The term 20/20 refers to the size of the letter which the average eye can read 20 feet away. In most standard vision testing, you can read the familiar Snellen eye chart at a distance of 20 feet. The upper number of the fraction expresses this distance. The lower number identifies the size of the letter you were able to read at this distance. Thus, 20/70 vision means you were able to read at 20 feet the line which a normal eye can read at 70 feet.
Often, this test is given by having you look into a machine which creates the same conditions optically. The results are roughly similar to those of the Snellen chart.
However, parents should not be lulled into the impression that their child has perfect vision because he scores a 20/20 on this vision test. All it measures is how well or poorly the child sees at that distance. And that’s all. Problems with near vision, eye coordination, and focusing ability are among the many problems NOT discovered in this test. Only a professional eye examination will protect the priceless miracle of sight.