It's safe to assume that you've come across the expressions twenty-twenty vision and visual acuity. As common as these terms are, do most people really know what they mean? Having a proper understanding of them will enlighten you as to how an eye specialist determines the state of your vision during an eye exam.
The term 20/20 vision actually refers to the clarity of your vision from 20 feet away. When you have 20/20 eyesight, that means that from a distance of twenty feet you are able to accurately see that which normal-sighted people can see from that distance. You may not know this, but 20/20 is really just a standard measurement. Many people can even see 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 might only be able to discriminate from 15 feet.
Each eye is tested on its own. When your optometrist instructs you to correctly read letters from the eye chart, the smallest letters you can clearly see determine the visual acuity of the eye being examined.
But 20/20 eyesight doesn't always mean that your eyesight is totally free of flaws, because it can only judge your distance vision. There are lots of equally vital sight skills; being able to focus on objects that are close by, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these are all very important to your general eyesight. Furthermore, someone with 20/20 vision may still have unhealthy eyes. Even those who have suffered damage to the nerves inside their eyes due to diabetes, high blood pressure, glaucoma, or other conditions are still able to have 20/20 vision, without the help of glasses. This is why an eye care professional always performs a comprehensive eye exam, rather than just a regular 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 from an eye chart!