You've probably come across the expressions 20/20 vision and visual acuity. As frequently used as these terms are, do people really grasp their meaning?
The term 20/20 vision refers to the clarity and sharpness of sight from 20 feet away. If you've been told you have 20/20 vision, that means that from a distance of twenty feet you are able to accurately see what should be seen from that distance. Alternatively, 20/100 eyesight indicates that to see what most people can see from 100 feet, you would have to stand only 20 feet away. Obviously, if this was the situation, it would mean that you would be pretty near sighted.
Both eyes are examined one after another. When the optometrist asks you to look at the eye chart and read out the letters, the smallest row that you can read accurately determines the visual acuity of the eye that's being tested.
However 20/20 vision doesn't always mean your vision is perfect, because it only assesses how accurately you see at a distance. Other necessary vision skills; your ability to focus on close objects, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision – these all contribute to your overall ability to see. Also, someone with 20/20 vision may still have unhealthy eyes. Those with damage to the retina from glaucoma, diabetes, high blood pressure, or other conditions are still able to have 20/20 vision without glasses. For this reason, an optometrist always conducts a comprehensive eye exam, as opposed to just a plain visual acuity test.
During your next eye exam, you'll understand why you need to read letters from the eye chart, and more!