my eyeglasses prescription has a total of 7 two or three digit numbers - for each eye there is “power”, “cyl” and “axis” (and there are also a bunch of other fields left blank) and there is also a separate “pupilar distance”. That, to my unwashed masses sort of eyes, suggests that the eyeglasses are almost like a unique snowflake, precisely matching so many parameters.

The problem is that such a naive view does not reconcile with the fact that Lions Club eyeglasses recycling program is capable of collecting millions of eyeglasses and finding recipients with matching prescriptions for majority of them. That suggests that the numbers on the prescription are not all that set in stone, and quite possibly they could be aggregated into broad categories of some sort so that within a given category all prescriptions would be thought of as equivalent, allowing glasses to be passed around between people.

Well, so what’s the straight dope here? Am I correct with the small number of categories out of huge number of possible parameter combinations hypothesis? Just how many such categories do Lions Club or similar outfits need and use? How do such categories work, e.g. is it a matter of integer-dividing each three digit parameter by 7 or another such coefficient in order to reduce the number of combinations? Or do they use some sophisticated multi-parameter formula to generate each new reduced-dimensional parameter?

Is the original hypothesis of uniqueness of glasses produced according to a prescription in itself valid, or are they actually also produced according to some granular categories computed from the parameters I see on prescription?