It's likely that you have run into the terms visual acuity and 20/20 vision. Yet, what do these terms actually mean?
20/20 vision is used to indicate the sharpness of your eyesight measured at a distance of 20 feet. When you have 20/20 vision, that means that from a distance of 20 feet you're able to clearly see what is normally seen from that distance. So, 20/100 eyesight indicates that to see what most people can see from 100 feet, you would have stand as close as 20 feet away.
Your eyes are examined separately. When your optometrist asks you to look at the eye chart and read out the letters, the smallest row that you can read accurately determines the visual acuity in the eye that's being examined.
However 20/20 vision doesn't always mean your vision is perfect, because it can only judge your distance vision. There are other really vital vision skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision – these are aspects of healthy vision. More importantly, someone who has 20/20 vision may have eye problems. Even people who have 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, your optometrist will always carry out a comprehensive eye exam, as opposed to just a simple eye chart exam.
The next time you find yourself having an eye test at the optometrist, you'll understand what we're looking for when we ask you to read letters aloud from an eye chart!