SNELLEN EYE CHART:

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.

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ASTIGMATISM:  is another situation, however.
Often referred to as "out-of-round."
Think of squeezing a tennis ball that has been cut in half.

Are the lines evenly spaced among the rays below ??