Have you ever had an eye exam? If so, your doctor has probably asked you to cover one eye and read letters from a special eye chart.

Eye charts usually contain several rows of letters. The top row begins with very large letters. The letters get smaller and smaller the closer the rows are to the bottom of the chart.

What does identifying your ABCs have to do with eyeglasses? Each line of the chart has been assigned a number. During the test, patients read each line of the chart until the letters become too small to read easily.

If you can read the line labeled 20 from a distance of 20 feet, this means you have 20/20 vision.

20/20 is considered normal vision. If you can only see the giant letter at the top of the chart labeled 20/200, you have 20/200 vision.

A score of 20/200 means you would need to stand much, much closer (20 feet) to read something most people can read from 200 feet away. People with vision scores of 20/200 or worse are considered legally blind.

There are many different medical reasons people wear glasses, but the simple answer is that glasses help people see better. Some people wear glasses for reading things that are far away or for driving.

Other people only wear glasses when they’re doing an up-close activity, like reading, writing or knitting. Some people wear glasses all the time.

People who have difficulty seeing objects that are far away are called “myopic” or “nearsighted.” This means they can easily see objects that are close to them, but when viewing objects in the distance, the image can appear fuzzy.

People who have difficulty reading and seeing things close-up are called “hyperopic” or “far-sighted.” Another condition that can cause people to need glasses is “astigmatism,” which occurs when part of the eye is irregularly shaped.

Thankfully, myopia, hyperopia and astigmatism can usually be corrected by wearing eyeglasses or contact lenses. Both eyeglasses and contact lenses use convex or concave lenses to bend the light entering the eye. This refocuses the eye, helping to correct vision problems.