You have most likely run into the expressions visual acuity and 20/20 vision. As common as these terms may be, do most people actually grasp their meaning?
The term 20/20 vision refers to the clarity of sight from 20 feet away. When you have 20/20 eyesight, it means that from 20 feet away you are able to properly see that which should be seen from that distance. Did you know that 20/20 isn't the best possible visual acuity? The truth is, a considerable number of people can even see better than 20/20; for instance, 20/15, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate as close as 15 feet.
Both eyes are tested one after another. When your optometrist instructs you to correctly read letters from the eye chart, the smallest row that you are able to read without error indicates the visual acuity in the eye that's being tested.
However 20/20 eyesight actually doesn't mean that your eyesight is perfect, because it can only judge how well you see at a distance. There are lots of equally crucial vision skills; the ability to focus on close objects, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these all contribute to your overall vision. Furthermore, someone who has 20/20 vision can have unhealthy eyes. Even those who have suffered damage to the retina as a result of glaucoma, diabetes, high blood pressure, or other conditions are still able to have 20/20 vision. For this reason, your eye care professional always performs a comprehensive eye exam, and not just a plain eye chart examination.
During your next eye exam, you'll know what we're testing for when we ask you to read from an eye chart!