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20/20, 20/40, and other such numbers, measure visual acuity, or sharpness of vision.

If a person has 20/20 visual acuity it means that when they stand 20 feet away from the chart they can see what a "normal" person can see at 20 feet. With 20/40 acuity, the person must move up to 20 feet to see what a "normal" person can see at 40 feet.

Visual acuity is just one part of vision. Other important vision parameters include: peripheral vision, depth perception, refractive error, focusing ability and eye health.

A person with excellent visual acuity may well have serious vision problems that interfere with daily activites. Indeed, a person with tunnel vision may have 20/20 acuity but be legally blind. Or a person with moderate hyperopia can have severe headaches and eyestrain while having 20/20 visual acuity.