# Thread: PD on wrap frames

1. ## Wrap PD's

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

2. 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?

3. 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?

4. Originally Posted by drk
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.

5. 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:

6. Originally Posted by drk
It also kills a lot of time that I should be working! :hammer:
It's staying sharp, not killing time.:D

7. ## Ok, I became Fed-up...

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

8. Originally Posted by 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
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.

9. 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

10. Originally Posted by 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
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:
1. Vertex
2. Prism (based on front curve)
3. Rx
My 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.

11. 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?

F

12. Originally Posted by 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.

13. 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?

14. Originally Posted by drk
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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