You have most likely come across the expressions visual acuity and 20/20 vision. Yet, do people understand what these terms truly mean?
The term 20/20 actually refers to the clarity and sharpness of vision measured at a distance of 20 feet. If you have 20/20 vision, that means that from a distance of 20 feet you can clearly see that which normal-sighted people can see from that distance. So, 20/100 eyesight would indicate that you would need to be as close as 20 feet away to see what a person with normal vision would see from 100 feet away. Obviously, in this scenario, it would mean that you would be extremely near sighted.
Both eyes are examined one after another. When your optometrist instructs you to read the letters on the eye chart aloud, the smallest row that you are able to read clearly determines the visual acuity of the eye being examined.
However 20/20 sight actually doesn't mean you have perfect vision, because it only assesses how well you see at a distance. There are several other necessary sight skills; being able to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these all contribute to your overall ability to see. More importantly, someone with 20/20 vision may still have unhealthy eyes. Even those who have suffered damage to the retina due to diabetes, high blood pressure, glaucoma, or numerous other conditions are still able to have 20/20 vision, without the help of glasses. And because of this, an optometrist always performs a comprehensive eye exam, and not just a regular eye chart examination.
The next time you find yourself having an eye test at the optometrist, you'll know what we're looking for when we ask you to read from an eye chart!