You have most likely come across the expressions 20/20 vision and visual acuity. As frequently used as these terms are, do people actually know what they mean?
The term 20/20 actually refers to the sharpness of sight from 20 feet away. If you have 20/20 vision, that basically means that from a distance of twenty feet you can clearly see what normal-sighted people can see from that distance. To give an extreme example, 20/100 eyesight indicates that you'd need to be as near as 20 feet away to see what a person with normal vision is able to see from 100 feet away.
Both eyes are tested one after another. When your optometrist instructs you to look at the eye chart and read out the letters, the smallest row that you are able to read clearly determines the visual acuity in the eye being tested.
It's important to recognize that 20/20 eyesight actually doesn't mean that your eyesight is perfect, and that's because it can only judge how well you see at a distance. There are several other necessary components to seeing well; being able to focus on close objects, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are aspects of good vision. And actually, a person who has 20/20 vision may still have unhealthy eyes. Even people who have damage to the retina from glaucoma, diabetes, high blood pressure, or a range of other diseases can still have 20/20 vision without needing to wear eye glasses. This is why your eye care professional should always conduct a comprehensive eye exam, as opposed to just a simple eye chart examination.
When you're having your next eye exam, you'll understand what we're testing for when we ask you to read letters aloud from an eye chart!