How Stuff Works says that a person with 20/20 vision can see what a normal person can see from 20 feet away. A person with 20/13 vision is above average because he can see what a normal person can see at 13 feet from 20 feet away.
How Stuff Works says that 20/20 eyesight is the average for people. But a person’s eyesight gets worse as the second number in this fraction goes up. For example, a person is considered legally blind if his or her eyesight is measured as 20/200. But the second number shows how well a person can see. The lower it is, the better. A person who could see 20/10 would have very good eyesight.