A person taking the test covers one eye from 6 metres or 20 feet away, and reads aloud the letters of each row, beginning at the top.

[4] Wall-mounted Snellen charts are inexpensive and are sometimes used for approximate assessment of vision, e.g. in a primary-care physician's office.

Whenever acuity must be assessed carefully (as in an eye doctor's examination), or where there is a possibility that the examinee might attempt to deceive the examiner (as in a motor vehicle license office), equipment is used that can present the letters in a variety of randomized patterns.

BS 4274-1:1968 (British Standards Institution) "Specification for test charts for determining distance visual acuity" was replaced by BS 4274-1:2003 "Test charts for clinical determination of distance visual acuity — Specification".

[citation needed] Visual acuity is the distance at which test is made / distance at which the smallest optotype identified subtends an angle of five arcminutes and the critical distinguishing features of the optotype subtend an angle of one arcminute.

[5][2] Snellen defined "standard vision" as the ability to recognize one of his optotypes when it subtended 5 minutes of arc.

Thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of one minute of arc.

Outside the United States, the standard chart distance is 6 metres (20 ft), and normal acuity is designated "6/6".

[7] Another calculation for United States clinics using 20-foot chart distances (slightly more than 6 m), and using a 17 mm model eye for calculations, and a letter which subtends 5 minutes of arc, gives a vertical height of the 20/20 letter to be 8.75 mm.

[8] Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription.

The largest letter on an eye chart often represents an acuity of 6/60 (20/200), the value that is considered "legally blind" in the US.

Commonly digital chart products support randomizing optotypes displayed to prevent patients from memorizing lines they have previously read.