It's safe to assume that you've stumbled upon the expressions twenty-twenty vision and visual acuity. As common as these terms may be, do people really grasp what they mean? Having a proper understanding of these terms will help you appreciate how an eye specialist determines the state of your eyes during an eye exam.
The term 20/20 indicates the clarity of vision measured at a distance of 20 feet. If you've been told you have 20/20 eyesight, it means that from a distance of twenty feet you are able to accurately see what normal-sighted people can see from that distance. So, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have stand 20 feet away. Obviously, if this was the situation, you would be very near sighted.
Both eyes are tested one after the other. When your optometrist instructs you to correctly read letters from the eye chart, the smallest letters you can clearly read determine the visual acuity of the eye that's being examined.
But 20/20 vision doesn't always mean that your eyesight is totally free of flaws, because it can only judge how sharply you see at a distance. There are other really important sight skills; the ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these are aspects of healthy vision. Furthermore, a patient with 20/20 vision may still have unhealthy eyes. Even those who have suffered damage to the retina as a result of diabetes, high blood pressure, glaucoma, or a range of other conditions can still have 20/20 vision, without the help of glasses. For this reason, an optometrist will always carry out a comprehensive eye exam, rather than just a plain visual acuity exam.
The next time you find yourself at an eye exam, you'll understand why you need to read letters from the eye chart, and more!