You've probably run into the terms twenty-twenty vision and visual acuity. Yet, do people know what these terms truly mean? Having a proper understanding of them will give you insight into how an eye care professional evaluates your eyes when you have an eye exam.
20/20 is used to indicate the clarity and sharpness of vision measured at a distance of 20 feet. If you have 20/20 eyesight, that means that from a distance of twenty feet you are able to accurately see that which should be seen from that distance. Alternatively, 20/100 vision indicates that you'd need to be as near as 20 feet away to see what a normal-sighted person is able to see from 100 feet away. Obviously, if this was the case, it would mean that you would be pretty near sighted.
Your eyes are examined separately. When the optometrist instructs you to read the letters on the eye chart, the smallest row that you can read clearly determines the visual acuity of the eye that's being evaluated.
However 20/20 vision doesn't necessarily mean you have perfect vision, and that's because it only determines how clearly you see at a distance. There are other really crucial vision skills; the ability to focus on close objects, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these are all extremely important to your general vision. Also, a person who has 20/20 vision can still have unhealthy eyes. Even people who have damage to the retina from glaucoma, diabetes, high blood pressure, or a range of other conditions might still have 20/20 vision, without the help of glasses. This is why an optometrist always conducts a comprehensive eye exam, rather than just a regular eye chart exam.
When you're having your next eye exam, you'll know exactly why you're asked to read letters off an eye chart, and more!