It's safe to assume that you've come across the expressions visual acuity and twenty-twenty vision. As common as these terms may be, do most people actually grasp what they mean? When you have a proper understanding of them, you will see why an eye care professional asks you to do more than simply read an eye chart.
The term 20/20 indicates the accuracy of sight from 20 feet away. When you have 20/20 eyesight, that means that from twenty feet away you're able to properly see what should be 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 person with normal vision would see from 100 feet away.
Each eye is tested separately. When you're asked to read the letters on the eye chart, the smallest letters you can properly see determine the visual acuity of the eye that's being examined.
It's important to recognize that 20/20 sight doesn't necessarily mean your vision is perfect, because it only assesses how sharply you see at a distance. There are other really vital sight skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these are aspects of healthy vision. And actually, a person who has 20/20 vision may have plenty of other eye-related health problems. Even people who have damage to the sensory nerves inside their eyes due to diabetes, high blood pressure, glaucoma, or other conditions can still have 20/20 vision without needing to wear eye glasses. And because of this, an eye care professional always conducts a comprehensive eye exam, rather than just a simple eye chart exam.
The next time you have a comprehensive eye exam, you'll know exactly why we're asking you to read letters from the eye chart, and more!