping of a frame would effectively tighten it against a client's temples, and you'll undertand conceptually how the finished frame PD becomes more narrow
> 
> Barry


Is there any calculator to determine the PD on wrap frames? Thanks.

Samuel

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## Leo Hadley Jr

This is a great subject, 
not only for the compensated PD, but the RX as well. I cant tell you how many times I have seen older licensed opticians rejecting these jobs because they do not believe compensations have to be made for the wrap. I think it is very important this subject be covered extensively in continuing education, trade magazines, and any other resource like Optiboard for the non licensed states. 
I believe Harry Chiling has a tilt and wrap calculator posted in the file section of OptiBoard. 
That may help out a great deal. :Cool:

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## AWTECH

> I hear a lot of talk about Rx calculator's for wrap frames, but not much about verifying PD. The common practice of verifying a PD is to take a ruler and measure the distance from the front of the frame. After tracing these frames we have found that the frame PD comes out narrow, so we will take a frame PD prior to blocking and inset the lens more depending on the difference. The PD then verifies correct.
> 
> I am questioning whether this is correct and how does it affect the patient? If you take a PD from the minus side you will get a narrower reading. By doing what we are doing is this creating too narrow of a PD for the patient? So, what is correct? Am I hallucinating here or is this a valid point?
> 
> Bob


Consider the a spherical lens with a mono PD of 30mm.  With a high wrap frame you may get a needed PD for lens mounting of about 31.5mm.  This will place the optical center in front of the pupil.  The object of this compensation would be to place the lens in front of the pupil and to do this you need to increase the PD during lens fitting.  If you measure a wraped frame link this you would find the distance, when measured on the bench, from the center of the frame to the center of the lens would be 31.5mm.  When placed on the patient and then measured keeping the PD stick on the same plane as traditional flat dress eyewear you would measure the PD as 30.0mm.

There are a lot more calculations that go into mounting lenses in wrap frames, but this should help you understand the basics of the PD.  

Base in prism accomplishes the same thing as increasing the PD.

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## bt5050

i think these explanations ( the photos where helpful for me ) - that r found on the ziess spazio site - 
it shows how not compensating the rx- or pd - alters the actual rx - and the pupil- oc relationship changes - 

http://www.solatechnologies.com/prod...optics/spazio/

at what power - should  compensation start being   used ? - it appears they may use - 3d- but i am unclear if this is just their example - or that is what they suggest  

at 1st - i was a bit concerned using  -these altered rx and induced prism - jobs - and it came from my exp. that a percribing MD and od - todl the pts -the glasses where made way wrong - and was not happy about it - 

although i got a little satisfaction - by calling him - and showing him how wraps are done - and that we where actually correct - -  it is not always conveyed to the the pts very well - and all they hear is - they made them wrong - 

so now  i make it a point to mention - at the time of sale - that the rx- will be compensated for the wrap - so they are actually made to the correct rx-  just in case in the small &#37; - they go back for a recheck - without seeing me 1st - and get misinformation 

have a great weekend - 
b

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## HarryChiling

I have suggested in te past that when doing a wrap the lenses be engraved or laser marked with something to give other a way of reversing the Rx, for example if the lens had engraving *T15W20* an optician can determine that the Rx was compensated for 15o tilt and 20o wrap compensation. Then if the formulas used to determine the compensations were reversed (simple as the compensations would be multiplying instead of dividing into the power matrix the compensations) then the patient upon dispense is given a small business card size CD with a compensation program on it (example: Darryl's or mine) wich also has a reverse function so that anyone down the line can reverse the compensations and see what the original Rx was. This would add great value to a lens package and cost very little to the manufacturer.

If any manufacturer applies any of these techniques to their lenses and comes up with some ultra premium product as a thank you please give me a hefty discount on it.

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## HarryChiling

To find the Rx that should be surfaced into the lens use this calculator:
http://www.optiboard.com/forums/show...43&postcount=1

Once the Rx is figured out Use this calculator to come up with you surfaceing ticket:
http://www.optiboard.com/forums/show...16&postcount=1

Now with the back curves figured out use this excell sheet to find out what second cuts (lenticular) to make to thin out the edges:
http://www.optiboard.com/forums/show...04&postcount=3

I will eventually incorporate all these calculators into one huge wrap and tilt calculator that will blockout the sun and bring about another ice age, but time is a prescious commodity right now. ;)

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## lensgrinder

> Base in prism accomplishes the same thing as increasing the PD.


Please correct me if I am wrong, but the prism is due to the lens tilt, thickness of the lens, and base curve.  If you increase the PD, the angle still remains the same.  The person is never going to look over the OC, because the light is coming in at an angle to the line of sight, not straight.  This is why you need the prism, to correct for the displacement and based on a 14mm vertex and 15 degrees of wrap you would need to increase the PD about 7mm per eye. Let me know your thoughts.

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## bt5050

then the patient upon dispense is given a small business card size CD with a compensation program on it

i think your idea with a engraving and something for the pt at disp ( ie card) - would be great - so it is apparent when the rx has been changed from the orginal - provided by the persriber - 

however - i dothink going as far as giving a pt a cd- is kinda over board - ( at least here where i am ) - since the od.md's expire there rx's - the pt would not have the rx for years and years - and need the rx converted back - 

furthermore - if we wanted to make another pr - it is most likely it will be in a differant frame - with differant wrap- and therefore we could easily work off the real rx - 


BUT I DO LOVE THE ENGRAVINGS - and dipense card - idea -!!!

b

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## HarryChiling

> Please correct me if I am wrong, but the prism is due to the lens tilt, thickness of the lens, and base curve. If you increase the PD, the angle still remains the same. The person is never going to look over the OC, because the light is coming in at an angle to the line of sight, not straight. This is why you need the prism, to correct for the displacement and based on a 14mm vertex and 15 degrees of wrap you would need to increase the PD about 7mm per eye. Let me know your thoughts.


The prism is for alighning the ray when leaving the lens to be parralel with the optical axis, the compensation to the PD would be for the ray from lens front through the lens IMO.  So if we were to consider this the compensation would be:

*tan(wrap)=dec/thickness*

for example a lens 2mm thick wraped 15o, would be:

_tan(15)=dec/2mm_
_tan(15)*2mm=dec_
_dec=tan(15)*2mm_
_dec=0.2679*2_
_dec=0.5358_

So add 0.5mm to your lens PD.




> however - i dothink going as far as giving a pt a cd- is kinda over board - ( at least here where i am ) - since the od.md's expire there rx's - the pt would not have the rx for years and years - and need the rx converted back -


The idea is so that the prescribing doctor can reverse the Rx themselves and would have the software for further use.  It would be more to add value than anything else.  You could hand out a card with the Prescribed Rx and the Compensated Rx, but if the card is lost the glasse are wrong again.  However if the engravings catch on then when a preescriber sees it they will know to use the software to get the original Rx.  The patient doesn't have to worry about anything and the prescriber now has a tool at their disposal whihc would further prommote their product.  Just an idea, I can dream can't I. :D

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## Darryl Meister

> The prism is for alighning the ray when leaving the lens to be parralel with the optical axis, the compensation to the PD would be for the ray from lens front through the lens IMO. So if we were to consider this the compensation would be


This only applies to prism induced by decentration. The tilt of the actual lens, with or without any decentration, also induces a prismatic effect that is independent of decentration and proportional to the front curve of the lens. Moreover, the tilt of a lens is further exacerbated by decentration, even when the PD is correct, since the decentration of a meniscus lens occurs about an arc. While I provide a fairly accurate first order approximation of these effects in my Rx Compensator program, my impression is that very few "wrap calculators" take these variables into account.

And this all assumes that the PD was correct in the first place. Since wrap tilt effectively reduces the optical center distance, by an amount roughly proportional to the cosine of the angle of tilt, this needs to be factored in before edging the lenses -- either in the compensation software or by the person laying out the lens for edging.

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## Barry Santini

> And this all assumes that the PD was correct in the first place.


Yes, we all need to review whether our traditional methods for "taking a PD" are, in fact...*representative* of the actual visual axis...




> Since wrap tilt effectively reduces the optical center distance, by an amount roughly proportional to the cosine of the angle of tilt, this needs to be factored in before edging the lenses -- either in the compensation software or by the person laying out the lens for edging.


This type of PD compensation is already on my wish list for Darryl's  next version of his Rx compensator...

How 'bout it, Darryl????!


barry

me

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## Darryl Meister

> This type of PD compensation is already on my wish list for Darryl's next version of his Rx compensator... How 'bout it, Darryl????!


I'll get it in there sooner or later... I just have to figure out the best way not to confuse people with it.

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## Samuel Jong

Darryl, is this kind of lens what we called Loop?

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## bt5050

hi everyone - 

have been looking around for a old trade mag. that i saw - that had a great write up on wrap frames - and it had great explanations on the optics outcome - and the ways these issues are handled - 

i was looking for this - to provide to my staff - to have them get a better understanding of what is going on - and why - 

i thought the pics - where great also - for those that need to see it - to understand it - 

any help would be appreciated - 
b

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## HarryChiling

I think it might have been a 20/20 magazine, I will look through my office for it I am almost sure I have an extra copy.

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## Fezz

I think a fellow Optiboarder=Barry Santini, wrote that one.

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## Barry Santini

> I think a fellow Optiboarder=Barry Santini, wrote that one.


Sorry, Fezz, as much as I'd like to take credit for it...I didn't write that one.  That was Dave Carr, and you can see it on the Jobson website for free.

Check it out: http://www.2020mag.com/CE/TabViewTes...6/Default.aspx

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## Fezz

OOPPPPSSSS!

I was thinking of your "Get Going With Goggles" January 2007 20/20 issue.

My bad!

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## DragonLensmanWV

> *The shortest distance between two points is a straight line.*
> When taking a Frame PD the key is not to bend your PD stick, if this requires you to take the measurement from the back (this method seems to be problematic due to the temples, sometimes), take DBL + A, use Frames Book measurements, then do so. 
> The patient's PD is the same, do not bend your PD stick, take it from the back if needed.
> 
> If you are using a compensated Rx, then you would apply ANSI to the compensated Rx, because this is what they are made to, not the original Rx.


That's a good argument for the wooden PD rulers that I use.

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## drk

A corollary that has nothing to do with this subject, directly:

Picture this cranial geometry with pupillometry, from an overhead view: 

Coronal plane of left orbit relatively planar to coronal plane of bridge, but coronal plane of right orbit curved away from coronal plane of bridge (say, from a front-on view, "flat face" from left eye to bridge and "rounder face" from bridge to right eye.) Would not the pupillometer reading be automatically correct? The vertex distance from right eye to pupillometer will be greater than the left, and the p.d will be read "too short" at the linear plane of the pupillometer, but the glasses will be worn in the same plane as the pupillometer, anyway, so all works out.

But, what if the silver top bar of the pupillometer is not "up" or up but not touching the forehead to make the pupillometer approximate the plane of the glasses frame, and the pupillometer "rolls" towards the right via the rounder forehead...will that not introduce somewhat significant error in the p.d. measurement? It would OVERESTIMATE the measurement on the eye that it was pushed too far towards, and UNDERESTIMATE the measurement on the eye that it was tilted away from.

Likewise, if the pupillometer measurement were done exactly right with the top bar touching, if the frame at dispense were not adjusted the exact same way, say with the vertex distance incorrectly short on one side from non-parallel temple splay, think of how that would effect the vision on narrow progressive corridors and reading areas! It would wreak havoc.





*******
For that matter, if an ophthalmic frame has more wrap (by design or by optician) than the flat pupillometer, the lenses will have the p.d.s effectively too narrow AND THE PROGRESSIVES INSET TOO FAR.

If the ophthalmic frame has less wrap than a flat pupillometer (think slightly negative face form on a fat head with a small frame with a memory metal bridge), the lenses will have p.d.s effectively too wide AND THE PROGRESSIVE OUTSET TOO FAR.

(To make this germane to the thread, consider wrapped sun progressives! What a source of multiple errors galore! It's suprising anyone can wear those without a physics software program! Not only would there be mind-numbing VARIABLE tilt effects from top to bottom, but the pd on those progressives are critical, and the frame wrap p.d. effect would about HAVE to be compensated for, in order to get binocular vision at near. Why do I even try to do it?  It seems like sheer folly!  FT-28 set low, here I come.)

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## Barry Santini

Pupilometers make certain assumptions during fitting that are not always representative.  And I was lookin at the Zeiss *EYE TERMINAL* as a possible solution, but I'm thinkin' how would one *verify* the finished eyewear, in situ, with the Eye terminal?  Revert to felt tip pen?  Well, then what's is the point of Eye Terminal?

My two cents, what's yours?

Barry

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## Angel of Grace

I think it would be great if the pupilometer had more wrap to it or a way of adjusting it to compensate for wrap and tilt.  Does anyone know of something like that available?  :drop: This would certainly help give us closer measurements.   Angel of Grace

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## HarryChiling

Would their be a point of that level of accuracy when the eye exam is rounded to the nearest 0.25 D in the first place and the standards for what is allowed to pass are getting looser?

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## drk

This is kinda the same problem that cartographers have: how do you "flatten the globe" into a 2-dimensional map and preserve correct distance relationships?

Answer: you really can't.  There are some ways they do it: a map that looks like a peeled baseball, a projection map that makes sizes non-proportional, etc.

What we face is taking a linear, one-dimensional X-axis measurement of a quantity that is better defined by two dimensions: X and Z.

Fortunately, this degree of precision is rarely needed.  We are stuck with linear measurements of the distance from the reference point (center of the bridge) to an approximation of the visual axis (Purkinje-Samson image #1, i.e. the pupil reflection).

Yes, monocular vertex distance error (Z-axis) may exist, but what degree of accuracy is needed on a pair of glasses that get smudgy and slip down the nose, anyway?

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## HarryChiling

> Yes, monocular vertex distance error (Z-axis) may exist, but what degree of accuracy is needed on a pair of glasses that get smudgy and slip down the nose, anyway?


My point exactly, the compensations performed on eyewear do make a significant difference in the Rx when tilt and faceform are introduced, but they are still only aproximates.  The goal is to get the Rx to within ANSI standards or better.  By the time the slight difference in PD is significant enough to create the needed prism to be outside of the ANSI standard, the processing techniques used today would become more of an issue, or the Rx would be outside of the acceptable range for wrap eyewear.

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## drk

Yup.

I guess it's good to discuss and understand variables, because they may occasionally come into play.

It also kills a lot of time that I should be working! :hammer:

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## HarryChiling

> It also kills a lot of time that I should be working! :hammer:


It's staying sharp, not killing time.:D

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## Barry Santini

And I've found that the typical trigonometric calculations for figuring how to compensate (outwardly)the PD with witch to block a wrap are...WRONG (or at least inaccurate).

In frustration, I decided that, as a visual learner, I needed to see this wrap-narrowing phenomenon in action in order to get my brain around it. So I decided that I needed to get a wrap-adjustable frame, i.e., a *folding* frame.  So I ordered an new (old?) Persol folding, zyl aviator sunglass.  I played around by putting in clear lenses, marking PDs (say 27, 30 & 35) and then adjusting the wrap angle of the frame to 15, 20 & 25 degrees,and this is what I've found:

ALL THE CALUCULATORS OVER-ESTIMATE THE AMOUNT OF *NARROWING* THAT OCCURS.  

For example, for a 20 degree wrap angle, the standard formula says spread the mon-blocking PD by almost 2 mm...but I found that 0.5mm (at a 35PD) worked just fine.

Why, I don't completely understand...yet!

But, like the wright brothers, I'm ready to question what I've been led to believe to get better answers.

Any Ideas?

Barry

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## AWTECH

> And I've found that the typical trigonometric calculations for figuring how to compensate (outwardly)the PD with witch to block a wrap are...WRONG (or at least inaccurate).
> 
> In frustration, I decided that, as a visual learner, I needed to see this wrap-narrowing phenomenon in action in order to get my brain around it. So I decided that I needed to get a wrap-adjustable frame, i.e., a *folding* frame. So I ordered an new (old?) Persol folding, zyl aviator sunglass. I played around by putting in clear lenses, marking PDs (say 27, 30 & 35) and then adjusting the wrap angle of the frame to 15, 20 & 25 degrees,and this is what I've found:
> 
> ALL THE CALUCULATORS OVER-ESTIMATE THE AMOUNT OF *NARROWING* THAT OCCURS. 
> 
> For example, for a 20 degree wrap angle, the standard formula says spread the mon-blocking PD by almost 2 mm...but I found that 0.5mm (at a 35PD) worked just fine.
> 
> Why, I don't completely understand...yet!
> ...


Barry:  I just checked an ICE-TECH wrap job with a 24 degrees of wrap angle and 10 degrees of panto and the mono PD increased by approximately 1.3mm per eye.  With your conclusion that 0.5mm worked fine and our calculation being 1.3mm or less for the 20 degree you are then dealing with 0.8mm or less difference.  Others who use a 2.0 mono PD change may have or may not have a reason for doing so.

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## Barry Santini

Thanks again, Allen, for your contribution. However, part of my puzzlement on this subject is that so many different calculations deliver significantly different results. And this is also true for the various wrap caluculators when optimizing the Rx for spherical designs.

I think my evaluations are crude.  my conclusion is affected by:

1. the size of the dot of my marking pen
2. observer parallax
3. interpolation of the result on a progressive protractor grid marked off in 1mm increments

I think that we all need to fundamentally understand the geometrical optics involved here before any of us pray too much at the altar of software optimization.

I'm in...let's discuss it!

Barry

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## HarryChiling

> Thanks again, Allen, for your contribution. However, part of my puzzlement on this subject is that so many different calculations deliver significantly different results. And this is also true for the various wrap caluculators when optimizing the Rx for spherical designs.
> 
> I think my evaluations are crude. my conclusion is affected by:
> 
> 1. the size of the dot of my marking pen
> 2. observer parallax
> 3. interpolation of the result on a progressive protractor grid marked off in 1mm increments
> 
> I think that we all need to fundamentally understand the geometrical optics involved here before any of us pray too much at the altar of software optimization.
> ...


What are we measureing here, the change as percieved on the spectacle plane or the effect on the plane the center of rotation lies on.  To be honest with you, I thinkt eh most complicated part of dealign with anything in wrap frames is that we are actually taking traditional Rx's and PD's which all used to lie or were assumed to lie on the specatacle plane and now introduce a tilt which causes the lenses to interesct many planes along the z axis, so we in effect have to start thinking in three demensions.  That's where some fo the differences come into play.  I comp:
VertexPrism (based on front curve)RxMy calculator assumes you are using the lenses true tilt, which is slightly off from the amount of the frames tilt.  Decentration of a lens causes a tilt all of it's own that has to be factored in, now that should get one to thinking that if the lens tilt should be compensated for horizontal decentration, then what about vertical decentration.  I am sure you can keep justifying the accuracy to a point where you might want to charge the patient a ridiulous amount of money just to compute their comped Rx.

I will draw up some diagrams when I get home to illustrate the decnetrations tilt and the lens tilt.  Make no mistake I use my calculator all day everyday, because it is accurate enough, their are some companies out their that don't comp the Rx at all so using any of the available calculators is better than nothing.  I am planing on updateing mine when I finally get the chance, I had thought about releaseing the source code at one point to allow others to improve upon the desing and put out more sophisticated applications and still might.  I e-mailed someone my source code at one point in hopes that they would port it over to Java or PHP (nudge nudge, Al :p:D).  Barry if you have any programing background let me know and I will send you the source code for you to modify and make better, but the terms are that you make any improvements available to me.

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## Favio Becker

I am currently preparing for my OSCEs, and am a little stumped on this subject.

I know this is an old topic, but I'm wondering if anyone could explain the maths behind the adjustment of the PD for the dihedral angle?

Is it similar to the compensation of heights for the pantoscopic tilt?

Thank you for your help,

F

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## drk

> *******
> For that matter, if an ophthalmic frame has more wrap (by design or by optician) than the flat pupillometer, the lenses will have the p.d.s effectively too narrow AND THE PROGRESSIVES INSET TOO FAR.
> 
> If the ophthalmic frame has less wrap than a flat pupillometer (think slightly negative face form on a fat head with a small frame with a memory metal bridge), the lenses will have p.d.s effectively too wide AND THE PROGRESSIVE OUTSET TOO FAR.
> 
> (To make this germane to the thread, consider wrapped sun progressives! What a source of multiple errors galore! It's suprising anyone can wear those without a physics software program! Not only would there be mind-numbing VARIABLE tilt effects from top to bottom, but the pd on those progressives are critical, and the frame wrap p.d. effect would about HAVE to be compensated for, in order to get binocular vision at near. Why do I even try to do it?  It seems like sheer folly!  FT-28 set low, here I come.)


Years later, I think I was exactly backwards on the direction of inset and wrap.

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## drk

Fabio:

I used to think you were just a hunky guy with long hair.  I know that beauty is a fleeting thing.  Welcome to our profession, where average to ugly is the norm.  

I think you're exactly right: the movement to lower the OCs with pantoscopic tilt (180-axis or horizontal or X-axis) tilt is to have the optic axis of the lens intersect the center of rotation of the eye, as it goes from straight-ahead gaze to downgaze for reading.

Likewise, the temporal movement of the OCs with frame wrap (dihedral angle or face-form or 90-axis or Y-axis tilt) is to have the optic axis of the lens intersect the center of rotation of the eye, as it goes from straight-ahead gaze to left and right gaze to be shifty and look for the fuzz. 

 There has been a formula posted in the past.  It's geometric and trigonometric.  You may have to dig on Steve's server.


Footnotes:
1. "the fuzz" means "the man", "the cops", or in your case, "the bobbies".
2. At least I think what I posted is correct.  We need peer-review on this one.  I miss Darryl.  Barry has absorbed most of this.  Harry is probably too busy being shifty to post.  Uillean may know something.  Robert may not be a specialist in this.
3. Is your hair still long?

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## optical24/7

> Fabio:
> 
> ...I know that beauty is a fleeting thing.  Welcome to our profession, where average to ugly is the norm.


Must be an Ohio thing.... Here, all the Opto folks are gorgeous! Just look at my selfie avatar!

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