Visual Acuity

Visual acuity (VA) is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain.

Visual acuity is a measure of the spatial resolution of the visual processing system. VA is tested by requiring the person whose vision is being tested to identify characters (like letters and numbers) on a chart from a set distance. Chart characters are represented as black symbols against a white background (for maximum contrast). The distance between the person's eyes and the testing chart is set at a sufficient distance to approximate infinity in the way the lens attempts to focus. Twenty feet, or six metres, is essentially infinity from an optical perspective.

(The difference in optical power required to focus at 20 feet (6.1 m) versus infinity is only 0.164 diopters.)

In an eye exam, lenses of varying powers are used to precisely correct for refractive errors. A pinhole is also used to largely correct for refractive errors. English alphabet letters are typically used (as in the classic Snellen chart) as most people will recognise them but other symbols (such as a letter E facing in different directions) are also used.

In the expression, 20/40 vision, the 20 is the distance in feet between the subject and the chart. The 40 means that the subject can read the chart as well as a normal person can read a chart that is 40 feet away. This is calculated by finding the smallest optotype they can identify and calculating the distance at which it has a visual angle of 5 arcminutes.

The letters in the lowest line in the Snellen chart are composed of lines that are separated by a visual angle of one arc minute. This corresponds to lines that are spaced only 1.75 mm apart. Each succeeding higher line on the Snellen chart contains letters whose lines are separated by correspondingly larger visual angles. A person who can correctly identify letters on the lowest line is able to discern individual lines that are separated by a visual angle of one arc minute.

The metric equivalent of 20/20 vision is 6/6 vision. At 20 feet or 6 metres, a human eye with nominal performance is able to separate lines that are one arc minute apart (equivalent to lines that are spaced 1.75 mm apart). A vision of 20/20 is considered nominal performance for human distance vision. A vision of 20/40 is considered half as good as nominal performance. A vision of 20/10 is considered twice as good as nominal performance.

The 20/x number does not directly relate to the eyeglass prescription required to correct vision, because it does not specify the nature of the problem corrected by the lens, only the resulting performance. Instead an eye exam seeks to find the prescription that will provide the best corrected visual performance achievable. This may be greater or lesser than 6/6 for many reasons. In other words, 20/20 vision or 6/6 vision does not necessarily correspond to the best possible visual acuity a subject may achieve, but once this standard is attained the subject is considered to have achieved "normal" visual acuity.