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Thread: Prism for thickness reduction, make or not?

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    Prism for thickness reduction, make or not?

    dear Colleagues,

    I have experienced different Free Form designs. Some of them with bigger prism for addition reduction (almos 2/3 of the addition) and some near to zero (or zero). Regarding end customer adaptation and usage only, what is the best for them?

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    Quote Originally Posted by Mauricio View Post
    dear Colleagues,

    I have experienced different Free Form designs. Some of them with bigger prism for addition reduction (almos 2/3 of the addition) and some near to zero (or zero). Regarding end customer adaptation and usage only, what is the best for them?
    Having just recently studied this, I can say that "equithinning" up to 2/3 of the strength of the addition (but not exceeding 3 D total prism) isn't expected to affect the patient in any negative way as long as both lenses carry an equal amount of prism. Practically speaking, I can't say that I can recall an incident where someone's adaptation was affected by yoked vertical prism, as long as it was equal. I've seen re-do's where the replacement lens has come in with different prism thinning than it's mate, though those usually don't make it out of our lab.
    I can think of at least a couple specific patients that have pretty finicky tolerances who don't seem to be affected by varying amounts of yoked prism, though in all honesty I hadn't considered it in that light until now. (we have a handful of patients who seem to always need 2 or 3 re-makes with every order, for one reason or another...)
    The main benefit is a thinner, lighter lens, and "my new glasses are too heavy" is probably one of our most common complaints so I can't see any reason not to do it.
    As a matter of fact, it's not even one of the parameters we specify from any of our suppliers: they all just do it automatically. With the result that I've never dealt with progressives that weren't prism-thinned.

    Edit: now that I'm thinking about it, I've just realized that I've recently seen prism thinning that exceeded 2/3 thirds of the add... IIRC it was 2.13 D prism with a +2.50 add... and it was one of those remakes that didn't get caught before the patient found it for us
    Last edited by Christopher; 11-28-2012 at 12:22 AM.

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    Christopher, welcome to Optiboard!

    You are stating conventional wisdom held for many years. We always think of prism if its equal (or yoked) as benign or harmless However, Jim Sheedy did a study and found that most patients could tolerate only 2 to 4 diopters of yoked prism. What that means is somewhere between 2 and 4 diopters of prism thinning at the OC there will be subjective loss of visual accuity, almost all patients will have loss at 4 D but some at only 2 D. In practical terms, 3 D could cause loss of VA for half of our patients. The problems is no OD tests a patient for loss of vision due to yoked prism, and the range is fairly wide.

    Years ago it was explained to me that Prism Thinning was NOT introduced for its cosmetic benefits, but rather because it improved yield on the generators. (because they were ground closer to center, more lenses would pass inspection). It was just SOLD as a cosmetic benifit. Executive bi-focals were popular at the time, and no one proposed prism thinning for those monsters. So the cost of progressive blanks being high at the time, lends some credence to that view.

    So with todays free-form technolgy, we can in theory adjust prism thinning and keep most patients under the 2D threshold if we want. Some prism thinning is desirable for previous progressive wearers because if you remove it entirely, their world can be VERY strange, and adapation is longer. Things move very differently when we take all prism thinning away.

    As far as I know (Darryl didn't answer my question directly re Zeiss) Seiko is on the only company I can confirm that is adjusting prism thinning based on the RX, and eliminating it entirely in the Supercede with higher powers. Its one reason high Myopes LOVE the Seiko Supercede.

    The added factor is that with ground lenses, most opticians fit below center. As well over time, frames tend to drop. So many patients have more yoked prism than the numbers would suggest with a lens 2+ mm below intended. As power low as -3.00 sphere could cause loss of distance visual acuity in some lenses, depending on the prism thinning applied and the patients tolerance to yoked prism. Which we simply don't know.

    I think its time to put prism thinning away really.


    Quote Originally Posted by Christopher View Post
    Having just recently studied this, I can say that "equithinning" up to 2/3 of the strength of the addition (but not exceeding 3 D total prism) isn't expected to affect the patient in any negative way as long as both lenses carry an equal amount of prism. Practically speaking, I can't say that I can recall an incident where someone's adaptation was affected by yoked vertical prism, as long as it was equal. I've seen re-do's where the replacement lens has come in with different prism thinning than it's mate, though those usually don't make it out of our lab.
    I can think of at least a couple specific patients that have pretty finicky tolerances who don't seem to be affected by varying amounts of yoked prism, though in all honesty I hadn't considered it in that light until now. (we have a handful of patients who seem to always need 2 or 3 re-makes with every order, for one reason or another...)
    The main benefit is a thinner, lighter lens, and "my new glasses are too heavy" is probably one of our most common complaints so I can't see any reason not to do it.
    As a matter of fact, it's not even one of the parameters we specify from any of our suppliers: they all just do it automatically. With the result that I've never dealt with progressives that weren't prism-thinned.

    Edit: now that I'm thinking about it, I've just realized that I've recently seen prism thinning that exceeded 2/3 thirds of the add... IIRC it was 2.13 D prism with a +2.50 add... and it was one of those remakes that didn't get caught before the patient found it for us

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    Sharpstick, thank you for sharing that information with us. I personally found it really interesting.

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    Quote Originally Posted by Robert_S View Post
    Sharpstick, thank you for sharing that information with us. I personally found it really interesting.
    Yes, thank you; from me as well. I'll need a bit of time to digest it, and do some digging around and I'll surely talk it over with my preceptor.

    FWIW, I've had some of my own experience dealing with yoked prism in a big ol' pair of aviator style sunglasses that I didn't order lenses for correctly. As a result, they ended up with a bunch of base up prism and I've found that I have a very difficult time walking or doing any other activities while wearing them. They work fine while I'm driving because my lower horizon consists of nothing but the inside of the car but as soon as I leave the car with them on, I end up misjudging my stride and the terrain.

    Thanks for the great insight into the issue. As far as conventional wisdom, as a student, unfortunately, that's all I have to go on for now.

    Sharpstick, would you be so kind as to elaborate a bit more on what you mean by "it improved yield on the generators. (because they were ground closer to center, more lenses would pass inspection)"? ( I'm not very familiar with the production side)
    Last edited by Christopher; 01-04-2013 at 12:30 AM.

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    Quote Originally Posted by Christopher View Post
    Sharpstick, would you be so kind as to elaborate a bit more on what you mean by "it improved yield on the generators. (because they were ground closer to center, more lenses would pass inspection)"? ( I'm not very familiar with the production side)
    A washing machine with all the weight on one side will simply shake more at 10,000 rpm spin cycle. When lenses are surfaced off center, even in free-form, yield (the number of lenses that come off perfect) drops because unwanted cyl and prism are more likely, as is chatter. Its one reason a lot of Free-form companies don't allow lenses to be decentered very far or at all, but if they do, they charge more (ala Autograph II). Some of the extra cost of the Auto II is calculated on lower yields because of decentering. It has to do with the pressure not being centered, and creating an imbalance. The drop in yield can be even more dramatic in ground lenses than Free-form, but its a cost the lab has to eat not only in blanks but in generator time.

    I heard it from an old Sola lab guy in Petaluma, who is now long retired or deceased, that prism thinning originally was business decision based on more on yield than anything else. Its definately not an optical benefit to the patient, esp in higher powers. There is a slight cosmetic benefit, but I have never once had a patient redo a progressive because only the bottom was too thick. Have you?

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    I have to disagree. It's certainly true that it's a good idea to minimize the amount of movement that a generator's cutter has to make (that's an in-and-out movement as it traverses the spinning lens), because as that increases, surface accuracy diminishes and roughness increases. And generally, it's true that decentration increases cutter movement. The negative effects are related to the steepness of the curves. However, one of the more significant problems in freeform production is the "center artifact" - the cutter velocity effectively drops to zero at the center of the blank, and machine software has to jump through hoops to keep the machine from producing an anomaly on the surface at that point. Cutting a freeform surface with a modicum of decentration moves the prism reference point (with all the other references) off the center of the blank, away from any center artifact, so that it's less likely to be directly in the patient's gaze. So, that's a good thing.

    Second, on freeform progressives, prism thinning actually reduces cutter movement (usually). The area of the lens where the add is located is flatter than the rest of it (that is, further from the cutter), so tilting that edge of the surface towards the cutter (which is what prism thinning would do) will usually diminish the amount of cutter movement. I suppose a lens for with-the-rule astigmatism could be an exception, depending on the relative amounts of add and cylinder.

    Lastly, I would note that if a lab is using the conventional rule of thumb of 0.6 prism diopter per diopter of add power, even a patient wearing a 3.00 add would get less than 2 diopters of yoked prism. Of course, in many instances, rather than using the rule of thumb, we'll be set to calculate an amount of prism that equalizes thickness top and bottom, which, if not limited by a "cap", could produce very large amounts of prism. Our customers who use that thinning mode usually employ such a cap.

    I didn't understand the comment about a patient rejecting a job "because only the bottom was too thick" - that implies too much thinning prism, does it not?
    Last edited by shanbaum; 01-04-2013 at 06:44 PM. Reason: Wrote "against" instead of "with." Sorry.

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    @Robert: to your last question, he may have incorrectly specified an OC height that deviated significantly from ".5B" resulting in what he described. Then it would only have been "yoked" if the total power in the vertical meridians were the same, and an imbalance only noticeable depending on power difference and OC deviation amount from .5B. As the imbalance increases, the glasses become less wearable.
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    Robert, you add some good points. If we move or reduce the center artifact, that would have a net positive result on "yield", would it not? How far does the center need to move?

    Quote Originally Posted by shanbaum View Post
    However, one of the more significant problems in freeform production is the "center artifact" - the cutter velocity effectively drops to zero at the center of the blank, and machine software has to jump through hoops to keep the machine from producing an anomaly on the surface at that point. Cutting a freeform surface with a modicum of decentration moves the prism reference point (with all the other references) off the center of the blank, away from any center artifact, so that it's less likely to be directly in the patient's gaze. So, that's a good thing.

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    Here is how I came to question the conventional wisdom of prism thinning many years ago. I ordered 2 pairs for the same patient, same material, same RX, same progressive. 1 Sun, one Clear. They came in fine, but the patient could not adapt the sun pair. I was stumped until I looked the prism thinning. They came in with different thinning amounts, because 2 different operators ran them, and 2 different sized frames. I reordered WITHOUT any prism thining, and the patient noticed improvment in BOTH pairs. After that, I ordered about half my grinders w/0 prism thinning (i ordered my last ground lens in 2005).

    Prism thinning is subjective. I can be done by the lab software, the operator, the conventional rule of thumb, or the manufacturers recommendation. There is no one standard.

    Quote Originally Posted by shanbaum View Post
    Lastly, I would note that if a lab is using the conventional rule of thumb of 0.6 prism diopter per diopter of add power, even a patient wearing a 3.00 add would get less than 2 diopters of yoked prism. Of course, in many instances, rather than using the rule of thumb, we'll be set to calculate an amount of prism that equalizes thickness top and bottom, which, if not limited by a "cap", could produce very large amounts of prism. Our customers who use that thinning mode usually employ such a cap.

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    Here is how I came to question the conventional wisdom of prism thinning many years ago. I ordered 2 pairs for the same patient, same material, same RX, same progressive. 1 Sun, 1 Clear. They came in fine, but the patient could not adapt the sun pair. I was stumped until I looked the prism thinning. They came in with different thinning amounts, because 2 different operators ran them, and 2 different sized frames. I reordered both WITHOUT any prism thining, and the patient noticed improvment in BOTH pairs. After that, I ordered about half my grinders w/o prism thinning (i ordered my last ground lens on August 30th of 2005). Prism thinning is subjective. I can be done by the lab software, the operator, the conventional rule of thumb, or the manufacturers recommendation. There is no "one" standard.

    Quote Originally Posted by shanbaum View Post
    Lastly, I would note that if a lab is using the conventional rule of thumb of 0.6 prism diopter per diopter of add power, even a patient wearing a 3.00 add would get less than 2 diopters of yoked prism. Of course, in many instances, rather than using the rule of thumb, we'll be set to calculate an amount of prism that equalizes thickness top and bottom, which, if not limited by a "cap", could produce very large amounts of prism. Our customers who use that thinning mode usually employ such a cap.

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    Quote Originally Posted by shanbaum View Post
    I didn't understand the comment about a patient rejecting a job "because only the bottom was too thick" - that implies too much thinning prism, does it not?
    I began ordering about half my grinders without prism thinning in 1999, I never had one patient complain about how thick the bottom was.

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    Robert,
    I am not proposing we eliminate prism thinning entirely, that would probably result in a huge amount of non-adapts for long term progressive wearers. But I do propose that we keep it under 1.5D of yoked prism up to 2mm above OC, (a little cushion to account for frame drop over time and fitting errors).

    Although most Free-form companies reduce prism thinning, only Seiko was transpartent with me on how they handled it. They deliberately adjust prism thinning to keep prism below 2D at OC, and eliminate it entirely on the Supercede over a -3.00.

    Other lens companies may do this, but I am provoking this discussion to find out which ones, and demonstrate the primary reason for prism thinning is really yield, with cosmetics only a secondary benefit. Opticians should know what they are getting and why, and most FF lenses can NOT be ordered with any specific prism thinning number, or if the optician wants, eliminated entirely.

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    This is an excellent discussion!
    I brought this up today at work with my preceptor, and he was like: "oh yeah. Prism thinning causes lots of problems." He seemed to take it for granted that it was just one more aspect to keep in mind when trouble shooting. He even mentioned my funky sunglasses as an example. (I think I've got him convinced to try specifying NO prism thinning on a high myope PAL order).

    Funny how it's passed off as no big deal as far as NAIT's curriculum is concerned (that conventional wisdom thing again). I guess that's where real-world experience comes in.

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    Van, I didn't mean to argue either in favor of or against prism thinning generally, and I think I probably agree with your overall position, which I take to be, that it should be minimized. I just wanted to clear up a few things about production. I would add that it's more important to minimize it on high-index (or more precisely, low-abbe) materials, where chromatic aberration is exacerbated.

    I know that many of our customers set their systems to not thin lenses unless they're plus in the total add power, or when they're just a little bit minus there.

    The reason I'm confused about your comment regarding "how thick the bottom was" is that prism-thinned lenses will have a thicker bottom edge, and in fact, one of the more serious production issues with non-thinned lenses is the knife edge at the bottom, which is easily chipped, or which may tear up a polish pad if not beveled (by hand, which no one likes having to do).

    In fact, with regard to generating, prism thinning on regular (front-side) progressives has the opposite effect to the one I described for freeform lenses - it creates additional cutter movement, not less, so one wouldn't expect prism thinning to improve yields from that standpoint. Most freeform systems, like most cut-to-polish systems (which face the same issues), use prism blocking to the extent possible, so that the surface doesn't have to be tilted in the generator. Of course, prism blocking introduces its own hassles; setting the blocker up is just another place for things to go wrong.

    With regard to your "center artifact" question, we don't have the freedom to move the center artifact on the surface - it's at the block center, which usually coincides with the blank center (if it doesn't, other bad things happen, when the cutter goes off and back on an eccentrically-blocked lens as it spins). Moving the surface matrix so that the center artifact is outside area of the lens through which the patient usually gazes is achieved by decentering the surface matrix. I don't think that there's an optimal amount of decentration, because there are several factors to consider. It would be good to move the matrix so as to get the center artifact into the less-usable part of the design - where there's unwanted astigmatism that makes that part of the surface unusable anyway. But generally, when we're working with lens design systems that support variable decentration, we decenter the matrix so that the blank center coincides approximately with the finish block center, at a point that minimizes the blank size requirement, so that the lens can be cribbed smaller - subject to some constraints intended to deal with the issues that you mention (i.e., as you decenter a steep curve, the in-and-out motion of the cutter can become excessive). Since most lenses are blocked in and up (in finishing) that movement does tend to move the blank center into the wasteland.

    With traditional surfacing, the surface issues are all fined (or "smoothed") away in seconds.

    Optotech's original freeform system used a raster-cut method, rather than spinning the lens, so it wouldn't have the center-artifact issue; but they've abandoned that approach with their newer machines. Their old system was slow compared to the competition's gear. So far as I know, they all spin, now.

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    Quote Originally Posted by shanbaum View Post
    Optotech's original freeform system used a raster-cut method, rather than spinning the lens, so it wouldn't have the center-artifact issue; but they've abandoned that approach with their newer machines. Their old system was slow compared to the competition's gear. So far as I know, they all spin, now.
    the diamond ball in optotech generator allow to generate lens with rustler cutting, this take a long time but give a better lens, in the same time you can use the same ball in a spiral cutting, you get a lower quality but faster.
    the problem is in the single diamond cutting machine which can make only spiral cutting but faster production time and little bit lower quality, and in this case you always find this central pint default, even if you make the linear calibration that they recommend.
    by the way they still can offer the diamond ball machine asm80 omega

    Hamza Mahmoud

    Mail : mahmoud.hamza@optylab.com
    Web : www.optylab.com

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    Thanks Robert, your great knowledge makes a valuable contribution...
    I know you are not arguing with me per se, but I am learning from your input so its appreciated, really.


    Quote Originally Posted by shanbaum View Post
    I didn't mean to argue either in favor of or against prism thinning generally, and I think I probably agree with your overall position, which I take to be, that it should be minimized. I just wanted to clear up a few things about production.

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    I mis-spoke, I was backwards, thanks

    QUOTE=shanbaum;448542]
    The reason I'm confused about your comment regarding "how thick the bottom was" is that prism-thinned lenses will have a thicker bottom edge, and in fact, one of the more serious production issues with non-thinned lenses is the knife edge at the bottom, which is easily chipped, or which may tear up a polish pad if not beveled (by hand, which no one likes having to do).
    [/QUOTE]

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    Base Study: Yoked prism has an negative effect on most users at 4D and almonst none (but not zero) at 2D. Somewhere between 2 and 4 D people will experience a loss of visual accuity.

    Sheedy, J. and S. Parsons. "Vertical yoked prism—Patient acceptance and postural adjustment." Journal of Ophthal. Physiol. Opt. Vol. 7 (1987), pp.: 255-257.

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    I mis-spoke, I was backwards, yes, non-prism thinned lenses are thicker at the top.

    Quote Originally Posted by shanbaum View Post
    The reason I'm confused about your comment regarding "how thick the bottom was" is that prism-thinned lenses will have a thicker bottom edge, and in fact, one of the more serious production issues with non-thinned lenses is the knife edge at the bottom, which is easily chipped, or which may tear up a polish pad if not beveled (by hand, which no one likes having to do).

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    Robert,
    I know that a Free-form progressives with both concave and convex back surfaces are now available starting with the Surmount, and some FF lens are now using Aspheric blanks. To some degree we can get Free-form lenses thinner than was possible before because our curve options are now enormous. With the control possible on new digital lenses, this will even further reduce the need for prism thinning would it not?

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    Quote Originally Posted by sharpstick
    You are stating conventional wisdom held for many years. We always think of prism if its equal (or yoked) as benign or harmless However, Jim Sheedy did a study and found that most patients could tolerate only 2 to 4 diopters of yoked prism. What that means is somewhere between 2 and 4 diopters of prism thinning at the OC there will be subjective loss of visual accuity, almost all patients will have loss at 4 D but some at only 2 D. In practical terms, 3 D could cause loss of VA for half of our patients
    Just to clarify, Dr. Sheedy actually conducted a sort of acceptance test using yoked prism; visual acuity was not measured. He basically found that subjects could not tell any difference with 2.0 diopters of yoked vertical prism, but that several experienced discomfort or postural adjustments with 4.0 diopters of yoked prism. (I think I mentioned this in a similar debate in another thread.)

    As Robert pointed out earlier, you are unlikely to see any more than 2.0 to 2.5 prism diopters of thinning in practice, even in the worst-case scenario. Consequently, the amounts of prism-thinning routinely applied to progressive lenses should not cause any visual discomfort. It can, however, significantly reduce thickness and weight in many prescriptions.

    Quote Originally Posted by sharpstick
    With the control possible on new digital lenses, this will even further reduce the need for prism thinning would it not?
    Not so much. As long as the lens provides an addition power in the lower half, there will always be a difference in thickness between the upper and lower edges of the finished lens blank, proportional to the add power. (Although in some prescription and fitting height combinations, vertical decentration may minimize the required thickness or thickness difference of the lens.)

    Best regards,
    Darryl
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    Quote Originally Posted by Mauricio
    I have experienced different Free Form designs. Some of them with bigger prism for addition reduction (almos 2/3 of the addition) and some near to zero (or zero). Regarding end customer adaptation and usage only, what is the best for them?
    I would recommend reviewing the following article on prism-thinning Understanding Prism-Thinning. There are a number of factors involved in determining the optimum amount of prism-thinning for a given job, including the prescription, addition power, fitting height, far PD, and frame size.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Darryl Meister View Post
    As Robert pointed out earlier, you are unlikely to see any more than 2.0 to 2.5 prism diopters of thinning in practice, even in the worst-case scenario. Consequently, the amounts of prism-thinning routinely applied to progressive lenses should not cause any visual discomfort. It can, however, significantly reduce thickness and weight in many prescriptions.
    Hopping into final inspection earlier today, most of the progressive powers over a +/- 3 actually had yoked prism over 3D prism at OC. Although that represents a small amount of RXs, it still represents a significant possible decrease in VA to those few patients. I would believe its effect is detrimental to high cylinder as well. I will do some tracking of RXs later this week. The other problem is that many Opticians still fit below the OCs, and the degradation of height of a lens over time.

    I have contacted most of my Free-form lens companies reps, and currently Seiko is the only one to respond, and they said they will deliberately adjust Prism thining in their higher powers to keep it under 2D at OC, and in the Supercede, eliminate it entirely over a
    -3.00 . The other manufacturers have said "they will get back to me", so I am still waiting to hear. Maybe they don't even know how their lens designs handle prism thinning.

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    Quote Originally Posted by sharpstick
    Hopping into final inspection earlier today, most of the progressive powers over a +/- 3 actually had yoked prism over 3D prism at OC.
    By "unlikely to see," I was referring to the probability. Putting aside the fact that prescription powers above +2.00 or +3.00 D represent a small percentage of lens sales, I just ran some surfacing calculations and found that, with a +3.00 DS Rx, I had to fit a +2.50 Add SOLA VIP at over 8 mm above the geometric frame center before I hit 3.0 diopters of prism-thinning.

    That said, I wouldn't recommend any thinning above 3.0 prism diopters. Further, your laboratory must be using automatic prism-thinning to arrive at such a high value, which means that you guys have also set the prism-thinning limit in your software to some very high value. I find this surprising, particularly given your personal reservations against any kind of prism-thinning.

    Quote Originally Posted by sharpstick
    it still represents a significant possible decrease in VA to those few patients.
    I am still not sure why you keeping referring to visual acuity. I haven't seen any research to suggest that 3.0 prism diopters would significantly reduce visual acuity. And, even if it did, your +3.00 DS wearer would experience even worse visual acuity while reading, without the benefit of prism-thinning, since the vertical prism (and, for that matter, lateral chromatic aberration) would be extremely high in the near zone, otherwise.

    Quote Originally Posted by sharpstick
    I have contacted most of my Free-form lens companies reps, and currently Seiko is the only one to respond, and they said they will deliberately adjust Prism thining in their higher powers to keep it under 2D at OC, and in the Supercede, eliminate it entirely over a -3.00 .
    You may have to clarify what you mean: Did you actually contact a Seiko laboratory, or just a laboratory that processes Seiko free-form lenses (in addition to other lens brands)? What this response really suggests to me is that they have set a thinning limit of 2.0 prism diopters (not unreasonable, but probably unnecessarily conservative) and they don't bother to thin minus lenses for whatever reason.

    Of course, I can only speak for Carl Zeiss Vision and our own product recommendations and lab processing guidelines.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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