This appears to be based on 20/20 (6/6) vision. People can have a visual acuity significantly better than that: > Healthy young observers may have a binocular

This appears to be based on 20/20 (6/6) vision. People can have a visual acuity significantly better than that: > Healthy young observers may have a binocular acuity superior to 6/6; the limit of acuity in the unaided human eye is around 6/3–6/2.4 (20/10–20/8) > The significance of the 6/6 standard can best be thought of as the lower limit of normal or as a screening cutoff. When used as a screening test, subjects that reach this level need no further investigation, even though the average visual acuity with a healthy visual system is typically better. https://en.wikipedia.org/wiki/Visual_acuity https://en.wikipedia.org/wiki/Visual_acuity I myself have been tested (twice) at around 20/8 (and was still better than 20/20 when I was complaining of blurry vision after an eye injury).