It's likely that you have heard the expressions visual acuity and 20/20 vision. Still though, do people understand what these terms really mean? When you really understand what they imply, you will know why your optometrist needs you to do more than simply read from the eye chart.
The term 20/20 is used to indicate the sharpness of your eyesight measured at a distance of 20 feet. If you've been told you have 20/20 vision, that basically means that from twenty feet away you can clearly see that which should be seen from that distance. And did you know that 20/20 is just a standard measurement? A large number of people have eyesight better than 20/20; for example, 20/15, so what they could see at 20 feet, a person with normal vision would only be able to discriminate at 15 feet.
Your eyes are tested separately. When you're asked to look at the eye chart and read out the letters, the smallest row that you are able to read properly indicates the visual acuity in the eye being tested.
However 20/20 vision actually doesn't mean you have perfect vision, and that's because it can only judge your distance vision. There are several other necessary components to seeing well; the ability to focus on close objects, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision – these are all very important to your overall vision. Also, a patient who has 20/20 vision can still have unhealthy eyes. Even those who have suffered damage to the retina due to glaucoma, diabetes, high blood pressure, or numerous other diseases might still have 20/20 vision, without the help of glasses. For this reason, an optometrist always conducts a comprehensive eye exam, and not just a plain visual acuity test.
So the next time you book yourself in for a comprehensive eye exam, you'll know exactly why you're being told to read letters from the eye chart, and more!