Vision Terms

Astigmatism

Astigmatism distorts or blurs vision for objects at any distance. Astigmatic vision is similar to the "funhouse" mirrors in which you appear too tall, too wide, or too thin. Large amounts of astigmatism are usually inherited, may be present a birth, and frequently remain unchanged throughout life. Small amounts of astigmatism are very common, and may be acquired any time in life. They often do not require correction with glasses or contact lenses.

The cornea is the clear dome that covers the colored part of the eye (iris). It is similar to a watch crystal. In the ideal case, the cornea is a perfect hemisphere, similar to the dome in a church. In many cases, the cornea is not a perfect hemisphere. It is elongated into an oval shape, like the dome over a football field. This elongation causes astigmatism.

A basic principle of optics is that curvature determines focusing power. A flat glass window has no curvature. It has no focusing power. The window does not distort our view as we look through it. A magnifying glass is curved. It makes object appear larger. The magnifying glass, because it is curved, has focusing power. Since it is round, the focus comes to a single sharp, clear point. If the magnifying glass were oval, it would produce an elongated spot which would result in a distorted image.

In astigmatism, the curvature of the cornea, and therefore its focusing power, is not the same in the horizontal, and vertical directions. Light rays entering the eye do not focus at a single point.

Farsightedness (Hyperopia)

In the farsighted eye, light is focused at a point beyond the retina, causing blurry close-up vision. People who are farsighted first have trouble with things held close to the eye like reading material, especially fine print. Then, as the years go by, they have difficulty with intermediate range like TV, and eventually, they need glasses to see things far away like street signs.

Nearsightedness (Myopia)

In the nearsighted eye, light is focused at a point in front of the retina, causing blurry distance vision. People who are nearsighted can usually see well close-up, but have trouble seeing things that are far away such as road signs or the TV. Nearsightedness can be treated with glasses, contact lenses or laser vision correction.

Old-Sightedness (Presbyopia)

As the normal eye gets to be forty years old, it develops old-sightedness (presbyopia), which makes it difficult to see reading material, but things at a distance like TV and street signs stay in focus. Old-sightedness should not be confused with farsightedness. The difference is that as a farsighted eye gets older, everything is out of focus whether it's up close like a book or far away like a street sign. The only treatment for old-sightedness is to purposely make one eye nearsighted. This is called monovision.