Everyone has run into the terms visual acuity and 20/20 vision. As common as these terms are, do people really know what they mean?
20/20 vision refers to the clarity and sharpness of your vision measured at a distance of 20 feet. If you have 20/20 eyesight, it means that from a distance of twenty feet you can clearly see what should be seen from that distance. Alternatively, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have to stand only 20 feet away. Obviously, if this was the case, you would be extremely near sighted.
Your eyes are tested separately. 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 being evaluated.
But 20/20 sight actually doesn't mean your vision is perfect, because it can only judge how accurately you see at a distance. There are lots of equally vital vision skills; being able to focus on close objects, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision – these are aspects of healthy vision. And actually, a patient who has 20/20 vision may have plenty of other eye-related health problems. Even people who have damage to the nerves inside their eyes due to diabetes, high blood pressure, glaucoma, or a range of other diseases can still have 20/20 vision without needing to wear eye glasses. This is why your optometrist should always perform a comprehensive eye exam, as opposed to just a regular visual acuity test.
The next time you find yourself at an eye exam, you'll understand why you're asked to read letters from the eye chart, and more!