It's likely that you have stumbled upon the terms 20/20 vision and visual acuity. But what do these terms truly mean? When you understand these terms, you will see why an eye doctor needs to assess more than just how well you read the eye chart.
20/20 vision refers to the clarity of eyesight measured at a distance of 20 feet. If you have 20/20 vision, that basically means that from a distance of 20 feet you can see what is normally seen from that distance. Did you know that 20/20 isn't the best possible visual acuity? Many people have vision that's better than 20/20; for example, vision that measures 20/15, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate as close as 15 feet.
Your eyes are examined separately. When the optometrist instructs you to read the letters on the eye chart, the smallest row that you are able to read without error indicates the visual acuity of the eye that's being evaluated.
But 20/20 vision doesn't necessarily mean that your eyesight is perfect, and that's because it only assesses your clarity of vision at a distance. There are other really important components to seeing clearly; being able to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these are aspects of healthy vision. More importantly, a patient who has 20/20 vision can certainly have eye problems. Even those who have suffered damage to the retina from glaucoma, diabetes, high blood pressure, or numerous other diseases can still have 20/20 vision without needing to wear eye glasses. And because of this, an optometrist will always carry out a comprehensive eye exam, rather than just a plain visual acuity test.
During your next eye exam, you'll understand what we're testing for when we ask you to read from an eye chart!