Snellen Chart - "20/20" (or "6/6") Vision

"20/20" (or "6/6") Vision

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 1 minute of arc.

In the most familiar acuity test, a Snellen chart is placed at a standard distance, 20 ft in the US. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of 20 ft.

Three lines above, the letters have twice the dimensions of those on the 20/20 line. The chart is at a distance of 20 ft, but a person with normal acuity could be expected to read these letters at a distance of 40 ft. This line is designated by the ratio 20/40. If this is the smallest line a person can read, the person's acuity is "20/40," meaning in a very rough kind of way that this person needs to approach to a distance of 20 ft to read letters that a person with normal acuity could read at 40 ft. In an even more approximate manner, this person could be said to have "half" the normal acuity.

At 20 ft, the letters on the 20/20 line should subtend 5 minutes of arc (such that the limbs of the letters subtend 1 minute of arc), which means that the chart should be sized such that these letters are 8.87 mm tall and the topmost "E" should be 88.7 mm tall.

Outside of the US, the standard chart distance is 6 m, normal acuity is designated 6/6, and other acuities are expressed as ratios with a numerator of 6. Many rooms do not have 6 m available, and either a half-size chart subtending the same angles at 3 m, or a reversed chart projected and viewed by a mirror is used.

Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription. During such examinations, acuity ratios are never mentioned.

The largest letter on an eye chart often represents an acuity of 20/200 (6/60), the value that is considered "legally blind." Some individuals with moderate myopia may not be able to read the large E without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses. By contrast, legally blind individuals have a visual acuity of 20/200 (6/60) or less when using the best corrective lens.