It's safe to assume that you've stumbled upon the expressions visual acuity and 20/20 vision. As common as these terms are, do most people really know their meaning?
The term 20/20 is used to indicate the clarity of your eyesight from 20 feet away. If you have 20/20 vision, it means that from a distance of 20 feet you can 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 be 20 feet away. Obviously, in this scenario, you would be very near sighted.
Each eye is evaluated on its own. When the optometrist asks you to read the letters on the eye chart, the smallest row that you are able to read clearly indicates the visual acuity in the eye being evaluated.
However 20/20 eyesight actually doesn't mean you have perfect vision, and that's because it only determines how good your vision is at a distance. There are lots of equally important components to seeing well; your ability 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. And actually, someone who has 20/20 vision can certainly still have unhealthy eyes. Even people who have damage to the retina as a result of glaucoma, diabetes, high blood pressure, or numerous other conditions are still able to have 20/20 vision, without the help of glasses. This is why your eye care professional will always carry out a comprehensive eye exam, as opposed to just a plain visual acuity test.
During your next eye exam, you'll understand what we're testing for when we ask you to read letters from an eye chart!