What is meant by the term, "20/20 vision"?

What is meant by the term, "20/20 vision"?

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20/20 vision is used colloquially to mean your vision's perfect. What it means technically, though, is you can see a particular letter of a certain size 2 feet away. That size was defined historically by what people thought normal vision was. In practice, 20/20 vision means your vision's really good. But it turns out that many people, especially after laser surgery or with a very well-fitted pair of glasses or contacts, see better than 20/20. What we call 20/15, or super vision. If your vision's worse than 2/2 -- say it's 2/4 -- what that means is that you can see at 2 feet what someone with normal vision could see at 4 feet. So you have to get closer. Or if you're 2/1, it means that you can see at 2 feet what somebody else could see 1 feet away. And so their vision is in a way five times better than yours. It's just a way to think about it intuitively.