20/20 vision explained

Herman Snellen, a Dutch ophthalmologist, designed the first eye chart, which measures visual acuity. Referred to as the Snellen Chart, it was developed in 1862 and is still used today. He set the baseline of measurement at 20 feet, the approximate distance at which light rays entering the eye are parallel and do not require our eyes to bend the rays to focus them upon the retina.

Snellen set the standard of 20/20 vision to represent "normal sight," or what a person with normal vision could see at a distance of 20 feet. (Weber, Humphrey, and Silver, 1997)

This means that a person with 20/40 vision must be 20 feet away to read what he or she should be able to read at 40 feet; a person with 20/85 vision must be 20 feet away to read what he or she should be able to read at 85 feet away, and so on.

This standard does not measure visual acuity at close distances. Similar charts were devised with smaller letters to read at close range for the measurement of near vision.

This Article Coutesy The Rebuild Your Vision Program

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