Your eye functions much like a camera. Light entering the eye first passes through the transparent cornea. The light is bent (refracted) by the cornea and the crystalline lens inside the eye. Light rays are then focused by the lens to make a clear image on the retina at the back of the eye. When the eye is working properly, a sharp image is formed on the retina, a layer of nerve cells lining the inside of the eyeball that can sense light. When the optical system of the eye does not focus properly, we say that the eye has a "refractive error". Refractive errors can be corrected with glasses, contact lenses, or surgery, to restore clear vision. This section describes the optics, anatomy, and the refractive errors of the eye.

Visual acuity is usually measured with a Snellen chart. The Snellen chart displays letters of progressively smaller size. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject see as 20 feet what a normal person sees at 40 feet. Another way of saying this is that a person with 20/40 vision has vision that is only half as good as normal - or, objects must be at half the normal distance for him to see them. A person with 20/20 vision is able to see letters 1/10th as large as someone with 20/200 vision. However, 20/15 vision is better than 20/20. A person with 20/15 vision can see objects at 20 feet that a person with 20/20 vision can only see at 15 feet.

Levels of Vision

20/20 - Normal vision. Fighter pilot minimum. Required to read the stock quotes in the newspaper, or numbers in the telephone book.

20/40 - Able to pass Driver's License Test in all 50 States. Most printed material is at this level.

20/80 - Able to read alarm clock at 10 feet. News Headlines are this size.

20/200 - Legal blindness. Able to see STOP sign letters.

Myopia (nearsightedness)
Approximately one in four Americans is nearsighted, totalling about 70 million people. Myopia or nearsightedness occurs because the overall shape of the eye is incorrect or the cornea does not have the proper curvature, thus making the image out of focus. When the eye is too big or the cornea is too steep, visual images are focused in front of the retina. Distant objects appear blurred or fuzzy because the light rays are not in focus by the time they reach the retina. The greater the myopia, the more the light rays converge and the more blurred distant objects are.

Myths: Myopia is not caused by reading at an early age, prolonged reading, reading in the dark, watching TV too closely, wearing glasses too strong, wearing glasses too weak, or other similar myths. However, myopia tends to run in families, so it is probably inherited. Myopia tends to start between the ages of 8 and 12 (earlier for high myopes), and increases as the eye grows in length during puberty. Changes in glasses or contact lens prescriptions are necessary during growth periods. It tends to stabilize after age 18.

Symptoms: People with myopia are unable to see things at a distance such as street signs, chalk boards, televisions, and clocks.

Treatment: The treatment for myopia includes lenses (glasses or contact lenses) that allow images to be focused on the retina. Usually after age of 18-21, the eye stabilizes. At this point, laser vision correction could become an option.

Hyperopia (Farsightedness)

Hyperopia is much less common than myopia. In hyperopia, the cornea is not steep enough and light rays hit the retina before they come into focus. Thus, vision images are focused behind the retina.

Symptoms: People with hyperopia can see objects at a distance, but have trouble seeing objects up close like books, newspapers, or magazines.

Treatment: Treatment for hyperopia include either glasses or contact lenses that is convex (thicker in the middle than on the edges). Corrective lenses should be worn for near tasks such as reading, but may not have to be used for tasks involving distance. Laser vision correction could also be an option.

Presbyopia (Literally called "old eye")

Normal vision occurs when the lens of the eye changes shape to accomodate for far away objects and near objects. The lens gets thicker for near objects and thinner for distant objects. By the age of 40, the lens start to lose its flexibility due to the onset of presbyopia.

Symptoms: People with presbyopia find it difficult to see things up close and might have a hard time switching from near to distant vision.

Treatment: Normally bifocal lenses are needed. Bifocal lenses allow the user to view distance objects through the top portion of their glasses, and near objects with magnifiers added to the bottom portion of their glasses. However a complete eye exam will be needed to ensure that the proper strength of the lenses allow for seeing at all distances.

Astigmatism
Astigmatism is quite common and is possibly present in 100% of the population to some degree. Just as people have different shaped hands, head, and feet, people also have different shaped corneas. The cornea should be curved equally in all directions. People with astigmatism have an abnormal curvature to the cornea that resembles the shape of a football rather than that of a basketball. Light rays focus at different places depending on their orientation, and objects at all distances appear blurred. Astigmatism may be accompanied by myopia or hyperopia.

Symptoms: Astigmatism may cause blurred vision, eye strain, or even headaches. It can also cause images to appear doubled, especially at night.

Treatment: Astigmatism can be corrected with glasses or contact lenses. Soft contact lens are not able to correct astigmatism unless they are a special type known as toric lens. Large amounts of astigmatism often can only be treated by rigid contact lenses. Laser vision correction could also be an option.