Snellen Chart and Visual Acuity

Measuring the sharpness of vision

Overview

Visual acuity is a measure of the ability of the eye to resolve fine detail and is the most commonly used measure of visual function. The Snellen chart, introduced by the Dutch ophthalmologist Herman Snellen in 1862, presents rows of letters, called optotypes, that decrease in size. Acuity is recorded as a fraction: 20/20 in feet, or 6/6 in meters, means a person can read at 20 feet what a standard observer can read at that distance, while 20/40 means reading at 20 feet what the standard observer can read at 40. In the United States, best corrected acuity of 20/200 or worse in the better eye is one criterion for legal blindness. LogMAR charts, such as the ETDRS chart, have equal steps between lines and are preferred in research. Pinhole testing helps distinguish blur due to refractive error, which usually improves through a pinhole, from disease. Acuity reflects central macular function and can remain normal in conditions that affect peripheral vision, such as early glaucoma. Many young adults see better than 20/20, which represents a reference standard rather than perfect vision.