what is visual acuity

Visual acuity is a measure of the ability of the eye to distinguish shapes and the details of objects at a given distance. It is usually tested by reading an eye chart, which contains letters or symbols of varying sizes, at a distance of 20 feet. The results of a visual acuity test are written as a fraction, with the top number always being 20, which signifies the distance between the person and the chart, and the bottom number being the last line read correctly. For example, a result of "20/20" means that the person read the line that those with normal vision can read. Visual acuity decreases as the bottom number gets larger, with a result of 20/40 meaning that the person can see at 20 feet what those with normal vision can see from 40 feet away. Visual acuity is affected by various factors, including vision conditions like nearsightedness, farsightedness, or astigmatism, or eye diseases. A comprehensive eye examination by an eye care professional can diagnose what is affecting a persons ability to see well, and in most cases, glasses, contact lenses, or surgery can improve vision to 20/20 or close to it.