Encyclopedia - Refractive Error

What makes the eye see?
How does the shape of the cornea affect vision?
What does an eye glass prescription mean?

Refractive error occurs because light rays are not properly focused on the eye's retina. To understand refractive error, it is first necessary to understand the visual function of the eye.

What makes the eye see?

The eye works like a camera with two lenses. The first lens is the cornea, a clear membrane that covers the front of the eye. The second lens is the eye's natural crystalline lens, which is located behind the pupil. The cornea is responsible for about 70 percent of the eye's focusing power, while the natural lens "fine-tunes" the image before it is focused on the retina at the back of the eye. The retina, like the film in a camera, receives light images and sends them through the optic nerve to the brain(See Anatomy of the Eye). If both lenses are working properly, the image is focused precisely on the surface of the retina, and you have perfect "20/20" vision.

How does the shape of the cornea affect vision?

Refractive error normally falls into one or more of the following categories: myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. If the cornea is too steep, light rays are focused in front of the retina, resulting in myopia. This condition calls for a corrective lens that is concave in shape. If the cornea is too flat, light rays are theoretically focused behind the retina, resulting in hyperopia. A convex lens is used to correct this condition. If the front of the cornea is unevenly curved, it causes the light rays to fall on different spots on the retina, resulting in astigmatism. A special cylindrical lens is used to correct astigmatism.

What does an eyeglass prescription mean?

An eyeglass prescription defines the amount of refractive error with a minus sign (-) for nearsightedness, a plus sign (+) for farsightedness, and a special cylinder reading (cyls, cx or x) for astigmatism. The measurement unit for defining the amount of refractive error is called a diopter, usually expressed as a "D" in a prescription. For instance, a "-4.00 D" indicates 4 diopters of nearsightedness, while a "+2.00 D" would indicate that the patient has 2 diopters of farsightedness. Astigmatism is also expressed in diopters, and it includes a cylindrical measurement in degrees, i.e., "-2.00 D cx 180." This cylindrical correction in a prescription lens compensates for the uneven curvature of the front of your eye.

If you need bifocals, your prescription will include the words "Add for near," followed by a prescription for the near (reading) addition to the lens.