Visual acuity tests are used to evaluate eyesight. Several types of visual acuity tests may be used.
The Snellen test checks your ability to see at distances. It uses a wall chart that has several rows of letters. The letters on the top row are the largest; those on the bottom row are the smallest. See a picture of a Snellen chart .
You will stand or sit 20 ft (6 m) from the chart and be asked to cover one eye and then read the smallest row of letters you can see on the chart. If you are unable to cover your eye, an eye patch will be placed over your eye.
Each eye is tested separately. You may be given a different chart or asked to read a row backward to make sure that you did not memorize the sequence of letters from the previous test.
If you wear glasses or contacts, you may be asked to repeat the test on each eye while wearing them.
Let your health professional know if you have trouble reading the letters on one side of the row, or if some letters disappear while you are looking at other letters. You may have a visual field problem, and visual field tests may be needed.
The E chart tests the vision of children and people who cannot read. The E chart is similar to the Snellen chart in that there are several rows, but all of the rows contain only the letter E in different positions. The top row is the largest and the bottom row of Es is the smallest. You will be asked to point in the same direction as the lines of the E. Similar charts use the letter C or pictures. These charts are also available in a handheld card. See a picture of an E chart .
The Near test uses a small card (Jaeger chart) containing a few short lines or paragraphs of printed text to test your near vision. The size of the print gradually gets smaller. You will be asked to hold the card about 14 in. (36 cm) from your face and read aloud the paragraph containing the smallest print you can comfortably read. Both eyes are tested together, with and without corrective lenses. This test is routinely done after age 40, because near vision tends to decline as you age (presbyopia).
If you cannot read any of the letters or print on these charts because of poor vision, your visual acuity will be tested by other techniques, such as counting fingers, detecting hand movements, or distinguishing the direction or perception of light sources (such as room light or a penlight held up close to the face).
Visual acuity tests usually take about 5 to 10 minutes. They may be performed by a nurse, a medical assistant, an ophthalmologist, an optometrist, a teacher, or some other trained person. Testing may be done at a doctor's office, school, workplace, health fair, or elsewhere.
We all want great vision. Why? So that we can have a competitive advantage. In prehistoric times it gave us the edge to see the wild animals earlier. As society became more organized a method of comparing vision was needed. The credit for the most commonly used method goes to Snellen. Hence the chart which displays the letters or numbers bears his name. It compares a person's vision to the that of the average for the population. 
