<P> Visual acuity is typically measured while fixating, i.e. as a measure of central (or foveal) vision, for the reason that it is highest there . However, acuity in peripheral vision can be of equal (or sometimes higher) importance in everyday life . Acuity declines towards the periphery in an inverse - linear fashion (i.e. the decline follows a hyperbola). </P> <P> Visual acuity is a measure of the spatial resolution of the visual processing system . VA, as it is sometimes referred to by optical professionals, is tested by requiring the person whose vision is being tested to identify so - called optotypes--stylized letters, Landolt rings, pediatric symbols, symbols for the illiterate, standardized Cyrillic letters in the Golovin--Sivtsev table, or other patterns--on a printed chart (or some other means) from a set viewing distance . Optotypes are represented as black symbols against a white background (i.e. at maximum contrast). The distance between the person's eyes and the testing chart is set so as to approximate "optical infinity" in the way the lens attempts to focus (far acuity), or at a defined reading distance (near acuity). </P> <P> A reference value above which visual acuity is considered normal is called 6 / 6 vision, the USC equivalent of which is 20 / 20 vision: At 6 meters or 20 feet, a human eye with that performance is able to separate contours that are approximately 1.75 mm apart . Vision of 6 / 12 corresponds to lower, vision of 6 / 3 to better performance . Normal individuals have an acuity of 6 / 4 or better (depending on age and other factors). </P> <P> In the expression 6 / x vision, the numerator (6) is the distance in meters between the subject and the chart and the denominator (x) the distance at which a person with 6 / 6 acuity would discern the same optotype . Thus, 6 / 12 means that a person with 6 / 6 vision would discern the same optotype from 12 meters away (i.e. at twice the distance). This is equivalent to saying that with 6 / 12 vision, the person possesses half the spatial resolution and needs twice the size to discern the optotype . </P>

What is the normal vision of human eye