Everyone has come across the terms visual acuity and twenty-twenty vision. As common as these terms may be, do people really know their meaning?
20/20 vision is used to indicate the accuracy of eyesight measured at a distance of 20 feet. If you've been told you have 20/20 vision, it means that from a distance of 20 feet you are able to properly see what normal-sighted people can see from that distance. And did you know that 20/20 is really just a standard measurement? A large number of people can even see better than 20/20; for instance, vision that measures 20/15, so what they could see at 20 feet, a person with normal vision might only be able to discriminate as close as 15 feet.
Each eye is tested on its own. During the part when you're asked to read the letters on the eye chart, the smallest letters you can clearly see determine the visual acuity of the eye being evaluated.
But 20/20 eyesight doesn't necessarily mean your vision is perfect, because it only indicates your distance vision. There are lots of equally vital sight skills; being able to focus on objects that are close by, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision – these are aspects of healthy vision. Also, someone with 20/20 vision can certainly still have unhealthy eyes. People with damage to the nerves inside their eyes from diabetes, high blood pressure, glaucoma, or numerous other conditions are still able to have 20/20 vision without glasses. This is why your optometrist always conducts a comprehensive eye exam, and not just a simple eye chart exam.
When you're having your next eye exam, you'll understand what we're testing for when we ask you to read letters aloud from an eye chart!