You've probably stumbled upon the terms 20/20 vision and visual acuity. As frequently used as these terms are, do you actually grasp what they mean? When you really understand them, you will get why an eye doctor asks to assess more than just how well you read from an eye chart.
20/20 vision actually refers to the accuracy of your eyesight from 20 feet away. If you've been told you have 20/20 vision, it means that from a distance of twenty feet you can see that which normal-sighted people can see from that distance. So, 20/100 vision indicates that to see what most people can see from 100 feet, you would have stand as close as 20 feet away. Obviously, if this was the situation, it would mean that you would be very near sighted.
Both eyes are tested one after the other. When your optometrist instructs you to look at the eye chart and read out the letters, the smallest letters you can properly see determine the visual acuity of the eye being evaluated.
20/20 eyesight doesn't necessarily mean your vision is perfect, because it can only judge how good your vision is at a distance. There are several other necessary components to seeing properly; your ability to focus on close objects, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these are all very important to your general vision. Also, someone who has 20/20 vision can certainly have plenty of other eye-related health problems. Even those who have suffered damage to the retina as a result of glaucoma, diabetes, high blood pressure, or a range of other diseases can still have 20/20 vision without glasses. For this reason, your optometrist will always carry out a comprehensive eye exam, and not just a regular visual acuity test.
The next time you find yourself at an eye exam, you'll know exactly why we're asking you to read letters off an eye chart, and more!