Health care professionals test visual acuity by evaluating how well a patient can see letters on a chart from different distances, states Healthline. The Snellen test is one common format that involves reading rows of letters or symbols of various sizes from a chart positioned 14 to 20 feet away.
Visual acuity tests determine the level of detail patients perceive when looking straight ahead, and they are usually performed as part of a routine eye exam, explains MedlinePlus. One method requires instructing the patient to cover one eye with a hand or paddle while identifying the letters on a standardized chart. As the letters become progressively smaller, health care professionals measure the point where the patient is unable to read the letters at the current distance.
A score of 20/20 is considered normal because the patient stood 20 feet from the chart and accurately perceived the amount of detail expected at this distance, according to the American Optometric Association. In contrast, a person with a 20/40 score must stand within 20 feet of the chart to read at an accuracy level considered normal for someone standing 40 feet away. Visual acuity tests do not provide a definitive measurement of vision, and they are typically performed along with examinations that test peripheral vision, depth perception, eye coordination and other skills.