It's likely that you have stumbled upon the expressions twenty-twenty vision and visual acuity. As common as these terms may be, do people really grasp their meaning? Understanding them will enlighten you as to how your eye care professional determines the state of your eyes during your eye exam.
20/20 vision is used to indicate the clarity of your eyesight from 20 feet away. If you've been told you have 20/20 eyesight, that basically means that from a distance of 20 feet you can see that which normal-sighted people can see from that distance. You may not know this, but 20/20 vision is just a standard measurement. A large number of people can even see better than 20/20; for instance, 20/15, so what they would be able to see at 20 feet, a person with normal vision might only be able to discriminate as close as 15 feet.
Your eyes are tested separately. During the part when you're asked to read the letters on the eye chart aloud, the smallest letters you can properly see determine the visual acuity of the eye being tested.
But 20/20 vision doesn't necessarily mean your vision is perfect, and that's because it only determines how accurately you see at a distance. There are lots of equally crucial vision skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are aspects of healthy vision. More importantly, a patient who has 20/20 vision can 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 other diseases are still able to have 20/20 vision, without the help of glasses. This is why your optometrist should always conduct a comprehensive eye exam, and not just a regular visual acuity examination.
The next time you have a comprehensive eye exam, you'll understand why we're asking you to read letters from the eye chart, and more!