It's safe to say that everyone has come across the terms visual acuity and twenty-twenty vision. As frequently used as these terms may be, do you actually grasp what they mean? When you really understand these terms, you will get why an eye doctor needs to assess more than just how well you read an eye chart.
The term 20/20 vision actually refers to the accuracy of your eyesight measured at a distance of 20 feet. If you have 20/20 vision, that basically means that from a distance of 20 feet you can clearly see what is normally seen from that distance. To give an extreme example, 20/100 eyesight would indicate that you'd need to be as near as 20 feet away to see what a normal-sighted person would see from 100 feet away.
Each eye is tested on its own. When the optometrist instructs you to correctly read letters from the eye chart, the smallest row that you can read accurately indicates the visual acuity in the eye being examined.
20/20 vision doesn't necessarily mean you have perfect vision, because, after all, it only assesses your distance vision. There are lots of equally necessary sight skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these all contribute to your overall vision. More importantly, a person who has 20/20 vision may have unhealthy eyes. Even people who have damage to the retina due to diabetes, high blood pressure, glaucoma, or numerous other conditions might still have 20/20 vision without needing to wear eye glasses. This is why your eye care professional will always carry out a comprehensive eye exam, as opposed to just a plain eye chart exam.
The next time you find yourself at an eye exam, you'll understand why you're asked to read letters off an eye chart, and more!