What is 20/20 and 20/30?

Sharpness of sight. 20/20, 20/30, measures of sharpness of sight, ability to identify small letters with high contrast at a specified distance. Visual acuity measures only the smallest detail we can see; it does not represent the overall quality of vision. 20/30, the smallest letter that they can see at 20 feet could be seen by a normal eye at 30 feet.