It's safe to say that everyone has run into the expressions visual acuity and twenty-twenty vision. Still though, what do these terms really mean? When you really understand them, you will know why your eye doctor needs you to do more than simply read from the eye chart.
The term 20/20 vision actually refers to the clarity of your eyesight measured at a distance of 20 feet. If you've been told you have 20/20 eyesight, that means that from a distance of 20 feet you can clearly see what is normally seen from that distance. So, 20/100 eyesight indicates that to see what most people can see from 100 feet, you would have be as close as 20 feet away.
Each eye is evaluated on its own. When the optometrist instructs you to read the letters on the eye chart, the smallest row that you are able to read properly determines the visual acuity in the eye that's being tested.
It's important to recognize that 20/20 eyesight doesn't necessarily mean that your eyesight is perfect, because it only determines your distance vision. There are lots of equally necessary vision skills; the ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these all contribute to your overall vision. More importantly, someone with 20/20 vision can certainly have eye problems. Even those who have suffered damage to the retina from glaucoma, diabetes, high blood pressure, or numerous other diseases are still able to have 20/20 vision, without the help of glasses. And because of this, your optometrist always performs a comprehensive eye exam, rather than just a plain eye chart exam.
When you're having your next eye exam, you'll know what we're testing for when we ask you to read from an eye chart!