When it comes to eye exams, the ophthalmologist will ask you to look for the smallest line of letters you can read and then ask you to read it. This is done to measure your visual acuity, or sharpness. If you can read the bottom row of letters, your visual acuity is very good. But eye charts don't measure peripheral vision, depth perception, color vision, or the ability to perceive contrast.
To assess your near vision, the ophthalmologist may use a small portable card called a Jaeger eye chart. During an eye test, ophthalmologists use eye charts to measure vision at a certain distance and compare it with that of other human beings. The rotating E chart has the same scale as a standard Snellen eye chart, except that all the characters on the chart are a capital E, rotated in different 90 degree increments. The classic example of an eye test is the Snellen eye chart, developed by Dutch ophthalmologist Hermann Snellen in the 1860s.
Whenever acuity needs to be carefully evaluated (such as in an ophthalmologist's exam), or when there is a possibility that the person being examined may try to deceive the examiner (such as in a motor vehicle licensing office), equipment is used that can present the letters in a variety of random patterns. In the United States, the standard eye chart location is on a wall that is 20 feet away from the eyes. The Snellen eye chart is read by covering one eye and reading aloud the letters on the chart, starting at the top and moving down. A young child has an eye test and doesn't know the alphabet or is too shy to read the letters aloud.
There are several versions of the Snellen eye chart for people who cannot read the letters of the alphabet. The next time you sit in your optometrist's office chair, you'll understand why you need to read the letters on the chart in front of you and what the results mean for your vision. The Snellen eye chart is used to measure visual acuity, which refers to how well you can see without glasses or contact lenses. If you're able to read the final result, you're probably a good guesser.