It's safe to assume that you've run into the terms visual acuity and twenty-twenty vision. But do people know what these terms really mean? Really understanding what they imply will give you insight into how an eye care professional assesses your vision during an eye exam.
20/20 actually refers to the accuracy of sight from 20 feet away. If you have 20/20 eyesight, it means that from a distance of twenty feet you can see what normal-sighted people can see from that distance. Are you aware that 20/20 isn't the best possible visual acuity? Actually, a considerable number of people have eyesight better than 20/20; for example, some people have 20/15 vision, so what they could see at 20 feet, a person with normal vision might only be able to discriminate as close as 15 feet.
Each eye is evaluated on its own. When you're asked to read the letters on the eye chart, the smallest row that you are able to read accurately indicates the visual acuity in the eye being evaluated.
20/20 eyesight doesn't always mean your vision is perfect, because, after all, it can only judge your distance vision. Other vital components to seeing properly; being able to focus on close objects, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these all contribute to your overall vision. More importantly, someone who has 20/20 vision can still have unhealthy eyes. Those with damage to the retina from diabetes, high blood pressure, glaucoma, or other conditions can still have 20/20 vision without needing to wear eye glasses. And because of this, your eye care professional will always carry out a comprehensive eye exam, and not just a plain visual acuity test.
When you're having your next eye exam, you'll understand what we're testing for when we ask you to read from an eye chart!