text: what is the scale using 20/20 called?  and what is the conversion to diopters? i know that diopter is found by 1/f where f is the focal length. also i'm somewhat a -4.00 diopter contact lens means my eyes are, hence the conversion question. the 20/20 "scale" is actually a way of saying how good your eyesight is overall. it is a practical test. the patient looks at the special, standard chart from a predetermined distance and the doctor measures how much he can read. there are many factors that determine the result, including myopia, catarract, age etc etc  on the other hand, diopters measure the ability of the lens to focus on the retina. nothing else. there can be no conversion from one scale to the other, but someone with myopia will have worse eyesight (20/20 scale) without correction.