Snellen Eye Chart was developed in 1862 by Dutch ophthalmologist Herman Snellen, to determine if humans have a normal vision which is rated at 20/20
However, in the case of John, a 20/15 vision means John can see a line in the eye chart at 20 feet away that the average person can only see when they are 15 feet away.
This shows John has a sharper than average visual acuity.
