Visual acuity is the measurement of the sharpness or clarity of a person’s vision, which is a fundamental aspect of eye health. Eye care professionals utilize a standardized measurement known as the Snellen fraction, most commonly seen as the 20/20 notation, to quantify this clarity. This fraction provides a simple, yet comprehensive, way to describe a patient’s distance vision relative to a defined standard. Understanding the meaning of the two numbers in this fraction is essential for interpreting the results of a routine eye examination.
The Meaning of the Numerator
The numerator, which is the top number in the Snellen fraction, represents the fixed distance at which the visual acuity test is performed. In the United States, this number is almost always 20, signifying a testing distance of 20 feet from the eye chart. This 20-foot distance is a standardized benchmark used across most eye clinics and medical offices.
The test is designed this way because 20 feet is considered to be “optical infinity,” meaning the eye’s focusing muscles are relaxed when viewing objects at this distance. The patient’s actual vision quality does not change the numerator, as it simply defines the physical setup of the examination.
The primary function of the numerator is to establish a consistent testing environment so that the denominator can accurately reflect a patient’s visual capability. If the eye exam room is shorter than 20 feet, mirrors or projection systems are used to make the letters appear as though they are 20 feet away to maintain the standard. In countries using the metric system, this numerator is standardized at 6, representing 6 meters, which is the exact equivalent of 20 feet.
Defining the Denominator
The denominator, or bottom number of the Snellen fraction, represents the distance in feet at which a person with “normal” visual acuity could read the smallest line the patient successfully read. This number defines the size of the letters the patient was able to see at the standardized 20-foot distance.
Normal visual acuity, or standard vision, is defined as 20/20. If a patient is only able to read a larger line of letters, the denominator will be a higher number, such as 40 or 100. This higher number indicates that a person with standard vision could have read that same line of letters from a much greater distance.
The size of the letters on the chart is precisely calibrated to subtend a specific angle at the eye at the distance indicated by the denominator. Therefore, the denominator is the relative measure, indicating how far away a person with standard vision would need to stand to match the patient’s performance at 20 feet.
Interpreting the Visual Acuity Score
The complete Snellen fraction synthesizes the two numbers to provide a practical assessment of a patient’s distance vision. A score of 20/20 means the patient sees at 20 feet what a person with normal vision sees at 20 feet, establishing the baseline standard.
A score worse than 20/20, such as 20/40, indicates the patient must be as close as 20 feet to clearly see an object that a person with standard vision could see clearly from 40 feet away. This score often represents a common threshold, as many jurisdictions require at least 20/40 vision to drive a motor vehicle without corrective lenses.
Vision that is significantly impaired is reflected by a very high denominator. For example, a score of 20/200 means the patient sees at 20 feet what a person with normal vision could see from 200 feet away. Corrected visual acuity of 20/200 or worse in the better-seeing eye is the threshold for being considered legally blind in the United States.
Conversely, a smaller denominator than 20 indicates better-than-average visual acuity. A score of 20/15 means the patient can see clearly at 20 feet what the standard person needs to move up to 15 feet to see.