You have most likely run into the expressions 20/20 vision and visual acuity. As frequently used as these terms may be, do you actually grasp their meaning? When you understand what they imply, you will know why an eye doctor asks to assess more than just how well you read from the eye chart.
20/20 is used to indicate the sharpness of your vision from 20 feet away. If you've been told you have 20/20 eyesight, it means that from a distance of 20 feet you can see what should be seen from that distance. So, 20/100 vision would indicate that you'd need to be as close as 20 feet away to see what a person with normal vision is able to see from 100 feet away.
Each eye is evaluated separately. When your optometrist asks you to correctly read letters from the eye chart, the smallest letters you can properly see determine the visual acuity of the eye that's being evaluated.
20/20 eyesight doesn't always mean you have perfect vision, and that's because it only determines how good your eyesight is at a distance. Other necessary components to seeing accurately; the ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these are aspects of good vision. Also, a person who has 20/20 vision may have eye problems. Even those who have suffered damage to the retina due to diabetes, high blood pressure, glaucoma, or a range of other diseases can still have 20/20 vision without needing to wear eye glasses. For this reason, your optometrist will always carry out a comprehensive eye exam, as opposed to just a regular eye chart examination.
The next time you book yourself in for a comprehensive eye exam, you'll know exactly why you're being told to read letters off an eye chart, and more!