You've probably stumbled upon the terms 20/20 vision and visual acuity. But do people understand what these terms actually mean? Really understanding these terms will give you insight into how your eye specialist determines the state of your eyes in an eye exam.
20/20 vision is used to indicate the accuracy of sight from 20 feet away. If you've been told you have 20/20 vision, that basically means that from 20 feet away you can see what is normally seen from that distance. You may not know this, but 20/20 vision isn't the best possible visual acuity. A large number of people have eyesight better than 20/20; for example, 20/15, so what they could see at 20 feet, a person with normal vision would only be able to discriminate as close as 15 feet.
Each eye is tested separately. 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 in the eye that's being examined.
However 20/20 vision doesn't always mean your vision is perfect, because it only determines your clarity of vision at a distance. There are several other important components to seeing well; being able to focus on objects that are close by, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision – these also contribute to your overall vision. Also, someone who has 20/20 vision can certainly still have unhealthy eyes. People with damage to the nerves inside their eyes from glaucoma, diabetes, high blood pressure, or a range of other conditions are still able to have 20/20 vision without glasses. For this reason, an optometrist always conducts a comprehensive eye exam, rather than just a regular visual acuity test.
The next time you find yourself having an eye test at the optometrist, you'll know exactly why we're asking you to read letters off an eye chart, and more!