The history of 20/20 being considered normal is quite interesting, essentially it comes down to being simply what Herman Snellen defined as normal (on the plus

The history of 20/20 being considered normal is quite interesting, essentially it comes down to being simply what Herman Snellen defined as normal (on the plus side, he had a few years practice as an ophthalmologist before he invented his chart and it was based on standardizing several other charts in use, so it was based on a lot of experience; on the minus side, it was invented in the 1860s). It’s been a while since I read up on it but my recollection is that the healthy eye in a young patient (“normal” in the sense of “lacking any deficiencies whatsoever”, not in the sense of “average”) should be around 20/12, with natural variation between 20/10 and 20/16, and 20/20 indicating (extremely minor) deficiencies. The theoretical perfect eye from an optical standpoint, something like if you were to grow an eye from the same materials as a real human eye but you could specify exact dimensions etc., is something like 20/7 (as in, you can’t get better optical resolution than that given the optical properties of our lenses and the distribution and size of our light receptor cells). Eyes are extremely complicated structures that are either very sensitive to environmental conditions, or that the body is very willing to cut corners on given the slightest provocation, so few people get that “perfect” vision. There is one very good reason for considering 20/20 to be “normal”, though - since it has been accepted as the standard for over 150 years, most of the human-designed environment has used it as a yardstick for designing visual elements such as roadsigns. The size and shape of roadsign lettering and their placement relative to the road feature they’re describing is all according to a standard that was worked out on the assumptions of (some multiple of) “normal” 20/20 vision, for instance. Likewise for just about every other thing you have to read. So it makes a lot of sense to use that as the threshold. In this sense perhaps it helps to read “normal vision” not as “a normal eye” or “the normal acuity among average healthy humans” but rather “the vision threshold needed to interact normally with the human world”.