Acuity Text

Definition: Visual acuity refers to the quality of a person's sight. It is typically expressed as a ratio, as in 20/20. This ratio enables ophthalmologists to compare a person's vision to that of average people. The first number represents what the examinee sees at a distance of twenty feet. The second number represents how far most people can be to see the same sized object.

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

VA is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. It is the most common clinical measurement of visual function. In the term "20/20 vision" the numerator refers to the distance in feet from which a person can reliably distinguish a pair of objects. The denominator is the distance from which an 'average' person would be able to distinguish —the distance at which their separation angle is 1 arc minute. The metric equivalent is 6/6 vision where the distance is 6 meters. Twenty feet is essentially infinity from an optical perspective (the difference in optical power required to focus at 20 feet versus infinity is only 0.164 diopters). For that reason, 20/20 vision can be considered nominal performance for human distance vision; 20/40 vision can be considered half that acuity for distance vision and 20/10 vision would be twice normal acuity. The 20/x number does not directly relate to the eyeglass prescription required to correct vision; rather an eye exam seeks to find the prescription that will provide at least 20/20 vision.