We've all run into the terms visual acuity and 20/20 vision. As frequently used as these terms may be, do most people actually grasp their meaning? When you understand them, you will get why an eye care professional asks you to do more than simply read an eye chart.
20/20 vision actually refers to the clarity of eyesight from 20 feet away. If you've been told you have 20/20 vision, that means that from twenty feet away you can clearly see what should be seen from that distance. 20/20 vision isn't the best possible visual acuity. A large number of people have vision that's better than 20/20; for instance, some people have 20/15 vision, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate at 15 feet.
Both eyes are tested one after another. When your optometrist asks you to correctly read letters from the eye chart, the smallest row that you are able to read without error determines the visual acuity of the eye being tested.
However 20/20 vision doesn't necessarily mean that your eyesight is flawless, because, after all, it can only judge your clarity of vision at a distance. There are lots of equally crucial vision skills; the ability to focus on close objects, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are all really important to your overall eyesight. More importantly, a patient with 20/20 vision can have eye problems. Those with damage to the sensory nerves within their eyes from diabetes, high blood pressure, glaucoma, or numerous other diseases can still have 20/20 vision without glasses. And because of this, an optometrist should always conduct a comprehensive eye exam, rather than just a regular eye chart exam.
So the next time you have a comprehensive eye exam, you'll know what we're testing for when we ask you to read letters aloud from an eye chart!