Everyone has stumbled upon the terms twenty-twenty vision and visual acuity. As frequently used as these terms are, do most people actually grasp their meaning? When you have a proper understanding of these terms, you will know why an optometrist needs you to do more than simply read from the eye chart.
20/20 vision indicates the accuracy of vision from 20 feet away. If you've been told you have 20/20 eyesight, that basically means that from twenty feet away you can clearly see that which is normally seen from that distance. You may not know this, but 20/20 isn't the best possible visual acuity. A large number of people have vision that's better than 20/20; for example, some people have 20/15 vision, so what they could see at 20 feet, a person with normal vision would only be able to discriminate from 15 feet.
Each eye is tested separately. During the part when you're asked to read the letters on the eye chart aloud, the smallest row that you can read properly indicates the visual acuity of the eye being evaluated.
But 20/20 eyesight doesn't necessarily mean you have perfect vision, because, after all, it can only judge your distance vision. There are lots of equally important vision skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision – these are all extremely important to your overall eyesight. Also, someone with 20/20 vision can certainly still have unhealthy eyes. Even those who have suffered damage to the sensory nerves inside their eyes from glaucoma, diabetes, high blood pressure, or other conditions might still have 20/20 vision without glasses. This is why your optometrist will always carry out a comprehensive eye exam, and not just a simple visual acuity test.
The next time you find yourself at an eye exam, you'll understand why you're being told to read letters from the eye chart, and more!