how many categories does Lions Club need to sort donated eyeglasses?

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?

Over the years, I have had 2 sets of glasses that were made incorrectly.

One had the astigmatism correction (I have no idea what parameter that is) 180 degrees off. The glasses were still wearable, but I saw colored halos around lights, and had some other anomalies. For 2 years. Sporacdically annoying, and thankfully, I only had one pair like that.

Another pair was slightly (according to them) over correcting my nearsightedness. Big problem, after just an afternoon I had a headache and could not stand to wear them. Went back to my old pair till they could make a new set for me.

I have no idea how the Lions do it, but apparently some parameters are much more critical than others.

For someone that has no glasses, a pair that’s “close enough” is still life-changing. Not everyone has astigmatism, and not everyone who does have it needs to correct it, so most of the glasses that the Lions collect probably have no cylinder or prism factor at all. They’d probably dread my glasses with an axis of 070 OD / 135 OS - that’s certainly in “unique snowflake” territory.

In total, the Lions receive a lot of glasses per year - they have 17 recycling centers worldwide and each typically gets anywhere from 100,000 to 1,000,000 pair a year, so we’re looking at millions of pairs coming in. They sort the glasses - metal frames get scrapped as they’re worth more in scrap metal than as glasses. At these quantities, there’s apparently a significant amount of gold and other precious metals that can be recovered. The money from this goes toward the operational expenses of the recycling centers. e.g.: rent, electricity, etc. Plastic-framed glasses get cleaned, measured, labeled and bagged.

I’d guess that the glasses that are in good enough to shape to re-use are boxed up fairly generically in ranges (-2.5 to -3.5 may be lumped together as “about -3” for example) and when a group sets out on a mission, they will be taking a few thousand pair, so there’s going to be a good variety to pick from to find what’s best for an individual.

At the end of the day, if a person needs -5 power, but all they have left is -4, those “wrong” glasses will still be a huge improvement over nothing.

The real answer is thousands. There are machines that can read the prescriptions including bifocals. The glasses are cleaned up, repaired, the prescriptions read. They are then filed by prescription. It only make sense that they need more of the ones that they get the most of. When they go on missions to fit the glasses, they take many more than they will be able to fit. Most of the glasses are fit to the exact prescription needed. As the needs get further from average, they do have to compromise only getting close. As mentioned, even that makes a tremendous difference.