You have most likely heard the terms 20/20 vision and visual acuity. Yet, what do these terms really mean? When you understand what they imply, you will know why an eye care professional asks to assess more than just how well you read an eye chart.
20/20 vision refers to the accuracy of your eyesight from 20 feet away. When you have 20/20 vision, that means that from a distance of twenty feet you're able to accurately see what should be seen from that distance. Did you know that 20/20 is just a standard measurement? Actually, a lot of people have vision that's better than 20/20; for example, some people have 20/15 vision, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate at 15 feet.
Each eye is evaluated on its own. During the part when you're asked to correctly read letters from the eye chart, the smallest letters you can clearly see determine the visual acuity in the eye that's being evaluated.
20/20 sight actually doesn't mean that your eyesight is flawless, because it only assesses your distance vision. There are lots of equally necessary vision skills; the ability to focus on objects that are close by, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision – these all contribute to your overall vision. More importantly, someone who has 20/20 vision can have eye problems. Even people who have damage to the retina due to diabetes, high blood pressure, glaucoma, or a range of other diseases are still able to have 20/20 vision without needing to wear eye glasses. This is why your optometrist always conducts a comprehensive eye exam, as opposed to just a simple eye chart test.
During your next eye exam, you'll know exactly why we're asking you to read letters off an eye chart, and more!