Visual function is described in terms of visual acuity and visual field. Visual acuity is a measurement of the ability to distinguish details and shapes. One way to measure visual acuity is with a standardized chart of symbols and letters known as the Snellen chart, invented in 1862 by Dutch ophthalmologist Herman Snellen. He derived a simple formula that determines the relation between the distance at which a letter is read by the patient to the distance at which that same letter is read by a normal eye.
Normal vision is designated as 20/20. Visual acuity that is less than normal is designated with a larger second number, such as 20/200. An individual with a visual acuity of 20/200 must stand at 6 m (20 ft) to see objects that a person with normal sight can see at 60 m (200 ft).
Visual field indicates the ability of each eye to perceive objects to the side of the central area of vision. A normal visual field is said to be 180 degrees in diameter, or half a circle. An individual with a visual field of 20 degrees or less who stands at a distance from a large clock and looks at the number 12 is unable to see the numbers 11 and 1 to either side of it.
In the United States, legal blindness is defined as a visual acuity of 20/200 or worse in the better eye with the best optical correction, such as eyeglasses or contact lenses. In the legally blind, the visual field is not better than 20 degrees in the better eye.