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Thread: Free Form. ANSI standards, are they good enough?

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    Free Form. ANSI standards, are they good enough?

    What are your thoughts on ANSI standards when it comes to free form lenses. Especially PD? I'm wondering if all this energy and research is put into trying to perfect a lens, and then PD is not exact, does it negate the whole process?

    Any thoughts or research on this and other issues in this area?

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    Found this:

    "For lenses produced with compensations to account for as worn correction , the tolerances in the tables in section 5.0 apply to those values specified by the manufacturer, and not to the prescribed prescription.” This is especially important with free form lens fabrication technology."


    http://www.thevisioncouncil.org/memb...%20Summary.pdf

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Being off PD alone, but within ANSI standards is, for 90% of all prescriptions, not a real factor...unless there is latent phoria issues. Shame on the online fulfiller that cant fabricate within 2.5mm.

    However, for mature progressive wearers (> 2.25 add) and wrap work, PD can be an issue.

    FWIW

    B

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    Quote Originally Posted by Barry Santini View Post
    Being off PD alone, but within ANSI standards is, for 90% of all prescriptions, not a real factor...unless there is latent phoria issues. Shame on the online fulfiller that cant fabricate within 2.5mm.

    However, for mature progressive wearers (> 2.25 add) and wrap work, PD can be an issue.

    FWIW

    B
    Thanks for reply Barry. From what I linked above, I'm getting the impression that the manufacture should override ANSI standards if they have tighter tolerances. Is this how you read this?

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    OptiBoard Professional RT's Avatar
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    From what I linked above, I'm getting the impression that the manufacture should override ANSI standards if they have tighter tolerances. Is this how you read this?
    No. This means that if powers were compensated for position of wear, that the "starting point" for power tolerances would be the compensated Rx, not the original Rx. For example, let's say that you have a -3.00 sphere, but because of position of wear the compensated Rx is -2.83 -0.24. You measure -2.85 -0.20. If you compared the lens versus the original Rx, you would conclude that the lens is not within the ANSI Z80.1 power tolerance. If you compare your measurement vs. the compensated Rx, you would conclude that it is within tolerance.

    In this case, the word "manufacturer" means "the lab".
    RT

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    My read is that the tolerance stays the same, but it's referenced to the compensated power instead of the Rx power, which is entirely logical. For example, if the Rx is -3.00 DS, and the compensated power is -3.12 D, the tolerance is referenced to -3.12, and is -2.99 to -3.25 (-3.12 has a tolerance of of ±0.13 D).
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    One eye sees, the other feels OptiBoard Silver Supporter
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    RT, looks like we take our coffee break at the same time!
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    OptiBoard Professional RT's Avatar
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    Funny we both chose -3.00 as our example.
    RT

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    Are you myope too? Or maybe just like minds...
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Thanks for your responses guys. What you say makes sense. I have to say though, in the back of my mind it doesn't completely sit well with me. I feel that the lense designer goes thru excruciating calculations to create a lense design and then the design can be robbed of perfection due to sloppy manufacture. If it was me, the guy doing the design, it would upset me, knowing my design has been compromised by loose ANSI standards. As Barry says, FWIW......

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    OptiBoard Professional RT's Avatar
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    I'm not sure that I understand the linkage between "loose ANSI standards" and a "compromised design". Nothing in the ANSI Z80.1 standard deals with replicating the design. ANSI Z80.1 sets out to provide power tolerance at the near and far reference points, and achieves that goal. ANSI Z80.1 also defines tolerances for PD. If your lab is a "sloppy manufacturer", it is unclear that is a function of the ANSI standard, and sounds like a sloppy lab.

    Exactly which tolerance in the standard do you feel is "sloppy", and what would you propose instead?
    RT

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    Quote Originally Posted by RT View Post
    I'm not sure that I understand the linkage between "loose ANSI standards" and a "compromised design". Nothing in the ANSI Z80.1 standard deals with replicating the design. ANSI Z80.1 sets out to provide power tolerance at the near and far reference points, and achieves that goal. ANSI Z80.1 also defines tolerances for PD. If your lab is a "sloppy manufacturer", it is unclear that is a function of the ANSI standard, and sounds like a sloppy lab.

    Exactly which tolerance in the standard do you feel is "sloppy", and what would you propose instead?
    If I was an optics engineer I would be in a better position to answer this question - I'm asking this question for educational purposes only. Not trying to make a statement on ANSI or anything. As a point of example, if PD is determined to be at certain point, then all other calculations will be based off that, especially in a high curve. Get the PD off by 1mm, and now everything is skewed by that.

    An extreme example of this would be a triangle. Image that the PD is supposed to be at vertex of the 60 degree triangle, and it is moved 1 mm off. The compensated power adjustments will be thrown off pretty dramatically I imagine.

    Anyway, I was hoping a free form optics engineer might be lurking and chime-in. Maybe they've had discussions on this topic in meetings, among co-workers, ect....

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    Quote Originally Posted by AustinEyewear View Post
    If I was an optics engineer...
    We would need a mathematician, probably a doctorate level differential geometry type dude.

    I suspect that the tolerances for fitting and manufacturing are taken into account when designing the optics.
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    Quote Originally Posted by Robert Martellaro View Post
    We would need a mathematician, probably a doctorate level differential geometry type dude.

    I suspect that the tolerances for fitting and manufacturing are taken into account when designing the optics.

    These CNC machines have precision to 1/100 diopter. So why not demand its capability ? I guess that is where I'm going with on this in a round about sort of way.

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    Quote Originally Posted by AustinEyewear View Post
    These CNC machines have precision to 1/100 diopter. So why not demand its capability ? I guess that is where I'm going with on this in a round about sort of way.
    I thought you might be referring to the snowball or butterfly effect.

    As far as accuracy to .01 D, I suspect we're seeing that, or close to it, but that that level of accuracy is unnecessary because we don't see a difference of less than about .25 D or so.
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    Quote Originally Posted by Robert Martellaro View Post
    I thought you might be referring to the snowball or butterfly effect.

    As far as accuracy to .01 D, I suspect we're seeing that, or close to it, but that that level of accuracy is unnecessary because we don't see a difference of less than about .25 D or so.
    What about light though? I would imagine that is also a factor. I'd guess 0.01D can effect light rays? Fovea not very big and these guys are taking a lot of parameters into account to bring that beam of light dead center......

    P.S. what snowball or butterfly effect?

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    Quote Originally Posted by AustinEyewear View Post
    P.S. what snowball or butterfly effect?
    You said, As a point of example, if PD is determined to be at certain point, then all other calculations will be based off that, especially in a high curve. Get the PD off by 1mm, and now everything is skewed by that.

    The best lens designers, at least on paper, have a pretty good handle on how to make a lens that will perform as well as technically possible, assuming that the optician starts with an ideal frame/lens/eye relationship, and is able to supply accurate data for the software to work with. I also believe that the more optimized/compensated lenses are much less tolerant of inaccurate data, compared to spherical best form surfaces and simple PALs. A 3mm position error on a semi-finished lens might be tolerated, but the same error on a highly optimized lens will probably cause symptoms.

    You can see some of this for yourself by inputting a large p-value and dioptric power into Darryl Meister's optical analysis program. Observe how the power error and oblique astigmatism reacts to changes in base curves compared to spherical surfaces.

    Hope this helps,
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    I also believe that the more optimized/compensated lenses are much less tolerant of inaccurate data, compared to spherical best form surfaces and simple PALs. A 3mm position error on a semi-finished lens might be tolerated, but the same error on a highly optimized lens will probably cause symptoms.

    You can see some of this for yourself by inputting a large p-value and dioptric power into Darryl Meister's optical analysis program. Observe how the power error and oblique astigmatism reacts to changes in base curves compared to spherical surfaces.

    Hope this helps,
    And I think the above is why we're seeing the compensated calcs from Shamir, MJ, Oakley and IOT being tapered back a bit.

    B

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    Quote Originally Posted by Robert Martellaro View Post
    You said, As a point of example, if PD is determined to be at certain point, then all other calculations will be based off that, especially in a high curve. Get the PD off by 1mm, and now everything is skewed by that.

    also believe that the more optimized/compensated lenses are much less tolerant of inaccurate data, compared to spherical best form surfaces and simple PALs. A 3mm position error on a semi-finished lens might be tolerated, but the same error on a highly optimized lens will probably cause symptoms.

    Hope this helps,
    Thanks Robert - This is why I asked the question in the first place. To me it seems that inaccurate input data, or inaccurate manufacturing are similar. If one provides a PD input that is off, or manufactures it off due to sloppy workmanship, the net effect is the same. Which is what has me asking if ANSI standards are "good enough" for highly optimized lenses......

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    I think the standards are "good enough" but if your lab isn't good enough to make the eye wear to your spec's it's time to find another lab.

    I think you'd be hard pressed to find a situation where anyone anywhere was prosecuted for being out of ANSI standards. It's up to you to ensure the glasses are within your tolerances!

    Here's a challenge- Bring your pupilometer or pd stick to the next gathering of opticians and ask them to measure your monocular pd and see if you get the same numbers from everyone.

    I'll bet design engineers are happy knowing they've created the best design they can and as long as the paycheck clears they don't lose sleep over errors made in fitting them.

    My 2 pennies.

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    Quote Originally Posted by Uncle Fester View Post
    I think the standards are "good enough" but if your lab isn't good enough to make the eye wear to your spec's it's time to find another lab.
    For sake of discussion, I should have started with the caveat that the lab is within tolerances and optician measurements are dead on. With those two parameters defined as "perfect", then the only problem would be that the lab does not manufacture perfect, but is still within ANSI standard. That would only leave the ANSI standard in question. ANSI standards go before committees, and have outside forces exerted on them. They often are a one size fits all parameter. Ect, ect.... They are slow to change and be updated, seldom keeping up with ever changing technology developments.

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    Quote Originally Posted by AustinEyewear View Post
    Thanks Robert
    Your welcome.
    To me it seems that inaccurate input data, or inaccurate manufacturing are similar. If one provides a PD input that is off, or manufactures it off due to sloppy workmanship, the net effect is the same.
    I doubt that the data from the optician will propagate through the system in a linear way, hence my analogy with aspheric lenses above. So I would think that to some degree, it might not just be GIGO, but more like garbage in, even more garbage out.
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    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by AustinEyewear
    I feel that the lense designer goes thru excruciating calculations to create a lense design and then the design can be robbed of perfection due to sloppy manufacture... These CNC machines have precision to 1/100 diopter. So why not demand its capability?
    Keep in mind that just because a free-form surfacing process is not subject to the rounding limitations of hard lap tooling does not necessarily guarantee that the process will consistently produce lenses to within 0.01 D of accuracy. It is still, after all, a lens surfacing process. A great deal of process engineering and quality control is actually required to produce consistently accurate lenses.

    No one would ever use 0.01 D as a specification of actual process capability for free-form lens surfacing, although we would expect that the free-form surfacing process will at least minimize the likeliood or magnitude of power errors due strictly to lens surfacing compared to traditional lens surfacing. But there are still several sources of potential variance involved when fabricating eyewear.

    The ANSI Z80.1 tolerances apply to the net errors of the finished, mounted lenses. Power errors are the net result of a combination of 1) surfacing errors in curvature on the prescription surface, 2) manufacturing errors in curvature on the factory-molded surface, 3) errors in lens thickness due to calculation or measurement, et cetera. So ANSI power tolerances must reflect the potential propagation of errors through the entire process.

    In fact, when fabricating progressive lenses, errors in lens centration due to edging are arguably just as crucial, if not more so, than the small errors in lens power due to surfacing. A PD error of 1 mm or more per eye can significantly reduce the binocular alignment of the intemerdiate and near zones of the lenses. This is also why progressive lenses have a slightly tighter centration tolerance in the Z80.1 standard.

    It is possible to pursue lens design goals that make the lens less sensitive to fitting and fabrication errors, although this will often necessitate some other optical compromise, such as a reduction in overall clarity in order to soften zone boundaries. But lens designers frequently try to design a lens that provides the best optics possible with the assumption that the lens will be measured and fabricated to a reasonable level of accuracy.

    Quote Originally Posted by Uncle Fester
    I'll bet design engineers are happy knowing they've created the best design they can and as long as the paycheck clears they don't lose sleep over errors made in fitting them
    I agree that many lens designers assume that eyecare professionals will fit and fabricate lenses accurately. But, then again, so do the patients who ultimately pay for these lenses.


    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Thanks for your response Darryl - you've given me some things to go study! Don't be surprised if I come back with questions :)

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    Quote Originally Posted by Robert Martellaro View Post
    You said, As a point of example, if PD is determined to be at certain point, then all other calculations will be based off that, especially in a high curve. Get the PD off by 1mm, and now everything is skewed by that.

    The best lens designers, at least on paper, have a pretty good handle on how to make a lens that will perform as well as technically possible, assuming that the optician starts with an ideal frame/lens/eye relationship, and is able to supply accurate data for the software to work with. I also believe that the more optimized/compensated lenses are much less tolerant of inaccurate data, compared to spherical best form surfaces and simple PALs. A 3mm position error on a semi-finished lens might be tolerated, but the same error on a highly optimized lens will probably cause symptoms.

    You can see some of this for yourself by inputting a large p-value and dioptric power into Darryl Meister's optical analysis program. Observe how the power error and oblique astigmatism reacts to changes in base curves compared to spherical surfaces.

    Hope this helps,

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