It's safe to assume that you've run into the terms visual acuity and twenty-twenty vision. As common as these terms may be, do people actually grasp their meaning? Having a proper understanding of these terms will help you appreciate how an optometrist evaluates your eyes during an eye exam.
20/20 actually refers to the clarity and sharpness of vision from 20 feet away. When you have 20/20 vision, that basically means that from a distance of 20 feet you can see what is normally seen from that distance. 20/20 vision is just a standard measurement. Many people can even see better than 20/20; for example, some people have 20/15 vision, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate as close as 15 feet.
Your eyes are examined separately. During the part when you're asked to read the letters on the eye chart, the smallest row that you can read accurately determines the visual acuity of the eye being examined.
It's important to recognize that 20/20 vision actually doesn't mean your vision is perfect, because it can only judge how sharply you see at a distance. There are several other necessary vision skills; being able to focus on objects that are close by, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these are aspects of healthy vision. And actually, a patient who has 20/20 vision may still have unhealthy eyes. Those with damage to the retina from diabetes, high blood pressure, glaucoma, or a range of other conditions 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 regular visual acuity test.
The next time you find yourself at an eye exam, you'll know what we're looking for when we ask you to read letters from an eye chart!