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Visual acuity

Visual acuity is the eyes 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. The terms 20/20 and 6/6 are derived from standardized sized objects that can be seen by a "person of normal vision" at the specified distance. For example, if one can see at a distance of 20 ft an object that normally can be seen at 20 ft, then one has 20/20 vision. If one can see at 20 ft what a normal person can see at 40 ft, then one has 20/40 vision. Put another way, suppose you have trouble seeing objects at a distance and you can only see out to 20 ft what a person with normal vision can see out to 200 feet, then you have 20/200 vision. The 6/6 terminology is used in countries using the metric system, and that represents the distance in metres.

This is often measured with a Snellen chart or LogMAR chart.

Another test of Visual acuity or function (EVTS/OptimEyes) uses targets which change in size, contrast, and viewing time was developed by Daniel M. Laby. This test of visual function has been shown to correlate to professional sports performance.