Eye Charts Explained

During an eye test, eye doctors use eye charts to measure how well you see in the distance, compared with other human beings.

The classic example of an eye chart is the Snellen eye chart, developed by Dutch eye doctor Hermann Snellen in the 1860s. There are many variations of the Snellen eye chart, but in general they show 11 rows of capital letters. The top row contains one letter (usually the "big E," but other letters can be used). The other rows contain letters that are progressively smaller.

During an eye exam, your eye doctor will ask you to find the smallest line of text letters that you can make out, and ask you to read it. If you can read the bottom row of letters, your visual acuity is very good.

Three types of chart:

- "Tumbling E" Eye Chart
- 20/20 Vision In An Eye Test Means
- Near Visual Acuity: The Jaeger Eye Chart

Eye charts measure visual acuity only. They do help your eye doctor figure out whether you need prescription eyeglasses or contact lenses for your distance vision. And they help the Department of Motor Vehicles to determine if you are required to use eyewear for driving, or if you shouldn't drive at all because you are legally blind.

But eye charts don't measure your peripheral vision, depth perception, color perception or ability to perceive contrast.

And they don't measure items related to the health of your eyes, such as your eye fluid pressure, whether you have glaucoma, how dry your eyes are or whether your retinas are in good shape.

So eye chart testing is just one component of a complete eye exam, which you should have every one or two years.