It's safe to say that everyone has run into the terms twenty-twenty vision and visual acuity. Yet, do people know what these terms actually mean? When you really understand what they imply, you will know why your eye care professional asks you to do more than simply read from an eye chart.
The term 20/20 indicates the clarity and sharpness of sight from 20 feet away. When you have 20/20 eyesight, that basically means that from a distance of twenty feet you can clearly see what should be seen from that distance. You may not know this, but 20/20 vision isn't the best possible visual acuity. Many people can even see better than 20/20; for example, vision that measures 20/15, so what they would be able to see at 20 feet, a person with normal vision might only be able to discriminate from 15 feet.
Both eyes are tested one after another. 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 of the eye that's being examined.
However 20/20 vision actually doesn't mean your vision is perfect, because it can only judge your clarity of vision at a distance. There are other really crucial vision skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these all contribute to your overall vision. Also, a person with 20/20 vision can certainly have eye problems. People with damage to the retina due to diabetes, high blood pressure, glaucoma, or a range of other conditions are still able to have 20/20 vision. And because of this, an eye care professional should always perform a comprehensive eye exam, and not just a plain eye chart exam.
So the next time you book yourself in for a comprehensive eye exam, you'll know exactly why you're being told to read letters from the eye chart, and more!