You've probably heard the expressions visual acuity and twenty-twenty vision. As frequently used as these terms are, do most people actually know what they mean? Understanding them will give you insight into how an eye specialist assesses your vision when you have an eye exam.
The term 20/20 vision is used to indicate the clarity and sharpness of your eyesight from 20 feet away. When you have 20/20 eyesight, it means that from a distance of 20 feet you can clearly see that which should be seen from that distance. 20/20 is just a standard measurement. The truth is, a considerable number of people have eyesight better than 20/20; for instance, 20/15, so what they would be able to see at 20 feet, a person with normal vision might only be able to discriminate at 15 feet.
Both eyes are tested one after the other. When you're asked to read the letters on the eye chart, the smallest row that you are able to read without error determines the visual acuity in the eye that's being evaluated.
However 20/20 vision actually doesn't mean your vision is perfect, because it only determines how well you see at a distance. Other vital components to seeing well; being able to focus on close objects, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these are aspects of healthy vision. Furthermore, a person with 20/20 vision can have plenty of other eye-related health problems. People with damage to the nerves inside their eyes as a result of glaucoma, diabetes, high blood pressure, or a range of other diseases can still have 20/20 vision without glasses. For this reason, an eye care professional will always carry out a comprehensive eye exam, and not just a simple visual acuity test.
During your next eye exam, you'll understand what we're testing for when we ask you to read from an eye chart!