Visual acuity is measured using a Snellen chart or other similar test, by counting fingers etc. It often is expressed as a fraction, such as 20/20 or 20/200. What do these numbers mean? Is 20/200 or 20/400 better? Is 20/20 vision really perfect vision?

According to Medterms.com, an on-line dictionary of common medical terms, visual acuity is "the clarity or clearness of the vision; the ability to distinguish details and shapes of objects; also called central vision."

Visual acuity is most often measured by the use of a Snellen chart. Anyone who has visited an ophthalmologist is familiar with this test. First invented over 150 years ago by Dutch ophthalmologist Dr. Hermann Snellen, this chart combines numbers and letters to measure central vision. Here's how it works:

Block characters are arranged on the chart in rows, beginning with the largest letter at the top. Each row gets progressively smaller, and each row contains more letters or numbers. The original Snellen chart consisted of only letters, but today's versions also contain numbers.

It is commonly believed that perfect vision is 20/20, as measured on the Snellen chart. This is a myth. The person who has 20/20 vision can see at 20 feet what the average person sees at 20 feet. There is a line on the Snellen chart that corresponds to 20/20 vision; if that line is the lowest line on the chart that a person can read, then that individual has 20/20 vision.

There are two lines below 20/20 on the chart. Many people can read the letters on these lines, and so have vision better than the 20/20.

A person is considered to be legally blind if he cannot read lines smaller than the 20/200 line, the largest letter on the chart. This means that the person sees at 20 feet what the average person can see at 200 feet. Most legally blind people are not totally blind.

If the individual cannot read the 20/200 line on the Snellen chart, the ophthalmologist will ask him to count fingers at specific distances. The visual acuity is then reported as "cf (count fingers) at 3 feet" or "cf at 10 feet" etc. If the person cannot count fingers, his vision will be reported as "light perception" if he can tell when a light is shining into the eyes, or as "totally blind" if he cannot tell when the light is on.

If the person cannot read, as in the case of a very young child, tests similar to the Snellen chart, but using different characters, are available. An example is the Tumbling E chart. E's are arranged on the chart with the legs pointing in different directions (up, down, left and right); large E's are at the top, and the E's get smaller on each subsequent row to correspond to the rows on the Snellen chart. The individual is asked to tell or show in which direction the legs of the E's are pointing.

Because visual acuity is only a measure of the clarity or sharpness of central vision, a person may see 20/20, as measured on the Snellen chart, and still have severe vision problems. The Snellen chart and similar tests are only one way in which the ophthalmologist measures vision, diagnoses problems, and prescribes glasses.