The phrase “20/20 vision” is the universally accepted standard for normal visual acuity, which measures the sharpness or clarity of distance vision. While commonly mistaken for “perfect eyesight,” it is specifically a fraction that represents a performance comparison against an established norm. This measurement system provides a standardized way for eye care professionals to assess how well an individual sees compared to the average population.
The Meaning Behind the Numbers
The 20/20 fraction represents a comparison of two distances. The numerator (the first number) is the distance in feet at which a person stands from the eye chart during the examination, which is typically 20 feet in the United States.
The denominator (the second number) represents the distance in feet at which a person with average, or “normal,” vision can clearly read the same line of letters. A score of 20/20 means the patient standing at 20 feet can read the line of letters that a person with normal vision is also expected to read clearly at 20 feet. In countries using the metric system, this standard is expressed as 6/6, signifying 6 meters as the standard testing distance.
How Vision is Measured
Visual acuity is formally measured using a standardized chart, most often the Snellen chart, developed in 1862. This chart features rows of block letters, or optotypes, that decrease in size down the chart. The design of these optotypes is mathematically precise, based on the concept that a person with normal vision should be able to distinguish a specific character at a certain distance.
During an exam, the patient is positioned 20 feet from the chart and asked to read the smallest line of letters they can clearly identify. Each line of letters is calibrated to correspond to a specific visual acuity score. The eye care professional determines the final fractional score by having the patient read progressively smaller lines to find the smallest detail the patient’s eye can resolve at the 20-foot testing distance.
Understanding Deviations from 20/20
Scores differing from 20/20 indicate a divergence from the established visual norm. For example, a person with 20/40 vision must stand at 20 feet to see what a person with normal vision can see from 40 feet away. This score implies the individual’s eyesight is half as sharp as the standard.
Conversely, visual acuity can be better than the 20/20 standard. A score of 20/15 means the person being tested can see clearly at 20 feet what an average person can only see clearly when they move to within 15 feet of the chart. A score of 20/200 or worse in the better eye, even with corrective lenses, is the common threshold used to define legal blindness in the United States.