There are many ways to measure visual acuity. One measure of visual acuity that is commonly used is Snellen visual acuity,1
named after Dutch ophthalmologist, Herman Snellen and was developed in 1862. Snellen visual acuity is measured as a fraction and is represented as the distance at which the test is performed over the distance at which the smallest optotype identified subtends an angle of 5 minutes of arc and the detail of that optotype subtends 1 minute of arc.
An optotype is a standardized letter, shape, picture, or number used to show the subject whose visual acuity is being measured. Using the metric system, the formula is the viewing distance (measured in meters) over the letter size (measured in M-units). So, 6/6 is the same as 20/20.
20/20 visual acuity means that a person can see small detail from 20 feet away the same as a person with normal eyesight would see from 20 feet. If a person has a visual acuity of 20/40, he is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away.
Average human visual acuity is said to be 20/20. However, many people can see 20/15 and a few that can even obtain 20/10 visual acuity. It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10.