The Snellen eye chart is read at what distance?

• A. 20 feet
• B. 10 feet
• C. 15 feet
• D. 30 feet

Answer: (A)

Testing distance is fixed so results are consistent. The Snellen chart is read from twenty feet away, the standard distance for assessing distance vision. At this distance, the letters are sized so that a person with normal acuity can read the line that corresponds to twenty feet; in chart terms, that line represents what a normal eye can see at twenty feet. The concept behind this is that the detail each letter contains subtends a specific small angle at the eye, so changing the distance would change how easy the letters are to read and thus alter the measured acuity. In metric settings, the equivalent testing distance is about six meters, but the common practice described here uses twenty feet.