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.
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 ??