Visual acuity
Visual acuity is the eye's ability to detect fine details and is the quantitative measure of the eye's ability to see an in-focus image at a certain distance. The standard definition of normal visual acuity (20/20 or 6/6 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. Visual acuity is often measured by an optometrist with the help of a Snellen chart.
In countries where the foot is a unit of measurement, acuity is expressed relative to 20/20. In countries which use SI units, visual acuity is expressed relative to 6/6 (where the 6 is measured in metres). For all intents and purposes, 6/6 vision is equivalent to 20/20.
A visual acuity of 20/20 means that if you and a "normal" person with good eyesight both stand 20 feet away from an object, you would see the same thing. If you have a visual acuity of 20/40 (6/12), then if you stood 20 feet away from an object and the "normal" person stood 40 feet away, you would both see the same thing: this suggests that you have worse eyesight than normal. 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 20/15 (6/4.5). Some birds, such as hawks are believed to have an acuity of around 20/2, which is significantly better than human eyesight.
Most humans have one eye that has superior visual acuity over the other. If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. A person with a visual field narrower than 20 degrees in diameter also meets the definition of legally blind.
Contrary to popular belief, possession of 6/6 vision does not mean that a person has "perfect eyesight". It simply indicates the ability to resolve static images from a distance, but those with 6/6 vision may suffer from other visual problems, such as color blindness, an inability to focus on nearby objects, or an inability to track fast-moving objects.
