20/20
Average decent vision requiring little correction.
An individual with 20/20 vision does not have perfect eyesight.
WORSE than 20/20
An individual with 20/40 vision must stand at 20 feet to see what an individual with 20/20 vision can see at 40 feet.
BETTER than 20/20:
An individual with 20/15 vision can stand at 20 feet to see what an individual with 20/20 vision can see at 15 feet.