Visual acuity measures the sharpness or clarity of a person’s vision. It represents the eye’s ability to distinguish fine details and shapes with precision. This clarity is determined by the eye’s optical elements, such as the cornea and lens, and the neural components, including the retina and the visual pathways to the brain. Measuring visual acuity is a foundational step in eye care to determine the presence of refractive errors, such as nearsightedness or farsightedness, and guide necessary correction.
Decoding the Snellen Fraction
The result of an eye test is often presented as the Snellen fraction, which provides a simple comparison to a standardized level of sight. The top number, or numerator, indicates the distance in feet at which the test is performed, typically 20 feet in the United States. This number is fixed and represents the viewer’s distance from the testing chart.
The bottom number, or denominator, represents the distance at which a person with “normal” visual acuity could correctly read the line you read. A result of 20/20 means you can see clearly at 20 feet what an average person can also see clearly at 20 feet. This fraction is the benchmark for distance vision, though some individuals can achieve vision better than this standard, such as 20/15.
A larger denominator indicates lower visual acuity because you must be closer to the chart to see what a person with 20/20 vision sees farther away. For example, 20/40 means you can only see at 20 feet what a person with normal vision could distinguish from 40 feet away. This suggests your vision is half as sharp as the standard and often indicates a need for corrective lenses.
When the denominator reaches 200, the vision is considered significantly impaired. A person with 20/200 vision must stand 20 feet away to see what a person with normal vision can see from 200 feet away. This threshold of 20/200 or worse, even with the best possible correction, is the designation for legal blindness in the United States.
How Visual Acuity is Measured
Measuring visual acuity follows a standardized methodology to ensure consistent results. The most recognized tool is the Snellen chart, developed by Dutch ophthalmologist Herman Snellen in the 1860s. This chart features rows of block letters, or optotypes, that decrease in size down the chart.
The person being tested is positioned 20 feet away from the chart, which establishes the 20-foot numerator in the resulting fraction. If the testing room is not long enough, a mirror system is often used to optically simulate the required distance. A consistent viewing distance is maintained to standardize the measurement.
The test is performed one eye at a time, with the other eye covered, to assess individual capacity. The patient is asked to read the smallest line of letters they can clearly identify, starting from the largest at the top. The room is illuminated properly to ensure the test is conducted under consistent, high-contrast conditions.
For individuals who cannot read the Roman alphabet, variations like the Tumbling E chart or charts with standardized pictures may be used. The patient indicates the orientation of the symbol, and the smallest line correctly read determines the denominator of the Snellen fraction, providing the final visual acuity score.
What 20/20 Doesn’t Tell You
Achieving 20/20 visual acuity is a good outcome, but this measurement is only one specific component of overall visual health. The Snellen fraction only assesses the sharpness of central vision under high-contrast conditions, typically black letters on a white background. It is a static measurement, focusing on the ability to see fine details at a distance.
The 20/20 score does not provide information about peripheral vision, which is important for mobility and spatial awareness. The test also does not measure the ability to perceive depth (stereopsis) or the capacity to differentiate between colors. These are separate visual skills not evaluated by the Snellen chart.
Other aspects of eye health are not addressed by the acuity score, such as the eyes’ ability to focus at different distances or their coordination when working together. Furthermore, the test does not screen for eye diseases or complex refractive errors like astigmatism. This emphasizes why a comprehensive eye examination is necessary beyond a simple acuity check.