It's safe to say that everyone has stumbled upon the terms visual acuity and twenty-twenty vision. But what do these terms actually mean? When you have a proper understanding of them, you will see why your optometrist asks you to do more than simply read the eye chart.
The term 20/20 indicates the sharpness of your eyesight measured at a distance of 20 feet. If you have 20/20 eyesight, it means that from a distance of 20 feet you're able to clearly see what normal-sighted people can see from that distance. To give an extreme example, 20/100 vision would indicate that to see what most people can see from 100 feet, you would have to stand only 20 feet away. Obviously, if this was the situation, you would be extremely near sighted.
Both eyes are tested one after the other. When the optometrist asks you to correctly read letters from the eye chart, the smallest row that you are able to read accurately indicates the visual acuity of the eye that's being evaluated.
However 20/20 eyesight doesn't always mean that your eyesight is totally free of flaws, because it can only judge your distance vision. There are other really vital sight skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these also contribute to your overall ability to see. Furthermore, someone with 20/20 vision can certainly have plenty of other eye-related health problems. Even people who have damage to the nerves within their eyes due to diabetes, high blood pressure, glaucoma, or other conditions might still have 20/20 vision, without the help of glasses. And because of this, an eye care professional always performs a comprehensive eye exam, as opposed to just a simple visual acuity test.
When you're having your next eye exam, you'll know exactly why you need to read letters off an eye chart, and more!