Everyone has heard the expressions visual acuity and 20/20 vision. Yet, what do these terms truly mean? When you really understand what they imply, you will get why an optometrist asks you to do more than simply read from the eye chart.
The term 20/20 refers to the clarity and sharpness of eyesight from 20 feet away. If you have 20/20 eyesight, it means that from 20 feet away you are able to clearly see that which is normally seen from that distance. To give an extreme example, 20/100 eyesight would indicate that you would need to be as close as 20 feet away to see what a normal-sighted person would see from 100 feet away. Obviously, in this scenario, you would be extremely near sighted.
Your eyes are tested separately. When you're asked to look at the eye chart and read out the letters, the smallest letters you can clearly read determine the visual acuity of the eye being examined.
20/20 sight doesn't necessarily mean your vision is perfect, because, after all, it can only judge how accurately you see at a distance. There are other really crucial components to seeing accurately; being able to focus on objects that are close by, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these all contribute to your overall ability to see. And actually, a patient who has 20/20 vision can have eye problems. Even those who have suffered damage to the retina due to glaucoma, diabetes, high blood pressure, or numerous other diseases are still able to have 20/20 vision without glasses. This is why an optometrist always conducts a comprehensive eye exam, as opposed to just a simple visual acuity exam.
When you're having your next eye exam, you'll understand why you're being told to read letters off an eye chart, and more!