Visual acuity (VA) measures the sharpness or clarity of a person’s vision, specifically their ability to distinguish fine details and shapes. This metric is a fundamental component of any eye examination, offering a standardized way to assess the performance of the visual system at a distance. VA testing involves determining the smallest object a person can clearly recognize at a specific distance under high-contrast conditions. The resulting measurement helps eye care professionals determine if a person has a refractive error, such as nearsightedness or farsightedness, which can often be corrected with lenses. While it measures the clarity of central vision, it is only one aspect of overall eye health and does not account for peripheral vision or color perception.
The Standard Method: Using the Snellen Chart
The most widely recognized method for measuring distance visual acuity involves the Snellen chart, introduced by Dutch ophthalmologist Herman Snellen in 1862. This chart features rows of block letters, formally called optotypes, which systematically decrease in size from top to bottom. The chart’s design follows precise geometric principles to ensure consistent proportions.
The standard procedure requires the patient to stand at a specific distance from the chart, typically 20 feet in the United States or 6 meters in metric-using countries. This distance is chosen because the light rays entering the eye from the chart are nearly parallel, which minimizes the need for the eye’s internal lens to adjust its focus, a process called accommodation. The test is conducted by covering one eye at a time and asking the patient to read the smallest line of letters they can clearly distinguish. The line successfully read determines the visual acuity score for that eye.
Interpreting the Visual Acuity Fraction
Visual acuity results are expressed as a fraction, known as the Snellen fraction, that compares the patient’s vision to that of a person with defined “normal” vision. In the US imperial system, this fraction is written as 20/X, where the numerator (20) represents the distance, in feet, the person is standing from the chart. The denominator (X) indicates the distance at which a person with normal vision could read the smallest line the patient was able to read at 20 feet.
A result of 20/20 is considered the standard for normal distance visual acuity. This means the patient can see at 20 feet what a person with normal vision also sees at 20 feet. If a person has 20/40 vision, they must move to 20 feet to see what a person with normal vision can see clearly from 40 feet away. Conversely, a score such as 20/15 indicates better-than-average acuity.
For countries using the metric system, the notation is 6/X, where the numerator (6) represents the testing distance in meters, and 6/6 is the equivalent of 20/20 vision. A larger denominator signifies poorer visual acuity, illustrating that the patient needs to be closer to the chart than the standard population to see the same detail. The smallest line read accurately determines the final fraction.
Alternative Measurement Techniques
When a person cannot complete the standard letter-based Snellen test, alternative charts and methods are necessary to assess visual function. For pre-verbal children or those with literacy or language barriers, charts use non-alphabetic symbols.
Non-Alphabetic Charts
The Tumbling E chart uses the letter E oriented in different directions, requiring the patient to indicate the direction the E is pointing. Other options include Lea Symbols (shapes like a circle, square, house, and apple) or Patti Pics (stylized pictures designed to follow the same geometric principles as Snellen’s optotypes).
Functional Descriptions
For patients with severe vision loss who cannot read the largest optotype, acuity is recorded using functional descriptions. These observations include “Counting Fingers” (CF), which is the distance at which the patient can count the examiner’s fingers, or “Hand Motion” (HM), meaning the patient can only perceive the movement of a hand in front of their face. The lowest level of measurable vision is “Light Perception” (LP), which indicates the ability to only distinguish the presence or absence of light. LogMAR charts, such as the ETDRS chart, offer a more precise, logarithmically scaled measurement used primarily in clinical research, providing a continuous numerical score that is more sensitive to small changes in vision.