What does it mean to have 20/40 vision?

A person with 20/40 vision sees an eye chart 20 feet away that a person with normal vision sees at 40 feet away. All 50 states require a person to have 20/40 vision or better to pass a driver's license examination without corrective lenses.

Optometrists test vision with the Snellen chart, which contains letters that get progressively smaller. Fighter pilots must have 20/20 vision, and newspaper stock quotes and telephone numbers in the phone book are printed for people with 20/20 vision. Newspapers write headlines for people with 20/80 vision. People with 20/200 vision can read STOP sign letters but are legally blind.