# Optical Forums > General Optics and Eyecare Discussion Forum >  Bifocal prismatic effect

## ge2007

Hi everybody!

I am studying DO and I cannot understant the base directions of the prism due to the segment in bifocals.

Can somebody please help me? :Eek:

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## Laurie

Hi ge2007,

Welcome to Optiboard!  

Think of a bifocal segment, regardless of the style, as a small plus lens (prisms attached base-to-base) at the bottom of the lens.   At the slight second when your eyes roll downward across the segment line, and prior to getting to the near optical center (NOC), they are looking through Base Down Prism.  Since the image will be displaced toward the Apex (up), it gives the visual sensation to things 'jumping up', which is why we call it Image Jump.  

The higher the Add power and the lower the NOC placement in relation to the seg-line the greater the image jump.  You can calculate it using Prentice's Rule, where the amount of image jump is equal to the add power, times the number of mm the NOC is below the seg line, divided by 10.  (others here can post the actual notations in formula style, but I am old.)...

We do not necessarily assign a base direction (it will always be BD), we simply recognize it, calculate it for the ABO and other state licensing exams, and explain to the patient/client how to learn to ignore it.

Best Wishes for your opticianry studies, you will find a bunch of helpful 'O's' here.

: )

Laurie

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## ge2007

Thank you Laurie!
I just hoped there was a specific rule to help me with the exercises... My problem with the prisms is DOWN or UP, In or OUT?!?!?

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## gmc

As Laurie said, the prism direction in the seg will always be base down. Different seg types have different OC placements relative to the line. A FT28 normally has a 3mm difference. A FT35 normally has the OC on the seg line. A Round 22 mm seg has an 11 mm difference.

Apply Prentice's rule: Power x decentration/10. Say you have a +2.50 add in a FT28. You have 2.5 x 3/10. You have 0.75 prism diopters BD.

Assume you have a +2.50 add in a round seg. You have 2.5 x 11/10 or 2.75 prism diopters BD.

Hope this helps.

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## chip anderson

I may be disagreeing with others here.   But in glass flat tops have base up prism,  round segments have base down.
In plastic straight tops have base up.  Probably round ones have base down, at least above center of seg.
If you ever find one a glass panafocal has no prism.

Chip

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## gmc

Consider a 28mm round lens made of high index material. Cut the lens off a few mm above the widest part of the lens.
(The seg line on the FT28 lens I checked appeared to be about 4mm above). You have a FT28 mm seg. Since the thickest part of the lens is at the center of the lens and you are looking above that point, you have BD prism.

I gave it the eyeball test and sure enough, the image was deviated up, indicating BD prism.

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## chip anderson

Still disagreeing:  A prism bends light towards it's base.   It's base is the thickest part of the prism.

Chip

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## optical24/7

Yes Chip, prism bends light towards it's base but displaces images towards it's apex, always.

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## gmc

> Still disagreeing: A prism bends light towards it's base. It's base is the thickest part of the prism.
> 
> Chip


Okay Chip, picture that 28mm round lens from the side. You have essentially two prisms base to base. Cut off the top of the lens 4mm above the thickest part. As soon as you cross the upper line, you are looking through a base down prism, albeit one with the top part cut off. Hence you have base down prism and the image is deviated up.

Pull a SF blank with a +3.00 add and check it out for yourself. Look at a straight line such as the edge of a desk. You will see the image clearly deviated upward as you cross the line. You have to drop down several mm to the OC where the line within the seg will align with the line outside the seg.

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## Wes

Chip, straight tops have base down above the center (or oc) of the seg as well.  Below the center of the seg is the base up prism.  Pretty much like every other plus lens. If the seg line were at the midpoint of the seg, you would be right.

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## optical24/7

> Chip, straight tops have base down above the center (or oc) of the seg as well. Below the center of the seg is the base up prism. Pretty much like every other plus lens. *If the seg line were at the midpoint of the seg, you would be right.*


Like an exec bifocal.

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## junebug72

i am going to have to agree with chip the image is deviated up because the seg has base up prism. the base of prism is it's thickest side so unles you turn it upside down how could the prism be base down?

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## junebug72

i don't think you have to worry about the bifocal so much as the optical center. prism is used to move objects to the oc of the lens. you want your prism out for an eye that goes toward the temple, base in for an eye that goes towards the nasal. up and down prisms are mainly used to avoid vertical imbalances. just remember the bigger the frame the more prism it will take to reach your optical center so i strongly suggest using the smallest frame possible for those needing prism it adds a lot of weight and thickness to their lenses. matter of fact prism is ground in the lens to even have an oc. base in for hypers' base out for myops'. it is calculated by the power, frame measurements and the patients pd. hope this helps.

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## Wes

Sorry junebug, but these are two of the most incorrect posts I've ever seen on OB.

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## optical24/7

junebug, read post #8 again.

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## chip anderson

I conceed that many of you seem to have a better understanding of this than I.  But I do remember from my early "education" in optics that I was taught to use a straight top to off-set the base down in the lower half of minus lenses and to use round top segs to off-set the base up prism in the lower half of plus lenses.
What's been said so far in this seems to contradict this.
Monday I intend to go to the lab and check a bunch of different style distance plano bifocals and git myself straightened out.

Chip

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## Wes

If you think of the seg as a plus lens by itself, it makes more sense.  A plus lens can be thought of as two prisms base to base, right?  In the portion below the oc of the seg, there is base up prism. In the portion above the oc of the seg, there is base down prism, again, like every other plus lens in existence.  

And again, prisms bend light in the direction of the base, but displace the image in the direction of the apex.

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## junebug72

i don't think u understand the question! the student is having difficulty understanding which direction to place his prism. how can u disagree with the soul purpose of prescribed prism being to place objects to an oc of a lens. the direction depends on where the object is displaced? i think most of these posts has probably confused him more. 
he'd be better off asking someone else.

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## junebug72

there is no such thing as base to base prism. prism has a base it's thicker side and an apex it's thinnest side without either you do not have prism.

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## Wes

Junebug, the confusion being generated on this thread is coming from you.  If this is a trolling attempt you succeeded.

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## junebug72

just because u say so where is the facts that prove me wrong. u're arrogance is no substitute for facts. how do you determine which direction to prescribe prism?

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## uncut

(Counts to 10 slowly)  Grabbing popcorn! :Rolleyes:

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## Steve Machol

> just because u say so where is the facts that prove me wrong. u're arrogance is no substitute for facts. how do you determine which direction to prescribe prism?


This attitude is not acceptable on OptiBoard. I suggest you rethink your participation in this thread and possibly the forum.

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## Wes

Reminder to self: never argue with idiots; they drag you down to their level and beat you with experience.

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## RetroRat

> Hi everybody!
> 
> I am studying DO and I cannot understant the base directions of the prism due to the segment in bifocals.
> 
> Can somebody please help me?



Hopefully this will help in your study. Good luck. 



Retrorat

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## RetroRat

Apologies for the dodgy paint image, gotta leave for work in 7 minutes! (Gotta down this coffee in 6!!!)

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## tmorse

> As Laurie said, the prism direction in the seg will always be base down. Different seg types have different OC placements relative to the line. A FT28 normally has a 3mm difference. A FT35 normally has the OC on the seg line. A Round 22 mm seg has an 11 mm difference.


Always base down? Not quite accurate.

If any type of segment has its optical centre on the seg line, then as soon as your eye crosses that line to look near, your eye will experience a Base Up effect from this style of seg. As in an Executive segment or a very large FT with OC on seg line.

To determine base direction, always consider *'where is the base in relation to the eye'*, and Not 'where is the eye in relation to the base. 

Remember it's only the eye that moves... up or down (or in or out), while the distance lens (and its bifocal segment) stays stationary, locked in position in the frame. :Smile:

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## gmc

Please forgive the crude drawings.

The first represents a round seg. The horizontal line represents the near OC. If you are looking through the seg above that point, you have base down prism and the image you see is deviated up. Since the prismatic effect is a function of power and decentration, this seg will have the most jump at the top of the seg. As your eye looks deeper into the seg, the prismatic effect lessens.

The second image represents a flat top seg with the near OC on the seg line like a FT35. There is no prismatic effect at the seg line. As your eye drops deeper into the seg, you have a base up prismatic effect.

The third image represents a flat top seg with the near OC below the seg line like a FT28. At the seg line and just below you have a base down prismatic effect. As your eye drops deeper into the seg, you have no prismatic effect at the near OC. If your eye drops below the near OC, you have a base up prismatic effect.

As Wes has said, the seg can be considered as an individual lens. The same rules for prism apply in the seg as they do in a normal lens.

tmorse is also correct. If the near OC is on the seg line, no prismatic effect is present there and there is BU prism below the seg line. Thanks for the clarification.

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## Diane

Gary,

I was doing a crude drawing at the same time as you.  I won't go into detail like you, just post my drawing.

Diane

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## braheem24

Posters arguing with the ABOMs are assuming the original plus lens was cut in half.

A FT28 is deeper then 28MM which means it is cut above the OC, Diane's drawing shows it nicely.

Shallower then the width = BU
Deeper then the width = BD

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## tmorse

What no one has mentioned yet is that the base direction of a segments prism depends on the *reading level*(or how far the eye drops down into the seg from straight ahead distance gaze) 
in order to read. All segments are plus powered,  and a seg line can be manufactured to cut at the optical centre of seg (Executive) or higher (as in FT28), or even have no seg line at all as in a round top, but the reading level... the distance that the eye moves downwards to read will indicate the ultimate base direction of the segs prism.

Say a FT28 has a seg OC 4mm below its seg line... and a person wants to read a book. And say the distance OC is also 4mm, but above the seg line.  When the patient drops down only 1mm from straight ahead, he cant read (hasnt reached the seg yet), and a similar effect exists when he drops down 2 and 3mm from straight ahead. But when patients drops 4mm down, his eye finally hits the FT28 line and he can begin to read. But he is slightly confused by the seeing both distance and near at the same time at this line, so he drops his eye another 1mm further into the seg. Now he experiences a combined prismatic effect of distance (down 5mm) and, at the same time, he is now located 3mm from the FT28 OC of this segs plus power. Think of the seg as a separate very small chopped plus lens glued to the distance lens to create this FT28 bifocal.  

Now forget about whatever distance prism this 5mm produces, since we are discussing only seg prism direction. So after dropping say 5mm down to read, his eyes line of sight is now located 3mm above the OC of this FT 28 bifocal, and this produces a BD prism effect. 

But no one says you can read only at 5mm down. Your reading level can be changed at will... you can drop your eyes all the way down to the bottom of the seg lens if you like, and still be able to read. But as you lower your reading level beyond 5mm, your eyes line of sight is getting closer and closer to the segs OC in this FT28. If you drop your eye to a new reading level of 8mm, you eye will be looking exactly through the FT28's OC, and no seg prism effect will be produced at all. And if you keep going down and drop even lower than 8mm reading level, you eye will be looking below the FT 28 OC and now experiencing a BU effect...the base direction has reversed!! As you read lower yet and you will experience a larger and larger BU effect, until your reading level finally reaches the bottom of the FT28 seg.

But an Executive bifocal with OC on seg line... as soon as your reading level drops to say 5mm, your eyes line of sight is now 1mm below this Executive OC and you will experience a BU seg prismatic effect. And a larger reading level than 5mm will place your eye even further from the Executives OC (located overhead), and produce a corresponding larger BU effect.

But you are reading quite a distance down from your distance OC, and in a bifocal that will produce a combined distance and near prismatic effect (assuming your have distance power in the 90 meridian) with the amount of seg prism actually produced at your chosen reading level.
Most textbook authorities suggest a reading level of 8-10mm for bifocals, but that reading level distance is not cut in stone. This combined prismatic effect can be left for another thread.

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## junebug72

my apologies i certainly over reacted. surely i couldn't be that wrong. but sense i'm not sure myself anymore i will humbly leave this one alone. good luck to all!
 sincerely,
junebug

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## fvc2020

lovely pic

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## sharpstick777

> i am going to have to agree with chip the image is deviated up because the seg has base up prism. the base of prism is it's thickest side so unles you turn it upside down how could the prism be base down?


Because the OC of the seg is below the segment line, not at it.   So until you hit the seg center, you have about 4-5 mm where the prism is BD.

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## tmorse

> Still disagreeing: A prism bends light towards it's base. It's base is the thickest part of the prism.




Chip,

Draw a plus lens (side view) and now a horizontal line straight through its optical center... no deviation seen. 

Now draw another horizontal straight line 10mm above and parallel to this first optical center line and label on it Object A. This ray goes through the plus lens 10mm above the optical center and it does deviate towards the base. 

Now draw your eye behind this lens and look at Object A... looking through the lens 10mm above the optical center produces this BD prismatic effect. 

But the eye behind this lens at this point sees the image of Object A as a light ray angled downwards into the eye, and if you extrapolate this bent line back (to in front of this lens), the original position of Object A will seem to you the observer as having been deviated upwards, towards the apex of this prism. 

So the rule is this... Above (or below OC or the right or left of OC) a light ray is indeed bent towards the base of the corresponding prism, but the image of an object *as seen by the observer*, will indeed be deviated upwards, toward the prisms apex.  

Unfortunately I dont have the computer skills to draw this picture for you .

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## tmorse

> I may be disagreeing with others here. But in glass flat tops have base up prism, round segments have base down.
> In plastic straight tops have base up. Probably round ones have base down, at least above center of seg. Chip


This belief that FT in glass or plastic have BU prism is probably the source of your difficulty in understanding the concept of segment prism base direction. 

Not just 'round ones' but all segment styles that have a seg OC below the top edge (or seg line) will produce this BD prismatic effect. But if you drop your eyes to a lower reading level (further downward and past the seg OC) this BD prismatic effect will lessen, become zero (0) at the seg OC and then change to a BU effect at all points below the seg OC. 

At the FT seg line, you will experience BD *as the 'plus' segment's contribution* to the combined prismatic effect of distance and near, regardless of lens material.    :Smile:

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## drk

I think a clarifying idea is that lenses are *gradient* prisms.

We're thinking in terms of "absolute" prisms, like a loose prism that has the same deviating power no matter where you look through it.

Not so a lens...it has anywhere from zero prism at the center to a maximum amount at the edge of the lens.

Not only is a lens a gradient prism, but depending where you look through it (not unlike turning a loose prism) it can be BI, BO, BU or a combination of anything!  It's a "concentric" prism.  (I made that term up. :Biggrin: )

Add the fact that with a segmented MF we're essentially adding a lens to a lens (distance power) we have a really fun combination of net prismatic effect.

And, of course, add the fact that segmented MFs have all sorts of shapes possible (old Ultex, round, FT, ribbon, exec) and that makes for a confusing thread.

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## Robert Martellaro

> But I do remember from my early "education" in optics that I was taught to use a straight top to off-set the base down in the lower half of minus lenses and to use round top segs to off-set the base up prism in the lower half of plus lenses.


You're right, in the sense that a round seg will tend to counteract the induced, yoked BU prism due to significant plus power found on the vertical meridian in the distance. A round seg will have an additive effect on a minus lens, contributing to the BD prism. A Straight Top is essentially neutral (depending on the reading depth and set placement), with the OC about 4mm below the top of a 28mm wide seg.

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## Diane

Attachment 8365Attachment 8368Attachment 8369Attachment 8366
Attachment 8367

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## braheem24

Diane, Linky no worky.

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## Diane



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## Diane

They loaded out of order....Oh well.

Diane

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## Wes

> They loaded out of order....Oh well.
> 
> Diane


No worries.  Can't win em all. 
 Good demo though. 
And Chip's bound to believe it since you posted it.

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