Encyclopedia - Visual Acuity

What is visual acuity?
How is visual acuity measured?

What is visual acuity?

Visual acuity is the medical term for sharpness of vision. It deals with the sharpness, or discrimination, of central vision, rather than the extent or clarity of peripheral vision. Refractive errors, which can be corrected with eyeglasses, are the most common cause of poor visual acuity. These include myopia, or nearsightedness; hyperopia, or farsightedness; and astigmatism. Myopia is a reduced ability to see distant objects clearly, hyperopia is a condition that initially causes difficulty in seeing nearby objects and progresses to affect distance vision, and astigmatism is blurred vision caused by abnormal curvature of the front surface of the cornea.

How is visual acuity measured?

Overall visual acuity is measured by using the Snellen Eye Chart, with the large E at the top followed by rows of letters where each row is smaller than the previous one. A chart using the letter E facing up, down, left, and right is used for children and those who do not read.

For the test, the eye chart is placed 20 feet from the person being examined. For someone wearing eyeglasses, the test first is performed without glasses. One eye is covered, and the person reads the lowest line on the chart that can be ascertained. Then the same procedure is followed to test the other eye. Last, the sequence is performed while the eyeglasses are worn.

A reading of 20/20 is normal vision, meaning that the smallest symbol that can be read at a distance of 20 feet is the same as the symbol that a person with normal vision would detect at that distance. A 20/40 reading means that the person can read at a distance of 20 feet what a person with normal vision reads at 40 feet. A person whose vision can be corrected to 20/200 vision in the better eye is considered legally blind.

Near visual acuity is measured with the Jaeger card, which has print samples of different sizes. The card is held 14 inches from the person’s eye for the test. A result of 14/20 means that the person can read at 14 inches what someone with normal vision can read at 20 inches. The results of visual acuity tests are used to prescribe eyeglasses or other corrective measures.