Yes
No
It doesn't matter
ANSI is already too loose on tolerance
With iterated, digital lenses promising a *precision* of surface manufacture that can deliver *calculated/compensated* (POV, etc.) powers to an accuracy of the 0.01 diopter, is it reasonable to expect these lenses to obey a parameter tolerance that is 1/2 to 1/3 of what current ANSI standars state?
And, if not, are we getting "our money's worth*?
Chime in, anyone (Darryl, Harry, drk, etc..)
Barry
Not all digital lens designs are backside only. ISO standards gives lens blank manufactures 0.09 diopter tolerance and that is plus or minus. Lens still have to be molded to some extent and there is variation in molding.
Soft lap polishing can cause variance that varies as the radius of the curve(s) varies and as to the flexural amount of the unsupported portions of the lens.
I think what is interesting is that most of us can only check the toric surface of a lens and most progressives have 25% (generous) of the lens outside of tolerance for what should be interpreted as a wave.
Labs using 1/10 tooling @ 1.53 index were already tooling to 0.05 diopters @ index 1.53(worst case due to rounding) so really we have only changed 0.04 diopters in quality as measured.
Yes it is reasonable to expect digital lenses to exceed ANSI tolerances!
My problem is = What reference do you use when evaluating if those tolerances are accurate for?
Do you use the original rx? Do you base it on the compensated or optimized rx? If the compensated rx, how do you determine that the compensation numbers are correct? How can you accurately measure and verify these lenses if you are using a standard lensometer. Does your lensometer factor abbe value?
Comments, bashes?
Both Comments & Bashes, Fezz!
To my point, the "adjusted" values are always offered in 0.01D increments. If standard ANSI tolerances are applied (for instance, 0.14D power error on *progressives*)...well then, so much for adjusted values, eh?
barry
Last edited by Barry Santini; 10-02-2008 at 06:29 PM.
;):cheers::cheers::cheers::D
Chris:Originally Posted by Chris Bowers
You reply is great! But I'm less interested in the *mechanics* than in the marketing promise. Your explanation from the lab side is illuminating.
Barry
You use the compensated Rx, just like you would use for vertex distance compensation. You base it off of what was made.
Unless you want to do the calculations yourself you have to trust your labs calculations.
I think that Chris makes a great point. ISO standards are much looser for progressives than conventional lenses, which is why ANSI has a different power standard for progressives.
Yes, the marketing promises made by manufacturers ought to be reflected in a much higher resultant tolerance. If not, I'm not sure I'm buying any of it nor asking my patients to do so.
I'm with Chris, don't really think any lens polished with a less than rigid polishing tool can ever be trusted to be truly accurate. I also don't trust any method that claims to be cut so fast or with so fine a cut that no or mimimal polishing is required. I believe this to be true whether the lens is ophthalmic, contact lens, telescope, or a shaving mirror.
Chip
Hey, I've already seen the following from a specialty lab:
Rx:
R +0.25 sph
L. +0.50 sph
Add: +1.75
47 year old women/nurse, using OU OTCs up till now. Obviously an latent/early hyperope. I "D & D'd" (discussed and demonstrated -re: Trial framed) the DV Rx. She liked it!
Autograph II PAL iterated lenses ordered, trivex Trans + AR into a Silhouette Enviso rimless (I like these mounted *tight*, that's why trivex was used).
*Compensated* Rx stated from lab was:
R +0.31 -0.01 x 90
L +0.51 -0.02 x 90
Add +1.78
Rec'd Rx (measured on both a manual lensometer and my humphrey, but the readings are from the humphrey):
R + 0.52 -0.12 x 86
L + 0.77 -0.12 x 88
(Add - OK, not relevant).
So... I'm spooked! The last thing I want to do is give this type of client even *more* plus for their first, full time progressive. So I arrange to return the lenses for verification of my findings, and I hear back that, yes indeed, they agree they are "strong". They remake them. The new lenses rec'd are:
R + 0.37 -0.09 x 86
L + 0.62 -0.06 x 94
Please note: The reading above were determined by finding the *best* possible spot on the lens/humphrey to make them close to tolerance. My manual instrument, although not capable of this precision, corroborated the extra plus/small cyl.
Lab explanation: The marking-engraving process can cause "ripples/volcanoes" which sometimes necessitate the lenses being "re-polished", and this is where the power error can originate from. Further, the engraver must take into account index, coating, local barometric pressure and relative humidity. (Sounds similar to the learning curve with LASIK, yes?). Interestingly, the engravings were *very* faint, which the lab explains is how they reduce the problem. On another pair of wrap-progressive lenses, I found the engravings so faint they reminded me of the worst-cases I'd seen for Percepta (those who have experience here will agree with me).
Q: What if the original Rx proves with her to react as over plussed (let alone dispensing the lenses rec'd)? Remember, this *is* a first progressive, and she's already stated she's heard the "stories of difficulty" (great!)
And don't forget: The POV compensation for Autograph (and others) assumes certain defaults for the fitting parameters (not supplied here)
BTW, did I mention that this women demonstrated a real habit of "tilting her head back" when I was taking the pupil height (and you also know about that!)
OK, lab people, see this from our side, as we try to see it from yours.
Bottom line: IMHO, NONE of this all matters, if I could be confident that that mild plus for distance was "right on the money", i.e., not over plussed.
But those of you who have read my stuff elsewhere already know my opinions about this. And I am not impuning the competence of the refractionists, only the outmoded paradigm that is the traditional refraction process today.
And, if in the end, we all agree (lab, dispenser, Dr.) that there is no *real* way to be sure that *any* starting point is guaranteed to be satisfactory with any *one* client, then think about the following:
Both the dispenser, lab and Dr. are "on the hook" to the spend extra time and expense (double-compound) fixing the problems that appear, and try to satisfy the patient (client?).
So...
Why is it that, with the insurances we (the eyecare industry) deal with (esp. "XYZ" company), the frame vendors and the lens *suppliers* lose NOTHING in profit for insurance-reimbursed transactions, but the ones who have the greatest liability for unexpected and unforseen costs (i.e., the Lab, dispenser, & Dr.) are the ones the insurance companies expect to take the hit in profits... 'cause if don't, we won't have the "privilege" of seeing those clients if we don't take their insurance.
(My arm hurts from being held behind my back!)
TOTAL NONSENSE!!!!
Barry
(long rant, but worth it!)
Last edited by Barry Santini; 10-04-2008 at 07:07 AM.
Barry, I would liken this to our earlier discussion regarding near PDs. Writing prescriptions in 0.01 D steps gets you closer to the right value for the patient. I often hear the argument that, if the tolerance is 0.13 D, it shouldn't matter whether the prescription is written in 0.01 D increments. I would argue just the opposite, however: Because the tolerance is 0.13 D, it is all the more important to start with precise powers in order to minimize the propagation of error. This will ensure that your final product is that much closer to the exact prescription powers required by the patient. Otherwise, you have a potential rounding error of +/-0.125 D during the refraction combined with a potential fabrication error of +/-0.13 D; if these errors are additive, they can easily exceed the patient's just-noticeable-difference threshold.
That said, we are not really talking about more precise prescriptions in this particular case. Compensated prescriptions are typically based upon a standard spectacle refraction written in 0.25 D increments. During the compensation calculations, new prescription values are determined by a computer out to a lot of decimal places. These new values are simply rounded to the nearest 0.01 D, since prescriptions are traditionally written out to two decimal places. Unless these values are deliberately rounded to the nearest eighth or quarter diopter, which in effect would be changing the compensated prescription, there is really no reason to expect these compensated prescriptions to be written in anything but 0.01 D increments.
Darryl J. Meister, ABOM
Darryl,
I agree with your statements. But, the cost and promise of an *optimized* , frame iterated, or POV compensated Rx (or all) rests, to a substantial degree, on the delivery of greater *precision*, which is supposed to result in a more optimized/accurate focus of the refraction.
Standard ANSI tolerances, applied on progressives, seem to suggest that, at least in low powers if nowhere else, the promised benefits could be absent, or at least, reduced.
Please help me if I'm my logic is still off base.
Barry
Last edited by Barry Santini; 10-06-2008 at 07:01 AM.
I don't disagree that some eye care professionals may expect tighter tolerances on free-form progressive lenses.
Keep in mind that a compensated prescription is just a small tweak to the original refraction to account for the power changes introduced by lens tilt through one particular point on the lens (within the distance checking circle). A free-form progressive lens that has truly been optimized for the position of wear applies this type of correction at multiple points across the entire lens surface using optical ray tracing. The interaction of the progressive optics with the optical errors introduced in the position of wear -- for a given prescription -- can become quite complex, blurring and distorting the shape of the central viewing zones of the lens.
In reality, the benefits derived from the "compensated prescription" are small compared to the benefits derived from optimizing the entire lens design for the position of wear. In fact, if you are using a free-form progressive lens that has simply had the prescription tweaked for lens tilt, without correcting multiple points across the lens using optical ray tracing, additional optical errors will actually be introduced over much of the lens. The correct prescription will only be experienced over a very small area of the lens. I will try to post some examples, once I return from VisionExpo.
Darryl J. Meister, ABOM
Thanks, Darryl. Honestly, I either didn't realize these facts, or I forgot them.
Barry
Too funny Harry. I'm right there with you.
Properly medicated for your protection.
THANK YOU first off for the discussion of this topic. this is the biggest issue I have with 'adjusted rxes' and the like it seems like the more I think about this stuff the less sense it makes. it seems to me the industry currently has a non committal, or even disjointed approach to the issue at large. Specific to the topic of tolerance of digital lenses (and this thread) I would have to think that a more narrow and precise set of standards must be required of adjusted rx lenses, and really all lenses in general (because of the concepts proposed by adjust rx lenses)
if we're really saying that adjusting an rx for tilt and wrap and all that is necessary in the first place, than why aren't we ready to admit that conventional lenses are incorrect by the same amount the corrected ones are better by? for example, if im willing to add x amount of prism, and y amount of cyl in a 'adjusted rx' and consider the vision better for it, why don't I consider the vision worse BY THE SAME AMOUNT in a non adjusted rx lens?
Also, if we are unwilling to adjust our tolerances by the same small amounts we now consider relevant to change the prescription by, then really what is the practical difference in the lenses? for instance if I'm acknowledging power differences and decentration values in tenths of diopters and millimeters, but my tolerance is still in eigths and whole millimeters, than how is my end product any different?
Its hilarious to me that on a standard lensometer that (I think) were all using, I cant even verify the powers or difference of powers so minutely detailed in these adjusted rxes. whats the point of grinding them, (and claiming they make vision better) if the standard equipment we use to check power doesn't even recognize the distinction?
Adjusted rx lenses seem to be an all or nothing thing.... it either has to be admitted that the changes were making are important enough that we need a whole new method of constructing and manufacturing glasses (with stricter standards and methods of measuring them) or we need to recognize that these new technologies make little or no difference at all...
I may be making this too abstract here, but doesn't this seem to representative of the dichotomy we've always had in our industry? medical device/ retail item, scientific procedure vs bottom dollar.... etc. If I try to make it less abstract, it seems the same: for every pt I have that can tell their left lens is .11 D strong, I have a Pt who can wear his wife's glasses for a month and not notice. who hasn't seen it all as a dispenser? Are adjusted rx lenses just an example of us grasping too high, being too precise, when the old adage "you see with your brain, not with your eyes' still fits?
zack
Fezz, we cannot analyze these lenses to the accuracy that they are made. But, we can measure the joy that these lenses bring to our patients.I can't tell you how many , hundreds, of patients we have put in Toledo Opticals Visionary Max Progressive and they went WOW !!!
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