You have most likely stumbled upon the expressions visual acuity and twenty-twenty vision. But do people understand what these terms truly mean?
The term 20/20 vision is used to indicate the clarity of vision measured at a distance of 20 feet. If you have 20/20 vision, that basically means that from 20 feet away you can clearly see what normal-sighted people can see from that distance. Alternatively, 20/100 vision indicates that you would need to be as near as 20 feet away to see what a normal-sighted person would see from 100 feet away. Obviously, if this was the case, it would mean that you would be extremely near sighted.
Both eyes are examined one after another. When your optometrist instructs you to look at the eye chart and read out the letters, the smallest row that you can read properly determines the visual acuity of the eye being tested.
However 20/20 eyesight actually doesn't mean that your eyesight is totally free of flaws, because it only determines how accurately you see at a distance. There are lots of equally crucial vision skills; being able to focus on close objects, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision – these are aspects of healthy vision. Also, a patient who has 20/20 vision may have eye problems. Even people who have damage to the retina as a result of glaucoma, diabetes, high blood pressure, or numerous other conditions are still able to have 20/20 vision, without the help of glasses. For this reason, your optometrist should always conduct a comprehensive eye exam, rather than just a simple eye chart test.
So the next time you have a comprehensive eye exam, you'll understand what we're looking for when we ask you to read aloud from an eye chart!