I made myself an eye appointment the other day, so I was thinking about 20/20 vision (which mine is decidedly not.)
20/20 means that someone who has ideal vision can see something clearly at 20 feet (in this case, the eye chart). The first 20 is the ideal, and the second 20 is that person’s actual performance. Twenty feet is apparently just the right distance to measure vision at because if you’re trying to focus on something at 20 feet, your eyes are focusing in the same manner as if they were trying to focus on something at any further distance. In other words, if they can’t do it at 20 feet, they’re not going to do it at all. Interesting side note: there is a metric equivalent, and it is 6/6 vision (“hindsight being six six” just doesn’t have the same ring to it, though).
You can visualize (see what I did there?) the way the measurement works by imagining that a person with perfect vision can see something clearly at 20 feet, and then the bottom number is how many feet they would stand away from the same thing they’re looking at to approximate your vision. So, if you have 20/15 vision, they can see it perfectly at 20 feet, but to see what you can see from 20 feet away, they have to move closer, to 15 feet away because you have sharper focus. That’s what it means to have better than 20/20 vision – your lenses can let you distinguish better from further away than is typical. In practical terms: you can see the smaller lines on the optical chart clearly (since we already found out that you don’t actually need to move yourself to test this, since the way your eye focuses at 20 feet (or 6 meters) is basically how it will focus for any further distance).
If you’re like me, on the other end of the spectrum, what I stand at 20 feet to see clearly, a person with perfect vision can see clearly from 425 feet. This is how we know that the model is not one that is regularly tested in the field with men in business suits and white lab coats making people stand at various distances and shout out what they spy. Because obviously I can see nothing clearly from 20 feet and my shouted answers would be along the lines of, “something blurry! Another blurry thing!” and so on. Another way to think of it is that I have to stand 425 feet closer to distinguish what someone with ideal vision sees clearly from 20 feet (which is why, if you are like me, your prescription is a negative number). I have bad eyesight. But what a terrific model, that it has that kind of inherent flexibility that is absurd on the surface of it, yet so effective at measuring.
My husband’s vision ranges from something like 20/15 to “I can’t find my keys, no I looked there already.” But I understand this to be fairly typical with significant others in any scenario.