Everyone has come across the expressions visual acuity and twenty-twenty vision. As frequently used as these terms are, do people really know what they mean? When you understand them, you will know why your eye care professional needs to assess more than just how well you read an eye chart.
The term 20/20 vision actually refers to the clarity and sharpness of sight measured at a distance of 20 feet. When you have 20/20 vision, that means that from a distance of 20 feet you are able to clearly see what is normally seen from that distance. So, 20/100 vision would indicate that to see what most people can see from 100 feet, you would have stand 20 feet away.
Each eye is tested on its own. During the part when you're asked to read the letters on the eye chart aloud, the smallest letters you can properly read determine the visual acuity of the eye that's being evaluated.
It's important to recognize that 20/20 sight actually doesn't mean you have perfect vision, because, after all, it can only judge your distance vision. There are lots of equally necessary components to seeing accurately; the ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are all extremely important to your overall eyesight. More importantly, a patient who has 20/20 vision can certainly have unhealthy eyes. People with damage to the retina from glaucoma, diabetes, high blood pressure, or numerous other conditions might still have 20/20 vision. This is why your optometrist always conducts a comprehensive eye exam, and not just a simple visual acuity exam.
When you're having your next eye exam, you'll understand why you're being told to read letters off an eye chart, and more!