June 18, 2009 
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.
What 20/20 Vision Means
In the United States, the standard placement of the eye chart is on a wall that’s 20 feet away from your eyes. Since many eye doctors’ offices don’t have rooms that are 20 feet long, in a smaller room the eye chart may hang behind the patient chair, using mirrors to make it appear in front of you at a simulated distance of 20 feet.
20/20 vision (or really, 20/20 visual acuity) is considered “normal” vision, meaning you can read at 20 feet a letter that most human beings should be able to read at 20 feet.
Eye charts can be configured in various ways, but generally, if you can read the big E at the top but none of the letters lower than that, your vision is considered 20/200. That means you can read at 20 feet a letter that people with “normal” vision can read at 200 feet. So at 20/200, your visual acuity is very poor.
In the United States you are considered “legally blind” if your best-corrected visual acuity (meaning, your best distance vision with eyeglasses or contact lenses) is 20/200 or worse.
To get a driver’s license in most of the United States, your best-corrected visual acuity must be at least 20/40.
Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity, but many animals do, especially birds of prey, which have been estimated to have an acuity of 20/5 or even better.
The “Tumbling E” Eye Chart
In these situations, a modification of the Snellen eye chart called a “tumbling E” chart may be used. The tumbling E chart has the same scale as a standard Snellen eye chart, but all characters on the chart are a capital letter “E,” in different spatial orientations (rotated in increments of 90 degrees).
The eye doctor asks the person being tested to use either hand (with their fingers extended) to show which direction the “fingers” of the E are pointing: right, left, up or down.
Studies have shown that visual acuity measurements using a tumbling E chart are virtually the same as those obtained from testing with a standard Snellen eye chart.
Eye Chart Limitations
Eye charts measure visual acuity only, so they are just one part of a complete eye exam. They do help your eye doctor figure out whether you need prescription eyeglasses or contact lenses for your distance vision.
But eye charts don’t measure your peripheral vision, depth perception, color perception or ability to perceive contrast. Also they don’t measure items related to the health of your eyes, such as your eye fluid pressure, how dry your eyes are or whether your retinas are in good shape.
Taken from: http://www.allaboutvision.com/
In some cases a standard Snellen eye chart cannot be used. One example is when the person having an eye exam is a young child who doesn’t know the alphabet or is too shy to read letters aloud. Other examples include when the person is illiterate or has a handicap that makes it impossible for him to cognitively recognize letters or read them aloud.