What limits visual acuity to 20/20?
20/20 is not the limit of human visual acuity!
"20/20" (or 6/6/ in the metric system) just means "normal distance vision." Young people often can see better than 20/20 with or without help from glasses or contact lenses. The lines of print on Snellen eye chart get smaller beneath the 20/20 line: 20/15 and 20/10. The more accurate and standardized EDTRS type charts display three lines at the bottom (under the 20/20 line) that get progressively smaller: 20/16, 20/12.5, and 20/10.
It is common for patients to think 20/20 means "perfect vision" or at least the best possible vision. This is probably because the standardization of eye charts was always difficult and never really much of a concern in the optical business of the last century. But the Snellen chart was developed almost 150 years ago. Vision has been extensively studied so that we now know a simple number ration like 20/20 does not tell the whole story.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic