Everyone has heard the expressions visual acuity and twenty-twenty vision. As common as these terms may be, do you actually know their meaning?
20/20 vision actually refers to the sharpness of vision from 20 feet away. If you've been told you have 20/20 vision, it means that from a distance of twenty feet you're able to clearly see what should be seen from that distance. You may not know this, but 20/20 is really just a standard measurement. Many people have eyesight 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 might only be able to discriminate from 15 feet.
Your eyes are tested separately. During the part when you're asked to correctly read letters from the eye chart, the smallest letters you can clearly see determine the visual acuity in the eye being tested.
20/20 eyesight actually doesn't mean your vision is perfect, and that's because it only assesses how sharply you see at a distance. There are several other important components to seeing well; being able to focus on close objects, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision – these all contribute to your overall ability to see. Also, someone who has 20/20 vision can have unhealthy eyes. Even people who have damage to the nerves within their eyes due to glaucoma, diabetes, high blood pressure, or a range of other diseases might still have 20/20 vision without needing to wear eye glasses. This is why your eye care professional always performs a comprehensive eye exam, as opposed to just a regular eye chart exam.
So the next time you have a comprehensive eye exam, you'll know exactly why you're being told to read letters off an eye chart, and more!