When someone looks through another person’s pair of glasses, it naturally appears very blurry. Of course, we go through a long time at the opthalmologist’s office going through tests to make sure we have the right prescription for our glasses.
Is it possible for any two people to share the same glasses prescription?
Yes, of course. It’s so possible for farsighted people that they come in common prescriptions you can buy right off the rack.
Ignoring astigmatism, an eye prescription consists of three numbers, the interpupilary distance (the distance between the centers of your eyes in millimeters), and the “spherical” strength of the correction for each eye, in “diopters”. Diopters generally range between around -6.0 (very nearsighted) to around 6.0 (very farsighted). So there are, say, 120 possible values for each eye, thus 14,400 total. The IPD for adults is around 55-75 millimeters, so multiply by another 20 possible values, thus 288,000 possible prescriptions (not all equally likely). A large number but obviously finite and among the world’s billions of people, thousands will have any particular prescription.
Adding astigmatism complicates things: there are another two numbers in a prescription for astigmatic eyes, the “cylinder” in diopters, which measures the strength of astigmatic correction, and the “axis”, a value in degrees which measures the angle of the astigmatic correction. These will increase the possible prescriptions by a factor of a few thousand, but again the total possible is again finite and smaller than the world’s population, so by the pigeonhole principle there must be even astigmatic people with the same prescription.
ETA: It appears that diopters are not measured to 1 decimal place as I indicated above, but to 2 places with the second always 0 or 5. So that doubles the total prescription for each eye.
It’s possible for two one eyed people to share a pair of glasses if they are very narrow minded.
Years ago, I was at a taco place with a few friends. The tacos aren’t important for this discussion, but they were delicious, so I mention them anyway. One friend commented that she thought I’d look good with black rimmed glasses, like our other friend was wearing. I normally wear rimless glasses.
So the other friend took his glasses off and handed them to me. I could see perfectly through them!
I don’t know if we had the exact same prescription, but it certainly was close.
Also of note, contact lenses are not (typically) made on demand - they are made in known prescriptions, and stocked in advance. This wouldn’t be practical if all, or even most, prescriptions were unique.
I recently commented on how cool a friend’s new glasses looked. She took them off and handed them to me. I tried them on and they were just plain glass! The frames were expensive designer frames and she was wearing them purely as an accessory.
I used to be mildly nearsighted, so that things didn’t objectively look blurry to me, but I had trouble reading road signs and such, so I got glasses for driving only. At first the difference was remarkable- even though I didn’t perceive things as blurry normally, I’d put the glasses on and everything was so sharp and clear it looked unreal.
I was visiting one of my sisters, who said she suspected she might be nearsighted, but didn’t have a prescription. So I gave her my glasses to try, and she was amazed at how sharp and clear everything was, just like I was the first time I tried them on. She kept looking around going “whoaaaaa…” somewhat comically.
So we seemed to have the same prescription, or nearly so, but then we’re related so maybe there’s a genetic component.
As the anectdotes above indicate, glasses don’t have to be exact to be helpful. Or even make one think they’re perfect, if someone who has not worn glasses before, tries on some that are fairly close to what they need.
Anyway, there’s other factors besides those @markn_1 posted above that could be included in glasses prescription. There are bifocals, where lenses have different diopters on the bottom and top of the lenses. These are very common among older people’s glasses, since virtually everyone gets presbyopia when they get into middle age. And then, somewhat rare, there could be a prism adjustment in one of the lenses for people who have double vision. Possibly even two prism adjustments, one for up-down and one for left-right.
My wife and I’s prescriptions are close enough we can use one another’s glasses in a pinch. When I wear hers one side is a little blurry and the other a bit too sharp, but it’s close enough in an emergency.
I’m sorry, but how can corrected vision be too sharp?
Okay maybe “sharp” wasn’t the correct word, but you know, it’s like looking through binoculars the wrong way. Everything gets weird and distorted.
There are charities that collect, refurbish and classify second hand glasses to match against prescriptions of people in developing countries - so it’s definitely possible for people to use a pair of glasses that were originally made for someone else.
Apparently it’s horribly inefficient though because of all the handling in the process - as I understand it, it would work out better to donate money for new glasses manufactured specifically, cheaply and more locally for the individual.
Now you’ve gotta tell us the name of the taco place.
Heh. I think it was Rocco’s Tacos. This was before the Great Taco Revolution™, and I’ve found much better places since then.
I assume too that eye problems fall on a curve, people are more likely to have lesser than extreme vision problems. So th majority of glasses wearers probably cluster around normal, and the odds of finding a close to matching prescription are better the less distorted your vision is. Not a lot of people need coke bottle-bottom lenses. I didn’t need to wear prescription glasses to drive until I was 30. (First glasses, I had gotten used to judging distance in lower light by blurriness. I put my new glasses on in the mall when I first got them, and it was sensory overload - everything appeared five feet away.)
Glasses are extremely valuable items in poorer countries. Several organizations will collect and redistribute peoples’ old pairs.
Some local eye clinics will also do so.
So not only can two people share the same pair of glasses, it’s a positive good. Please clean out your drawers if you have old pairs and get them where they are needed.
As Mangetout noted above, it’s much better to donate money than glasses. According to a paper in Optometry and Vision Science, recycling used glasses costs twice as much as just manufacturing new ones, when you consider the cost of collecting, shipping at least twice, determining the prescription, sorting, and discarding damaged ones (over 90% of donated glasses are unusable). It may make you feel good to donate used glasses, but it’s a waste of money at every step along the way to getting them to an end user.
I can only find one study that says this, and it was of a test sample.
That study may be valid in the larger scale, but I never accept one small test study as the final answer for any issue. If you can find better proof I’ll certainly evaluate it.
Just FYI, my wife and I already donate large sums of money to various relief groups so doing more is a perfectly acceptable alternative.
I see it as a trade-off. Encouraging local manufacturing has to be a plus, but too much outside money coming in will distort the market too if not carefully managed. My one day’s earnings could exceed a third world inhabitant’s monthly wage.