Visual ACUITY refers to a standard that represents the clearness or sharpness in vision or sight. Visual acuity may be refered to as the "resolution" in vision.
The most common standard for measuring visual acuity is "SNELLIN" Acuity. The Snellin standard is usually used to define "distance" visual acuity. Under the Snellin standard the object is placed 20 feet from the observer. The measurement is always in reference to this 20 foot distance. As an example, consider a person who is said to have 20/40 visual acuity. 20/40 means that the person being tested had to move up to 20 feet from the object in order to see a letter which was actually placed 40 feet from the observer.
The test objects that are traditionally used to subjectively test visual acuity are: Capitol letters, Rotating "C"s, Tumbling "E"s (for children).
Another standard for measuring visual acuity is JAEGER acuity. This is defined in terms of a metric reference. The jaeger standard is most often used in measuring "near point" vision, rather than distance vision.
TECHNICAL DEFINITION: The reciprocal of the minimum angular separation of two lines just resolvable. It is measured in minutes.