It's safe to assume that you've stumbled upon the terms visual acuity and 20/20 vision. Yet, what do these terms actually mean? Really understanding what they imply will give you insight into how your optometrist determines the state of your vision during an eye exam.
The term 20/20 vision is used to indicate the clarity and sharpness of your eyesight from 20 feet away. If you've been told you have 20/20 vision, that means that from twenty feet away you are able to accurately see that which should be seen from that distance. Alternatively, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have be 20 feet away.
Both eyes are tested one after the other. During the part when you're asked to look at the eye chart and read out the letters, the smallest letters you can properly read determine the visual acuity in the eye that's being evaluated.
However 20/20 sight doesn't necessarily mean you have perfect vision, because, after all, it can only judge your distance vision. There are other really vital sight skills; the ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are all extremely important to your general vision. Furthermore, a patient who has 20/20 vision may have unhealthy eyes. People with damage to the retina as a result of diabetes, high blood pressure, glaucoma, or numerous other conditions are still able to have 20/20 vision without glasses. And because of this, your optometrist always performs a comprehensive eye exam, as opposed to just a plain eye chart test.
During your next eye exam, you'll know exactly why you need to read letters off an eye chart, and more!