You have most likely heard the expressions twenty-twenty vision and visual acuity. But do people understand what these terms actually mean? When you understand what they imply, you will see why an optometrist asks you to do more than simply read from the eye chart.
20/20 vision is used to indicate the accuracy of sight measured at a distance of 20 feet. If you have 20/20 vision, that means that from a distance of 20 feet you can 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 stand as close as 20 feet away.
Both eyes are examined one after another. When the optometrist asks you to read the letters on the eye chart aloud, the smallest row that you can read accurately determines the visual acuity in the eye that's being evaluated.
20/20 sight doesn't necessarily mean you have perfect eyesight, because it only determines your distance vision. Other crucial sight skills; the ability to focus on close objects, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these are all really important to your general vision. Also, a person with 20/20 vision can certainly have unhealthy eyes. People with damage to the sensory nerves within their eyes from glaucoma, diabetes, high blood pressure, or other diseases can still have 20/20 vision. For this reason, your optometrist will always carry out a comprehensive eye exam, rather than just a simple eye chart test.
The next time you have a comprehensive eye exam, you'll understand what we're testing for when we ask you to read letters from an eye chart!