It's common knowledge that 20/20 vision is as good as it gets. But where does this phrase originate?

20/20 vision

20/40 is worse than 20/20/

And 20/20 is not as good as it gets. Mine is 20/15 with correction.

It means that you can see at 20 feet what is at 20 feet. I can only get my vision corrected to 20/25 which means I can only see at 20 feet what others can see at 25 feet away.

The guy who devised the eye chart test back in 1862 set a rather low standard for what a “normal” eye ought to be able to resolve at various distances. In fact, the average healthy eye does considerably better than 20/20. 20/20 is actually the average for 60–70 year olds.

It’s a little more complicated, but it just has to do with testing. You know those eye tests sheets with the letters on them? 20/20 vision means that at from 20 ft away, you can read the letters of a certain size accurately. This isn’t the bottom of the chart and so you can get higher numbers, like @tedibear‘s 20/15 (and mine used to be similar with correction, but I think I’m just 20/20 at the moment). 20/20 is the average for good vision, not the best. In a similar manner, 20/400 is ‘legally blind’, but can be corrected to 20/20 or close enough so that they’re not functionally blind.

Oh, and more generally, the first number (almost always 20) is the distance you are at from the test. The second number is the range that the average population (or, apparently, the average population of 60–70 year olds) can see that from. So a 20/15 means that, at 20 feet, you can read something most can only read at 15 feet. Conversely, someone who’s legally blind has to get within 20 feet of something most people can read from 400 ft away.

20–20 is best for navigating and surviving in our world. I use to work with a woman who had 20–15 vision without her glasses and she couldn’t see well anything close to her. She had to wear glasses to make her vision 20–20 so that she could do her work.

The first number refers to a subject’s distance from the reading chart, while the second number refers to the distance from which a normal person could see a particular line of said chart. So if someone has 20/20 vision, that means that they can see from 20 feet away what a normal person could see from 20 feet away. As such, 20/20 is average vision in just the same way as 100 is the average IQ score. A change in the second number is not so much a matter of better or worse as it is a matter of how you will manage in a world that has been organized around the average person (as @gondwanalon notes). Were the average visual acuity different, we simply would have designed things like computer, road signs, and books differently.

Marilyn vos Savant once remarked that she wished we would call an IQ of 100 “perfect intelligence” just as we call 20/20 vision “perfect vision.” I’ve always thought this was a lovely idea.

The first number is the number of feet away. The second number is a representation of the letters on the line. They go all the way from 10 to 200. My vision is 20/15 with correction.

20/20 is just an average standard for good eyesight. Many people have much clearer, sharper vision.

Before I had laser eye surgery, I’d been 20/600. After surgery, it was 20/25. That procedure was one of the best things I’ve ever done.

But if you can see at 20 ft what avg people can see at 15 ft (=20/15), you could be farsighted, which is not ideal either.

Now that optometrists are wiser, they should come up with a different metric. Maybe something about the min and max distance that one can read. That would quantify nearsightedness and farsightedness.

Near- and far-sightedness refer to refractive errors, but visual acuity also depends on the health of the retinas. Some people cannot see 20/20 even with the image perfectly focused on their retinas because of retinal problems.

I discovered (when I flunked the eye test for drivers’ ed at age 16) that I was nearsighted and seeing 20/200 at distance. With correction I was able to see almost 20/15, but at my age now I happily settle for 20/20.