It's likely that you have stumbled upon the terms twenty-twenty vision and visual acuity. As common as these terms may be, do you really grasp their meaning? Understanding these terms will help you appreciate how an optometrist determines the state of your vision in an eye exam.
20/20 vision indicates the clarity and sharpness of vision from 20 feet away. If you have 20/20 vision, it means that from a distance of twenty feet you can see what is normally seen from that distance. Are you aware that 20/20 is really just a standard measurement? Many people have eyesight better than 20/20; for example, some people have 20/15 vision, so what they could see at 20 feet, a person with normal vision might only be able to discriminate as close as 15 feet.
Both eyes are tested one after the other. During the part when you're asked to look at the eye chart and read out the letters, the smallest letters you can clearly read determine the visual acuity in the eye being evaluated.
It's important to recognize that 20/20 sight doesn't necessarily mean you have perfect eyesight, because, after all, it only indicates your distance vision. Other vital sight skills; being able to focus on objects that are close by, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are aspects of good vision. Also, a person who has 20/20 vision can have plenty of other eye-related health problems. Even people who have damage to the retina from diabetes, high blood pressure, glaucoma, or a range of other conditions are still able to have 20/20 vision. For this reason, your optometrist should always perform a comprehensive eye exam, rather than 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 from the eye chart, and more!