What Exactly Is 20/20 Vision?

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.

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.