What does 20/20 vision mean?

The top number 20 refers to the testing distance of the eye chart. Optical infinity is a term that describes the minimum distance that an needs to be from your eye object so that it can be seen without any accommodation effort, or focusing of your eye muscles.

Twenty feet (6 metres) is the minimum distance between an object and the eye to be effectively at optical infinity; that's why this has become a standard testing distance for eye charts. The bottom number represents the size of the letter or target that you view.

Years of research have yielded an average-size letter that you should be able to see at normal vision. (Technically, a given testing distance if your eyes have a 2020 letter subtends 5 minutes of arc at 20 feet.)

 This letter size was given the designation 20. When you mathematically divide 20 (the size of the letter) by 20 (feet) you get a result of I, so that 20/20 vision becomes an index of normalcy.

If, for example, you have 20/40 vision, then 20 divided by 40 equals ½, and it could be said that your vision is half as good as it should be.

Another way to interpret 20/40 vision is to say that you need to double the size of a 20/20 letter, or double the visual angle that a 20/20 letter makes, in order for you to be able to read that letter at twenty feet.

Very simply, a person with 20/40 vision can see an object clearly from twenty feet that a person with "normal" eyes can see clearly all the way from forty feet. Eye charts were designed by analysing all of the letters in the alphabet and categorizing each letter according to how difficult it is to read at the testing distance.

The letter L, for example, is not confused with many other letters and is therefore easy to spot at a distance. The letter E, however, can look like a B, F, P, R, S, or Z at a distance, and it's usually the first letter that is missed on a line. Each line on an eye contains letters from chart easy and hard categories, so that if you misread a letter on a line, the specialist performing the test can determine the significance of your error.  Missing a hard letter like E is not as significant as missing an easy letter.