Visual acuity
From Freepedia
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, 6/6 or 0 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc.
Measurement
Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols such as Landolt Cs or E hooks.
In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal number.
Using the foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the metre, visual acuity is expressed relative to 6/6. 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). Recent developments in optometry have resulted in corrective lenses conferring upon the wearer a vision of up to 20/10. Some birds, such as hawks are believed to have an acuity of around 20/2, which is significantly better than human eyesight.
In the decimal system, the acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A value of 1 is considered to be normal, and smaller values suggest impaired vision. There are many people reaching values around 2.
Many 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.
