Let's take a poll, and please post a rationale underpinning your answer
Barry
How a given Rx is obtained in an exam room setting
How a given Rx is verified in a typical lensmeter
The differences between flat and curved lenses
The 20' testing distance not being equal to infinity
Let's take a poll, and please post a rationale underpinning your answer
Barry
No box for none of the above?
I don't understand the question!
;):cheers::cheers::cheers::D
I choose #1. It is designed to compensate for any variation between the 13mm vertex and perpendicular alignment of the phoroptor. If you fit a patient at a vertex of 11mm with 12 degrees of panto, the viewed Rx will have changed if made exactly to the phoroptor reading.
A lack of planning on your part DOES NOT constitute an emergency on mine!
Actually I think this means compensation for vertex, wrap, pantoscopic angle, etc. None of which is an option in the little boxes.
Chip
Is one answer and one answer only the right answer?Originally Posted by Barry Santini
Agreed. :hammer:
Sorry for the confusion, but some of it was intentional on my part. Chip's answer is the closest to the traditional answer. But, I attended a seminar on digital/free-form/POW progressives on Thursday nite, and the speaker stated with authority that POW compensations were releated to lensometry verification vs. fitting, which I thought was an interesting confusion of this concept.
More interesting, as I thought about POW, was why there isn't (isn't?) a compensation for the traditional testing distance of 20 feet not being fully corrected for the wearer's distance vision.
I now ask for opinions on why this should not be so, as long as we're compensating for optimal optics and vision.
What really drives this subject is the realization that, with free-form fabrication and optimization (particularly) of progressives, you actually have two different classes of lens corrections that can result from a specific Rx. For example:
Given Rx:
R -5.75 -0.50 x 170
L -6.25 -1.00 x 20
Add +2.75
This Rx, was POW compensated with Shamir Autograph II (1.60 index, abbe 36), using the following POW values:
VD = 14mm
Panto = 6 degrees
Panoramic angle (wrap) = 5 degrees
Here is the POW verification figures:
R -5.67 -0.37 x 153
L -6.16 -0.93 x 28
Add: +2.55
FYI, the lens powers happen to have toleranced about +0.11D weak in both spheres, using my Abbe-toggled Humphrey Lab 360 lensmeter
Now, here's the rub: (wait for it...)
If I had ordered these as Shamir Element, the given Rx would have been the *only* values used to fabricate, and there would be *no* POW compensation (even if defaults were used).
So...two pairs of "digital*, free-form, back-side-optimized progressive lenses for the same patient would have *very* different values in the same frame.
And help us all if the given Rx was undercorrected for any reason.
You see my dilemma?
Thoughts?
Barry
I don't see where there is a conflict between what you call the "traditional answer" and the "new" one. When Rx's are compensated for vertex or tilt, the point is invariably to produce a lens that will produce the refracted power in the as-worn position, and the compensated power is invariably that which is measured in the lensometer.
Freeform lenses are somewhat different in that a design might be altered based on some of those parameters as well. And, freeform designers can go a step further in "compensating" the Rx for other factors - for example, lifestyle (active or sedentary), which might result in modifications of the design that produce a lens that measures differently for that reason (by moving the designed Distance Reference Point away from a DRP that's inked on the lens by a fixed-form stamp, for instance).
As for the 20-foot question, I think that the answer is that the difference between 20 feet and any longer distance is insignificantly small. However, some freeform designers are at least thinking about the need to be able to specify working (as-used) and refracted object distances and viewing angles in order to provide a higher order of customization in their designs.
Although I agree with most of your analysis, your answer hints, and I do not mean this at all disrespectfully, that you've not had to deal with front-line dispensing issues, particularly trouble shooting progressives.
In theory, it is all nice and good if the client does not return with perceived issues with vision. This goes for whether the Rx is delivered as written, compensated (for whatever reason), or even completly "wrong".
The real issue for dispensers is when a client *does* return with a vision issue. With the added variables that POW compensations brings in, and if serendipitously paired with an unfavorable ANSI fabrication tolerance (as in this example), it becomes even more of a challenge to invetigate and resolve a lens vision/adaptation issues.
Every pair of eyewear delivered presents risks
1. Will they see satisfactorily?
2. Will they fit comfortably?
3. Will they (or others) like their appearance?
These risks alone, for my money (and using the rationale of Wall Street), underpin why Rx eyewear *must* be priced they way it conventionally is.
With respect to the example above:
Since almost *all* POW minus-sphere compensations result in verification values *less* than the original spheres, and with refraction protocols both delivering Rxs intentionally undercorected (for full DV by 0.16D at 20 feet) and always trying to ensure that *no* accomodation is stimulated during testing (whether or not this actually corresponds to what the wearer's vision system focus bias normally is), I think that it's now tantamount to a mandate that manufacturer POW algorithyms include compensation for the DV "overpluss" that is so typical of refractions today. Of course, age and primary use also enter into this equation as well.
For my money, lens design and fabrication optimazation and precision now exceed the precision of the exam room by orders of magnitude. I'd love to see the latest technology finally enter the refracting lane in a more mainstream way.
More thoughts?
Barry
Last edited by Barry Santini; 12-13-2008 at 02:04 PM.
To truly understand POW we have to remove the 2 dimensional thinking of most opticians and introduce the 3 dimensional world we live in. The phoropter is traditionally set in a plane positioned 12-14mm in front of the patients eyes and perpendicular to the line of sight. The glasses we fit are traditionally set at a vertex to either match the 12-14mm vertex distance or we compensate for the difference in higher powers by adjusting the prescription so that the effect matches the same prescription had the vertex matched. POW is the additional measures of how this plane is tilted so that compensations for additional power and astigmatism that is caused by this tilt can be eliminated.
The answer I am assuming is going to be number one since the reference plane of our original prescription is going to be the positioning of the doctors phoropter.
Last edited by HarryChiling; 12-13-2008 at 11:50 AM.
Ok, so, to simplify this POW discussion:
If we accept what Mr. Shanbaum says (and what he stated is correct),
then...
The Rx of every single progressive of external design (non-back-side, which is just about every progressive lens ever made until recently)
is just plain wrong, since there was no POW compensation!
So much for previous, earnest discussions of the differences in merit for these lenses.
What does this all mean for the Sheedy Progressive comparative study?
Barry
That's a stretch Barry. Keep in mind the older molded designs required a 10-12o tilt, and about 5o dihedral. The difference was that the older designs had to be worn in a specific way for the best possible optics through them, the newer designs if fit correctly will eliminate the need for global variables and take into account the actual variables. More accuracy = better. We have already accepted that the older designs were not as accurate as even they could have been. The limited number of base curves meant that there was a thresh hold for error that we were willing to accept to allow the price to be acceptable. I believe the same applies for FF products, the wows you get from element could be because you tend to fit it the most because of price, then the tough fits you reach for the Auto II as mentioned in your previous posts. If you weren't open minded or smart you could end up drawing the conclusion that if the Auto II's have a higher percentage of remakes they must be worst than the element, ignoring the fact that they get fit on your tough to fit patients. I welcome the element and these other new to market designs that are bringing affordability to the table. I truly would like to offer FF, but it's gotta make cents (not a misspelling).
Harry:
So, are you implying that traditional progressives, when fitted using your average supplied POW parameters, are, in effect, *close* to POW compensated, and therefore need not much further POW compensation?
????
Methinks that it is mostly the back-side, astigmatism and aberration-optimized correction potential that is responsible for most of the perceived wearer benefits for FF progressives.
POW then, is, at best, icing on the cake.
At worst, it is a recipe for compounding the negative effects of the often undercorrected Rxs we routinely receive.
Your thoughts are always interesting, Harry!
It's simply a little more accuracy. Imagine a lab that says to you I can make all your Rx's exactly as you describe them to the .01 D and it'll cost you nothing. Beats the ANSI we currently use, now if that lab tells you it's gonna cost another $100.00 to do it you'd have to think about it their is a value to accuracy we will each determine what this value is ourselves and price our product accordingly.
Harry and all:
I think that the promise POW compensations make for either additional precision or accuracy is easily outweighed or offset by the imprecision, inaacuracy and non-repeatability of the common refraction paradigm that exists today.
The differences in given Rxs alone with even one office, by different doctors, due to the effects of Dr's "personal discretion" on the Rx alone make POW compensations look more like scooping snow with a spoon off of the peak of Mt Everest. (?!)
Again, and not to belaber the point, but I feel that POW compensations *must* include at least the option for correcting the Rx to full infinity, if desired.
FWIW
Barry
Of course to accomplish this a stated vertex distance from the prescriber wouldn't hurt.
Chip
And how long before the eyeglasses on a typical patient fail to stay exactly at the pre-determined vertex distance... they will invariably loosen up, slide down the nose and 'loose' some of that minus power (most are myopes).
You must not be too good at adjusting frames, Mr. Morse.
Besides we are only responsible for seeing that they are in excellent working condition when the patient recieves them. Much like a car dealership is not responsible for damage after it leaves the lot.
Chip
My 'temple-bending' skills are exemplary. :D:D
I thought 20 feet was optical infinity, as in all light rays 20 feet out will have become parallel.
As to the total question, I can see the point made about how the glasses are verified. To put it simply, a compensated Rx is what the lensometer will tell you the power is when the effective power in position of wear is exactly as prescribed.
Interesting thread!
Yes, it is an interesting thread!
Barry
Light rays are deemed parallel at 20 feet (6 meters). But the vergence power of a light ray is given by
n
L=____ and with light ray in (air) n =1.0
l(m)
L = Vergence power in diopters and l= object distance in meters.
Since 20 = 6 meters , with rays still slightly diverging at this distance
hence the - sign.
1.0
L = ______ = -0.16D
-6m
So these rays are not really parallel at 20 feet. But let's say infinity is 10,000,000,000+ meters, then
1
L = _____________
-10,000,000,000+m
this fraction is about as close to zero (parallel rays) as we would want, but it's still not truly zero.:cheers:
So then, to allow for rounding, we would have to conduct exams in a lane 656 feet long (200 meters). That will give an error of .005 diopters. Rounded up, that will be an error of .01 diopters which I think we can all live with. All freeform lenses can now be made to that tolerence. I want to be the one that sells the moving sidewalks to the ODs to take patients to the chair.
On a side note, how big would the '20/20' line be at 200 meters? I'm thinking eye exams would need to be done at drive in theaters to achieve this type of accuracy.
A lack of planning on your part DOES NOT constitute an emergency on mine!
This perspective is consistent with what Darryl Meister had stated regarding the postion of the ANSI committee:
Namely, it is assumed that 0.25D is the smallest unit of optical vergence that represents a "just noticeable" difference to the human eye. It is logical then, that a dioptric value almost 50% less would be *unnoticeable* to the human eye, and therefore light rays eminating from 20 feet are *deemed* parallel to the human eye, for all intents and purposes.
However, factors such as spherical aberration of the human eye (which greatly contributes to the phenomenon known as night myopia), contrast sensitivity and brain's evolved cortical desire for superior contrast all conspire to make the assumptions that a value of 0.25D as a just notiecable difference for human distance vision, or that 0.16D vergence is equivalent to optical inifinity, IMHO, somewhat flawed.
FWIW
Barry
PS - See the Tag Heuer "night glass" for more information here.
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