Equithin and Prism Thinning revisited

[Dr. Bill Stacy]

I see there's an old thread on this subject, but since it's inactive, decided to start a new one (hope that's not a violation of any protocol here).

Anyway, I detest the concept of adding prism for cosmetic reasons unless that's what the patient wants. I think ANSI was wrong to allow the practice, as prism is should be a prescription item, to be prescribed by the doctor, not inserted at the whim of some nameless lens designer nerd somewhere in the world.

That aside, my main complaint is that it seems to have become an automatic add on by surfacing laboratories. This is sick. It should be an option to be chosen, not one that needs to be over-ridden on every single order by the ordering individual. Worse, labs seem to be unable or unwilling to make it an opt-in or opt out choice at all.

My worst case scenario was a guy who came in with +6 O.U. progressive Rx. The moment he put them on he exclaimed "not again!" and threw them on the counter. On lensometry check, they were spot on the Rx as ordered, except for 6^ base down O.U. The amount of prism distortion was breathtaking.

He had had the same exact experience at another provider, more than once. That got my attention.

Since then, I have made it a habit of specifying "NO THINNING PRISM, NO EQUITHIN" in the special instructions area of all digitally surfaced lenses. What a royal pain. I suspect a lot of "non adapts" are happening around the world because of this "feature".

[wmcdonald]

That is an excessive amount of prism. It should be approximately .66(add), and should have not real effect on visual outcomes since prism is in the same direction. An add of +2.00 D would only require prism thinning of 1.32 prism diopters. This would be virtually undetectable and should not cause this kind of reaction. It will also improve the cosmetics of the spectacles. The real problem comes when a single lens is replaced, and one has thinning, and the other does not. Then there is a problem. That is easy to read in the focimeter. While most labs automatically use prism thinning all to do not, so specifying it should be necessary, but not as a part of the Rx. It limits the Optician too much. I hope this is helpful.

[Dr. Bill Stacy]

I don't think it's dependent on the add, but the distance Rx. The problem is ANY amount of prism causes prism distortion, which you might think is not a big deal, but I do. I only use prism when I have to not only because of prism distortion, but because of other effects it can have on spatial orientation, color fringing, etc.

But I get that opticians want to use it so glasses will look better. Fine, I'm sure that's why ANSI did what they did with it.

I hope opticians will honor my "no prism thinning, no equithin" comments on my written Rxs. I care how my patients look, but I care more about how they see.

[sharpstick777]

Prism thinning is a consequence of the front side add actually sitting well behind the spherical distance surface. As Sheedy discovered as little as 2D of yoked prism can reduce DVA substantially in some people. All progressive inherently induce unwanted prism in the corridor, but prism thinning exaggerates this.

Some Free-form lenses by Seiko offer ZERO prism thinning automatically (Surmount, Superior) and with Shamir it can be over ridden at the lab. The lens geometry of backside free-form reduces the need for prism thinning, but will reduce your ability to decenter slightly (your 60 eye aviator will require prism thinning for cut out). Any patient over + or - 3D Total power is at risk of prism thinning negatively affecting their DVA.

[sharpstick777]

The amount of prism thinning is dependent on the add power in traditional progressives. The displacement of the add in traditional lenses is relative to the add only (higher add = more displacement) in pure free-form its dependent on the distance RX.

[wmcdonald]

Excellent response, sharpstick! I was referring to traditional progressives, and the overall effect of prism thinning is minimal at best in most cases. I did not consider freeform and their differences. This +6.00 will be a different animal then. I enjoyed learning about the differences in free form designs.

Dr. Stacy, Opticians fill the Rx as you write it. But in a traditional progressive, the general rule was: add(.66) and that was the amount of prism thinning utilized. But with these newer designs, I can see your issue, and appreciate your response. Just note no prism thinning and I know it will be followed.

[MakeOptics]

I don't think prism thinning is the issue here, the distance reference point on a lens is well above the prism reference point so even if the lens has "No prism thinning" the patient is still going to view through a point at least 4mm above the PRP (average 4mm fit point) and then the center of the distance reference point is going to be about 3mm above that where you check in the lensmeter.

So in your example of a +6.00 lens with prism thinning:

Prism = +6.00 x 0.7cm = 4.2 PD

add to that the 2/3 prism thinning (I am assuming based on your 6PD total, the add)

Prism Thinning = 2.75 * 2/3 = 1.83PD

In Myopes the prism thinning actually counters the amount of prism from viewing off axis but in hyperopes the effect compounds which is where the issue really is. You could yolk it the other way which would be horrible cosmetically providing better distance viewing but then all the sacrifice goes into the reading with a significant loss in acuity in reading.

An option is to use a lens with a shorter drop and short corridor with no prism thinning, or even a slight yolk in the other direction add some base up prism if reading is not a primary concern.

FYI - Seiko has some designs with the PRP that coincides with the fitting point.

[Wes]

This +6 BD reading was likely read about 8 -10 mm above the prism reference point, and not at it. Also, if the fitting cross was fit low, as so many opticians do, the problem will be exacerbated. Also, keep in mind heavier glasses slip down more. Try reading at the prism reference point, and tell us how much prism you see.

[drk]

Nice thread.

Now tell me this: why do we so oftentimes get unwanted verticals on PALs, as though they can't get the prism thinning right?

[Dr. Bill Stacy]

[QUOTE=MakeOptics;510500]I don't think prism thinning is the issue here, the distance reference point on a lens is well above the prism reference point so even if the lens has "No prism thinning" the patient is still going to view through a point at least 4mm above the PRP (average 4mm fit point) and then the center of the distance reference point is going to be about 3mm above that where you check in the lensmeter.

I don't get this jargon. My habit is to neutralize a the lens at the fitting cross, which is where I expect the patient's pupil to be when the eyes are in the primary position of gaze. This is where I expect to find any prism that I have prescribed, or no prism if I have not prescribed prism. In the case I mentioned above, this is where I measured the unwanted 6^ down o.u.

Now it is true that I found the optical center way down at the bottom of the lens, which he might use occasionally for reading, but these were sunglasses, so I wanted it where I wanted it, at the fitting cross, not in the reading zone, since he wanted these for driving a car.

It is also true that I have found that prescribing a little prism thinning of my own works well to thwart the lens designing nerds. If this patient had given me a chance, I'd have remade the Rx with maybe 0.5 base down o.u. This over-rides their equithin formulas, at least for now. (Until they find out that I'm doing that and add their calcs on top of mine).

And all of this also applies to digitally surfaced SV, BIs and TRIs, where the nerds will go next if they haven't already.

In which case we can throw out the concept of Optical Center Height???

[ml43]

[QUOTE=Dr. Bill Stacy;510509]

I don't think prism thinning is the issue here, the distance reference point on a lens is well above the prism reference point so even if the lens has "No prism thinning" the patient is still going to view through a point at least 4mm above the PRP (average 4mm fit point) and then the center of the distance reference point is going to be about 3mm above that where you check in the lensmeter.

I don't get this jargon. My habit is to neutralize a the lens at the fitting cross, which is where I expect the patient's pupil to be when the eyes are in the primary position of gaze. This is where I expect to find any prism that I have prescribed, or no prism if I have not prescribed prism. In the case I mentioned above, this is where I measured the unwanted 6^ down o.u.

Now it is true that I found the optical center way down at the bottom of the lens, which he might use occasionally for reading, but these were sunglasses, so I wanted it where I wanted it, at the fitting cross, not in the reading zone, since he wanted these for driving a car.

It is also true that I have found that prescribing a little prism thinning of my own works well to thwart the lens designing nerds. If this patient had given me a chance, I'd have remade the Rx with maybe 0.5 base down o.u. This over-rides their equithin formulas, at least for now. (Until they find out that I'm doing that and add their calcs on top of mine).

And all of this also applies to digitally surfaced SV, BIs and TRIs, where the nerds will go next if they haven't already.

In which case we can throw out the concept of Optical Center Height???

no offense, but hire an optician that knows lens design and you will solve most of the problems.

or spend a few days with your lab rep and get all the specifics on the designs you like to use, and how to order them so you get what you want.

[Craig]

[QUOTE=Dr. Bill Stacy;510509]

I don't think prism thinning is the issue here, the distance reference point on a lens is well above the prism reference point so even if the lens has "No prism thinning" the patient is still going to view through a point at least 4mm above the PRP (average 4mm fit point) and then the center of the distance reference point is going to be about 3mm above that where you check in the lensmeter.

I don't get this jargon. My habit is to neutralize a the lens at the fitting cross, which is where I expect the patient's pupil to be when the eyes are in the primary position of gaze. This is where I expect to find any prism that I have prescribed, or no prism if I have not prescribed prism. In the case I mentioned above, this is where I measured the unwanted 6^ down o.u.

Now it is true that I found the optical center way down at the bottom of the lens, which he might use occasionally for reading, but these were sunglasses, so I wanted it where I wanted it, at the fitting cross, not in the reading zone, since he wanted these for driving a car.

It is also true that I have found that prescribing a little prism thinning of my own works well to thwart the lens designing nerds. If this patient had given me a chance, I'd have remade the Rx with maybe 0.5 base down o.u. This over-rides their equithin formulas, at least for now. (Until they find out that I'm doing that and add their calcs on top of mine).

And all of this also applies to digitally surfaced SV, BIs and TRIs, where the nerds will go next if they haven't already.

In which case we can throw out the concept of Optical Center Height???

I am not so sure why you are antagonistic against the lens designers and opticians? Why would you read it at the fitting ht when it is read 4mm below that on most designs? You are looking for it to be wrong.
I used to order 0.25 bi prism OU when changing BC to help turn off the Shamir calcs for compensated lenses but have never heard of an issue with prism thinning and the resultant optics.

How long have you been an OD and doing the glasses?

We look for everything to work and if it does not give us great near and far vision we have an issue; but we have never had this be a consumer issue.

Hope you find a new lab/optican who will help you understand that everything is some sort of compromise optically and you may be creating an issue that does not exist for the consumer.

[Speed]

I have it in mine. Our lab software allows us to set that pre account. If a dispenser does not want it, we just turn it off on their account master page.

[Dr. Bill Stacy]

[QUOTE=Craig;510521]

I am not so sure why you are antagonistic against the lens designers and opticians? Why would you read it at the fitting ht when it is read 4mm below that on most designs? You are looking for it to be wrong.
I used to order 0.25 bi prism OU when changing BC to help turn off the Shamir calcs for compensated lenses but have never heard of an issue with prism thinning and the resultant optics.

How long have you been an OD and doing the glasses?

We look for everything to work and if it does not give us great near and far vision we have an issue; but we have never had this be a consumer issue.

Hope you find a new lab/optican who will help you understand that everything is some sort of compromise optically and you may be creating an issue that does not exist for the consumer.

I admit to being antagonistic towards lens designers, at least whomever it is that's responsible for forcing everyone into having unneeded and/or unwanted vertical prism in their glasses. I am not antagonist towards opticians unless they say something stupid or ignorant, but I'm the same way towards optometrists and ophthalmologists when they do it.

I've been an O.D. for 48 years and "doing the glasses" for 49 (as an optician 1 year before graduating from U.C., and then as an O.D. I own and operate a practice with an in-office finishing lab). I've employed several O.D.s and opticians over the years.

I do understand compromise and do it when it's needed. I don't compromise if it's not needed or necessary.

The people who designed equithin/thinning prism had an elegant idea but they either forgot or didn't know about prism distortion. I probably shouldn't call them nerds, although some people consider that a term of endearment.

Lots of the 3 O's I've come across don't know much or anything about prism distortion.

If anyone reading this doesn't know what prism distortion is, it's the same phenomenon that is used in elementary science classes to completely split white light into a rainbow with a strong prism, just not as obvious in our context.

Sorry if I offended anyone.

[Robert Martellaro]

I don't think prism thinning is the issue here, the distance reference point on a lens is well above the prism reference point so even if the lens has "No prism thinning" the patient is still going to view through a point at least 4mm above the PRP (average 4mm fit point) and then the center of the distance reference point is going to be about 3mm above that where you check in the lensmeter.

Right. The DRP (distance reference point) is where we measure the distance power. Align the upper circle on the template or the the yellow circle on the lens against the lens stop for this measurement. The FP (fitting point) is aligned with the pupil center on the primary gaze and is the fitting cross on the lens or template. The PRP (prism reference point) is centered between the alignment reference marks. Prescribed, prism-thinning, or unwanted prism is measured here.

My habit is to neutralize a the lens at the fitting cross, which is where I expect the patient's pupil to be when the eyes are in the primary position of gaze.

Reading the distance power at this point will add plus to the true power because of the large aperture size of the lensometer.

This is where I expect to find any prism that I have prescribed, or no prism if I have not prescribed prism.

The lens dioptric power will influence the result if we measure for prism at this point. We will only get an accurate reading at the PRP (see above).

Glad to see you back Dr. Stacy. Too bad about sci.med.vision- nothing there the last time I looked.

[Craig]

[QUOTE=Dr. Bill Stacy;510531]

I admit to being antagonistic towards lens designers, at least whomever it is that's responsible for forcing everyone into having unneeded and/or unwanted vertical prism in their glasses. I am not antagonist towards opticians unless they say something stupid or ignorant, but I'm the same way towards optometrists and ophthalmologists when they do it.

I've been an O.D. for 48 years and "doing the glasses" for 49 (as an optician 1 year before graduating from U.C., and then as an O.D. I own and operate a practice with an in-office finishing lab). I've employed several O.D.s and opticians over the years.

I do understand compromise and do it when it's needed. I don't compromise if it's not needed or necessary.

The people who designed equithin/thinning prism had an elegant idea but they either forgot or didn't know about prism distortion. I probably shouldn't call them nerds, although some people consider that a term of endearment.

Lots of the 3 O's I've come across don't know much or anything about prism distortion.

If anyone reading this doesn't know what prism distortion is, it's the same phenomenon that is used in elementary science classes to completely split white light into a rainbow with a strong prism, just not as obvious in our context.

Sorry if I offended anyone.

Thanks for the response. Does the material used have anything to do with the prism distortion effect? We do 96% trivex and have no issues with any optics but once you get into poly all sorts of issues are magnified and problematic.
What material are you using? I am guessing mostly cr-39 so this is not part of your issue with a relationship between abbe value and negative optical results.

Hope you work another 20 years, then you might go pt.

[Dr. Bill Stacy]

Right. The DRP (distance reference point) is where we measure the distance power. Align the upper circle on the template or the the yellow circle on the lens against the lens stop for this measurement. The FP (fitting point) is aligned with the pupil center on the primary gaze and is the fitting cross on the lens or template. The PRP (prism reference point) is centered between the alignment reference marks. Prescribed, prism-thinning, or unwanted prism is measured here.

Reading the distance power at this point will add plus to the true power because of the large aperture size of the lensometer.

The lens dioptric power will influence the result if we measure for prism at this point. We will only get an accurate reading at the PRP (see above).

Glad to see you back Dr. Stacy. Too bad about sci.med.vision- nothing there the last time I looked.

Yes of course, which is what I do for sph, cyl and axis measuring; I slide down to the fitting cross to check for prism. I had not heard about the PRP before arriving at this thread, but I understand that is where we should expect zero prism if there is prism thinning rather than at my favorite point, the fitting cross???

It sounds like this new terminology is only used for PALs, not for any other lens designs. Is that correct?

And they're dumping MRP? (major reference point)

It is good to be back and yes, too bad about sci.med.vision, but I guess the entire Usenet thing is pretty much gone.

Glad to hear from you.

[Dr. Bill Stacy]

[QUOTE=Craig;510536]

Thanks for the response. Does the material used have anything to do with the prism distortion effect? We do 96% trivex and have no issues with any optics but once you get into poly all sorts of issues are magnified and problematic.
What material are you using? I am guessing mostly cr-39 so this is not part of your issue with a relationship between abbe value and negative optical results.

Hope you work another 20 years, then you might go pt.

Thanks for that, maybe 1 more year, maybe 8 more. As long as I'm having fun and am getting smiles and/or hugs from patients.

For weak powers and no prism, I use poly a lot. As powers and/or prisms rise, poly use goes down due to the ABBE.

For moderate powers Trivex is my fav.

Higher powers, 1.67 index is usually my choice.

I use cr39 where safety and lens thickness are not big issues.

I'm impressed with the high trivex use you have. I sure don't like how hard it is on the Santinelli rough wheel, and can't conceive of using it above -6 on anyone but tiny people.

[MakeOptics]

Yes of course, which is what I do for sph, cyl and axis measuring; I slide down to the fitting cross to check for prism. I had not heard about the PRP before arriving at this thread, but I understand that is where we should expect zero prism if there is prism thinning rather than at my favorite point, the fitting cross???

It sounds like this new terminology is only used for PALs, not for any other lens designs. Is that correct?

And they're dumping MRP? (major reference point)

It is good to be back and yes, too bad about sci.med.vision, but I guess the entire Usenet thing is pretty much gone.

Glad to hear from you.

Just to add some clarification the PRP is where the lens should be checked for prism, if you order without "prism thinning" then it should read zero at this point. On PAL's this point is between the two laser engravings on the lens, this is why my above post may be the root of your problems if you order with zero prism the lens you get back will have zero at the prism reference point bu the fitting cross and the distance reference point will exhibit prism as it is off axis from the optical center of the lens or PRP in the case of zero prism. When you adjust to have zero prism at the distance reference or the fitting cross then the near reference point or reading area will be that much further from the optical center having all of the prism you removed from the distance at the near. In sunglasses I can see how that was problematic for the patient. With most labs I have used you can just tell them to globally set "No Prism Thinning" on your account and that should solve the problem but you still need to deal with prism when viewing through the different zones of the lens.

The reference to "prism distortion" you refer to is chromatic aberration since we are talking about the splitting of white light into the component colors, the reason why this change in terminology is important is that actual distortion error is due to the increase in prism as we move further from the optical center causing the pincushion or barrel distortion effect, that is another byproduct of prism that some designers address. Chromatic aberration can only be corrected by using better quality materials, distortion error can be reduced or minimized by varying the curves with aspherics two different animals.

I also find the term nerd endearing, so I don't think it's offensive to use but I am not a lens designer. On a side note plus lenses should use higher index materials more judiciously then minus lenses in my opinion due to center thickness.

PS - The Vision Council standardized many of the terms used here and it helps to use them to aid in clear communication, here is a copy of the standard for your enjoyment:

http://www.thevisioncouncil.org/site...l%20June16.pdf

[Craig]

Buy a trivex roughing wheel from Santinelli. They sell them because of me and they work amazing on all materials

[idispense]

[QUOTE=Dr. Bill Stacy;510531][QUOTE=Craig;510521

I've been an O.D. for 48 years and "doing the glasses" for 49 (as an optician 1 year before graduating from U.C., and then as an O.D. I own and operate a practice with an in-office finishing lab). I've employed several O.D.s and opticians over the years.
.[/QUOTE]


49 years, eh ? Never heard of PRP ? How many glasses were bang on Rx when measured at the fitting cross ? What compensated power lens would be bang on prescribed Rx at that point ?

[Robert Martellaro]

Yes of course, which is what I do for sph, cyl and axis measuring; I slide down to the fitting cross to check for prism. I had not heard about the PRP before arriving at this thread, but I understand that is where we should expect zero prism if there is prism thinning rather than at my favorite point, the fitting cross???

No, the PRP is where the degree of prescribed, unprescribed, or yoked/thinning prism is measured.

It sounds like this new terminology is only used for PALs, not for any other lens designs. Is that correct?

Same for SV, except substituting pupil height for fitting point. Segmented multifocals still use some of the older terms like segment heights and OC heights.

And they're dumping MRP? (major reference point)

Yup, vague and ambiguous. I think it was Rodenstock that introduced the change to PRP about 15 to 20 years ago.

It is good to be back and yes, too bad about sci.med.vision, but I guess the entire Usenet thing is pretty much gone.

Dust in the wind. SMV was taken over by nutters and pseudosciencers; OB is moderated, keeping that kind of thing to a minimum.

[Speed]

When we first introduced prism thinning at our lab years ago, we had to decide how to introduce this. We could either send a letter or otherwise contact our customers to see if they would chose this as an option or just implement. We decided to just turn this on for everyone and field any calls or questions that may come. Calls? We got zero calls. That told us that MOST people did not even know where to check the prism on progressives. That may still be the case. Most "vertical imbalance" problems are due to this ignorance of proper Rx verifications procedures. Most of the true V.I. problems are (as stated above) due to incomplete information on half pair orders (please include the prism for thinning). If you aren't comfortable checking progressives get some help from your providers. Then train your help.

[MakeOptics]

Yup, vague and ambiguous. I think it was Rodenstock that introduced the change to PRP about 15 to 20 years ago.

The '87 ANSI shows what's now referred to as the PRP being the "Error Free Point" after this point in time the adoption of the PRP came about more than likely due to the tighter prism tolerances referenced in a '95 memo by Dick Whitney to the ANSI committee.

[Dr. Bill Stacy]

Thanks to everyone for the re-education. If I understand the PRP correctly, any prism showing up there is the vector sum of any prescribed prism plus any equithin prism plus any lab error prism, so if I request "no thinning prism" and have not prescribed any prism then it should read close to zero prism at that point.

This means that even without thinning prism, if I want zero prism at the fitting cross, I'd have to calculate the amount based on the usual 4 mm distance from the MRP and the fitting cross. For an Rx of R +1.00 sph. and L-1.00 sph. this would mean prescribing R 0.4 base up and L .4 base down. I think that 4 mm separation was an arbitrary thing based on the premise that "it has to go somewhere" and for an average user, that's probably as good a place as any. And I think different lens designers have different thinning prism formulae, so it's a crapshoot if not removed entirely.

Maybe for people using the Rx mostly for reading, they might want to lower it and for those using them mostly for driving they might want to raise it, but that might be asking too much for the design gnomes (trying to decide if that's a better term than nerds, or maybe elves would be better).

I'm sure if I made any errors above I'll be corrected.