You've probably run into the expressions twenty-twenty vision and visual acuity. As frequently used as these terms may be, do you really grasp what they mean?
20/20 vision actually refers to the clarity of sight measured at a distance of 20 feet. If you have 20/20 vision, it means that from a distance of twenty feet you're able to clearly see what should be seen from that distance. To give an extreme example, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have stand 20 feet away. Obviously, in this scenario, it would mean that you would be pretty near sighted.
Each eye is evaluated on its own. When the optometrist instructs you to read the letters on the eye chart aloud, the smallest letters you can clearly see determine the visual acuity in the eye being tested.
However 20/20 eyesight actually doesn't mean you have perfect eyesight, because, after all, it can only judge your clarity of vision at a distance. There are several other crucial components to seeing accurately; being able to focus on close objects, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these are aspects of healthy vision. Also, a person with 20/20 vision can have unhealthy eyes. Those with damage to the retina from glaucoma, diabetes, high blood pressure, or a range of other diseases might still have 20/20 vision without needing to wear eye glasses. This is why an optometrist always performs a comprehensive eye exam, and not just a plain visual acuity exam.
When you're having your next eye exam, you'll know what we're testing for when we ask you to read letters from an eye chart!