Visual acuity is the most common measurement used to assess the clarity and sharpness of a person’s eyesight at a distance. The standard of “20/20” is widely recognized as the benchmark for normal sight. This fractional measurement represents a specific degree of visual performance.
The Meaning of the 20/20 Fraction
The expression 20/20 is a fractional representation of visual acuity, which compares a person’s sight to a standardized norm. The first number, the numerator, represents the distance in feet at which a person stands from the eye chart during the test, typically 20 feet. This distance is fixed for the measurement. The second number, the denominator, indicates the distance in feet at which a person with normal vision would be able to read the same line of letters.
A score worse than 20/20, such as 20/100, means the tested person must move to within 20 feet to see what a person with normal vision sees from 100 feet away. The larger the denominator, the poorer the distance visual acuity. Conversely, a score better than the norm, like 20/15, indicates the person can see clearly at 20 feet what the average person would need to move to 15 feet to see.
How Visual Acuity is Tested
Visual acuity is typically measured using a standardized tool called the Snellen eye chart. The chart features rows of block letters, called optotypes, that decrease in size from the top row to the bottom. The largest letter on the chart corresponds to a visual acuity of 20/200.
During the test, the patient stands or sits 20 feet away and is asked to read the smallest line of letters they can distinguish, first with one eye covered, then the other, and sometimes with both eyes together. The line that the person can accurately read determines their visual acuity score. The score is recorded as either “uncorrected visual acuity” if tested without glasses or contact lenses, or “corrected visual acuity” if tested while wearing their prescriptive correction.
Other standardized charts, such as the LogMAR chart or those using Tumbling E’s or shapes, are sometimes used, particularly for children or individuals who cannot read letters. The Snellen method remains the most common way to get the fractional measurement. The goal of vision correction is often to improve corrected visual acuity to 20/20 or better.
Acuity Versus Overall Eye Health
While 20/20 is the recognized standard for normal distance vision, it does not represent “perfect” vision. Many people have sharper visual acuity, such as 20/15 or 20/10. The measurement is limited because it only assesses the sharpness and clarity of central vision at a fixed distance.
Visual acuity testing does not evaluate many other functional aspects of sight that contribute to overall eye health. These factors include peripheral vision, depth perception (the ability to judge distances), and color vision.
Having 20/20 vision does not guarantee the absence of eye disease. Conditions like glaucoma, cataracts, or diabetic retinopathy can affect internal structures, such as the optic nerve or retina, without immediately reducing distance visual acuity. Regular comprehensive eye examinations are necessary to check these other aspects of vision and eye health beyond the 20/20 standard.