It's safe to say that everyone has stumbled upon the expressions visual acuity and twenty-twenty vision. Yet, do people understand what these terms really mean?
The term 20/20 vision is used to indicate the clarity and sharpness of eyesight 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 clearly see what is normally seen from that distance. You may not know this, but 20/20 vision is just a standard measurement. A large number of people have vision that's better than 20/20; for instance, 20/15, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate at 15 feet.
Each eye is evaluated separately. When the optometrist asks you to read the letters on the eye chart, the smallest letters you can properly read determine the visual acuity in the eye being tested.
However 20/20 eyesight actually doesn't mean your vision is perfect, because it can only judge how clearly you see at a distance. There are other really necessary vision skills; the ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these also contribute to your overall vision. Furthermore, a person with 20/20 vision may have plenty of other eye-related health problems. People with damage to the nerves inside their eyes from glaucoma, diabetes, high blood pressure, or numerous other conditions can still have 20/20 vision. For this reason, your eye care professional always performs a comprehensive eye exam, and not just a simple visual acuity test.
During your next eye exam, you'll understand why you need to read letters off an eye chart, and more!