Eyesight

Eyesight is assessed by visual acuity testing. At your eye exam, you will be shown charts of letters or numbers across the room and up close. Distance visual acuity is usually tested at a distance of 20 feet (or a simulated distance of 20 feet using a set of mirrors). Near visual acuity is typically tested at a distance of 16 inches, which is an average reading distance.



The use of letters of incrementally smaller sizes to evaluate a person's eyesight began in the early 1800's. The measurement of visual acuity that is used most frequently is the Snellen fraction. It is named after Dutch Ophthalmologist Hermann Snellen who developed the classic letter chart in 1863. Since then, people have come to recognize that the Snellen fraction "20/20" means perfect eyesight.



Snellen determined the size of the letters on his test chart by comparing the eyesight of a large number of his patients with that of his assistant, who could see distant objects very clearly. (Historical accounts don't explain how Snellen knew his assistant had "perfect" eyesight.)