My understanding is that if you have 20/20 vision, you can see at 20’ what you should be able to see at 20’. If your vision is 60/20, you can only see at 20’ what you should be able to see at 60’. I’m sure somebody else can explain this better. My question is, why is 20/20 vision considered normal? How is it decided what I “should” be able to see at 20’? Is this the average visual ability of the population, and did they just measure a lot of people to come up with it?
Yes, it’s the (ideal?) uncorrected unimpaired visual ability of the population, and yes, they measured a whole bunch of people to come up with it.
You can’t pilot a fighter aircraft for the U.S. Armed forces unless you have it, among other things.
Oh, btw… in your example 60/20, it’s only the left eye that’s impaired. Each number represents the corresponding eye, with an understood underlying 20 as the denominator.
Actually you can be a pilot and not have 20/20 vision. If you are correctable to 20/20, you can get a wavier for glasses. As people get older their vision can start to fail and the military is not going to waste all that training, i.e., money.
Nickrz, do you have a citation for that? I was pretty sure that visual acuity is measured and recorded separately for each eye. I don’t ever recall hearing of someone legalling blind in both eyes getting a score of 240/240.
20/20 means that you can see clearly at 20 feet, what the average population can see clearly at 20 feet. 20/40 means that you have to stand at 20 feet to clearly see what the average population can see clearly standing at 40 feet.
240/240 vision would be a different way of stating ‘normal’ since it would imply that you can see clearly at 240 feet, what the average population can see clearly at 240 feet.
If someone has 20/60 (left eye) and 20/100 (right eye) vision, they might express it as 60/100 (with the 20’s implied), but that would be technically incorrect and shouldn’t show up on a patients documents as such.
BTW, in countries where metric is standard 6/6 is used as the equivalent to 20/20.
Gilligan’s example shoud’ve been printed as ‘20/60’, rather than ‘60/20’, and Nickrz comments are off-the-wall; substitute Stephen’s post; it is correct.
Well, yeah, 20/20 is considered standard good eyesight (uncorrected or corrected with a lens), but, of course, many people have eyes that resolve 20/15 or even 20/10. My right eye, lens-corrected, is good for 20/15, but my left eye is almost entirely shot, including central vision, from glaucoma.
20 ft or 6 m is chosen because it is close enough to infinite distance, in terms of focal adjustment of the ocular lens and convergence of the two eyes, that effects of these functions may be ignored.
Of course, it’s stupid that optometrists bother expressing the numerator 20 or 6, rather than just stating a decimal fraction, but of course, “20/20” has become their an advertising banner for them in the US, and the extension of its use to investigative-news TV programs, etc. tends to keep them “institutionalized”, rather than replaced by home-computer programs or coin-op machines.
so how would you write a blind person’s vision? would it be ~/20 or 0/20? (for each eye)
[~ substituted for the infinity symbol because i’m lazy]
Sorry I got the numbers backwards; 20/60 it is, then. Anyway, as an average, is this the mean, median, or mode? Mean or median seems to imply that there’s a lot of people with much better eyesight than 20/20, to balance what seems to be the large number of people who have worse. I am guessing then that when they measured this, a majority had 20/20, a few had better vision, and large minority had worse.
IIRC, I heard many years ago that the originator of the measurement just took a dozen or two of his grad students as the ‘standard’. Same thing for 37C as ‘normal’ body temperature. (And the ‘.6’ on 98.6 is just from converting the approximate 37C far too precisely into Farenheit.)
Bob the Random Expert
“If we don’t have the answer, we’ll make one up.”
Umm, no, that’s not how it works. The 20/X convention is only used when giving someone a vague, general assessment of their eyesight. It describes the resolving power of both eyes working together.
For instance, my ophthamologist says that my uncorrected eyesight is about 20/400. This does not mean that I have one perfect eye and one bad eye; if that were the case, I’d only have to wear one contact lens. But I really do need two, and they’re very nearly the same strength.
Of course I don’t fit in; I’m part of a better puzzle.
I thought Nick was a little off base there, but I couldn’t find any documentation, and my “batting average” on this MB has been too good lately, so I kept my mouth shut.
Anyway, getting back to the OP, I believe that 20/20 is based on the the average healthy eye. The glasses I used to wear were actually a stronger perscription than I needed. (My eyes are something like 20/500, but the glasses corrected to better than 20/20.) I recently switched to contact lenses after switching to a new doctor. At first I thought the prescription he gave me wasn’t strong enough, but after following it up (and getting used to the contacts) I found that my new prescription was more appropriate for my eyes.
For various purposes, people of different levels of ‘low vision’ get labled as being ‘blind’ in different degrees/senses.
I believe someone who is "totally blind’ or ‘blind’ for all practical purposes is not given any numerical label, but if you want authority on such things, ask your question in the newsgroup
, which is frequented by optometrists.
Let’s see, if one isn’t lazy, do any of these work?:
I strongly suspect that rjk is at least roughly right about the true origin of the level of eyesight corresponding to 20/20. Again, you could check in the above newsgroup, but don’t ever believe all an optometrist will tell you.
Well, you can read a value during the use of both eyes, but only DMVs have a use for that. Optometrists will generally take such readings for each eye alone, since that’s what’s needed to assess how your vision can be improved. But I believe many optometrists don’t bother reading these values on your unaided eyes; they just get such readings for your ‘best-corrected’ vision, i.e., with their lens prescriptions doing their work on your vision. Otherwise, they may also be taken for whatever prescriptions people, for one reason or another prefer, although they are not the best corrections for acuity (20/x). Yes, of course, the worse-than-20/20 reading of uncorrected binocular vision can be divided in fault between the two eyes in any proportion, depending on the particular case.
The first digit is always 20; the second digit refers to the number of feet away you should have been able to read the line on the chart that you did read at 20 feet. Excellent eyesight could be 20/15; mine is around 20/750 in the right eye, uncorrected (what “e”? what chart? what wall?), left eye significantly better at 20/475. When a person is said to have 20/20 visiion, it is generally assumed to mean “at least that good in both eyes”.
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I feel terrible,my vision is 20/650!No, I’m not legally blind.My sister-in-law has 20/10 vision,meaning she can see at 20 ft. what most can see at 10!
Well yeah, I did sort of goof there, huh?
I guess what I meant was each eye uses the same nomenclature - 20/whatever. You can be 20/60 in your left eye and 20/20 in your right eye. Which is, incidentally, the reason why good binoculars have two methods of focus - one diopter adjustment for the right eye, and one for both eyes together. People rarely have the same capability in both eyes.
When I wanted to join the Air Force in 1975, I qualified for any job they had - except pilot. That was because I did not have 20/20 vision. It was true back then, have they changed the rules? I guess I goofed on that one as well. I should have said “become” a pilot.
Of course, Nick, you could always get yourself a lawn chair …
Bob the Random Expert
“If we don’t have the answer, we’ll make one up.”
The military still requires 20/20 uncorrected for pilots, at least at the start of training.
For pilots in general, the eyes must be ±3 diopters uncorrected(roughly 20/100) in order to pass a commercial medical (meaning airline pilot, or any other pilot who flies for hire). And they must be correctable to 20/20 with glasses or contacts.
For private pilots (at least in Canada) your vision must also be ± 3 diopters, but if your vision is worse you can get ‘flexibility’ applied to give you your license. Generally, anyone can get a license as long as they can correct to 20/30 or better, no matter how bad the uncorrected vision is. You can even get a pilot’s license if you have monocular vision (i.e. one eye missing or blind).
If you are color blind you can’t fly at night, because many of the signals on runways are color coded.