It's likely that you have heard the terms visual acuity and 20/20 vision. But what do these terms really mean? When you have a proper understanding of what they imply, you will get why an optometrist asks to assess more than just how well you read from an eye chart.
The term 20/20 vision indicates the clarity of your eyesight measured at a distance of 20 feet. When you have 20/20 eyesight, that means that from twenty feet away you are able to properly see that which normal-sighted people can see from that distance. So, 20/100 vision would indicate that to see what most people can see from 100 feet, you would have stand as close as 20 feet away.
Both eyes are tested one after another. When your optometrist instructs you to read the letters on the eye chart, the smallest letters you can clearly see determine the visual acuity of the eye that's being examined.
20/20 sight doesn't necessarily mean you have perfect vision, because it only assesses your distance vision. There are lots of equally crucial vision skills; the ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these are all really important to your general vision. More importantly, a person with 20/20 vision can certainly still have unhealthy eyes. Those with damage to the retina from glaucoma, diabetes, high blood pressure, or a range of other conditions are still able to have 20/20 vision without glasses. And because of this, your eye care professional always performs a comprehensive eye exam, and not just a regular visual acuity test.
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!