The visual acuity test is a fundamental part of an eye examination, designed to measure the clarity and sharpness of your distant vision. This test, most commonly performed using a standardized Snellen chart, determines how well you can discern letters or symbols at a distance. To ensure accurate and comparable measurements, the patient must be positioned at a very precise distance from the chart. This fixed separation is necessary so the eye’s focusing mechanism, known as accommodation, is relaxed to get a true reading of distance vision.
The Standard Distance for Visual Acuity
The universally accepted distance for measuring visual acuity is 20 feet (6 meters). This measurement was chosen for a reason related to the physical structure of the eye. At 20 feet, the ciliary muscles inside the eye, which change the lens’s shape to focus on near objects, are considered to be in a relaxed or “at rest” state.
When these muscles are relaxed, the eye is not actively focusing, allowing the examiner to measure the eye’s refractive state for distance vision without interference. This fixed distance ensures that any variation in the score is due to the patient’s actual vision. During the examination, the patient is asked to stand or sit at this distance and read the smallest line of letters they can distinguish on the chart.
Interpreting Your Eye Exam Score
The result of the visual acuity test is commonly expressed as a fraction, such as 20/20, tied to the standard 20-foot testing distance. In this fraction, the numerator (20) represents the testing distance. The denominator represents the distance at which a person with normal vision could clearly read the same line of letters you read.
A score of 20/20 indicates you can see clearly at 20 feet what a person with normal vision can also see at 20 feet. If your score is 20/40, you must stand as close as 20 feet to read a line that a person with normal vision could read from 40 feet away. A score of 20/100 signifies that the letters you make out at 20 feet are large enough for a normal-sighted person to read from 100 feet away.
This fractional notation provides a standardized measure of your distance vision relative to the norm. While 20/20 is the benchmark for standard visual acuity, it only measures clarity at a distance. It does not account for other aspects of vision, such as peripheral awareness, depth perception, or color vision.
Adjustments for Smaller Spaces
Many eye examination rooms are not 20 feet long, challenging the maintenance of the distance standard. To overcome this spatial limitation, eye care professionals use optical techniques to simulate the required distance, most commonly involving mirrors or specialized projectors.
Using Mirrors (Folded Optics)
In the mirror setup, the chart is mounted behind the patient, and the patient views its reflection in a mirror placed in front of them. The distance light travels from the chart to the mirror and back to the patient’s eyes is precisely calculated to equal a total path of 20 feet. This method, sometimes called “folded optics,” allows a standard 10-foot room to function as a 20-foot testing lane.
Using Projectors
Projectors are also calibrated to display the letters at the correct proportional size for a shorter room, such as 10 or 14 feet. This calibration mimics the visual angle of the letters as if they were viewed at 20 feet.
Separate from distance vision, near-vision acuity is tested using small hand-held cards. These cards are typically held at a standardized distance of about 14 to 16 inches, measuring your ability to focus on objects at a reading distance.