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The eye functions much like a camera. The camera's lens, located in front, focuses the image on the film in the back of the camera. In the eye, there are two lenses-the cornea (outer lens) and crystalline lens (inner lens)-which focus the image on the retina, that lines the back wall of the eye (analogous to the film of a camera).

In the eye, the pupil acts like a camera shutter, gauging which light rays are allowed to enter the eye through the cornea. The electrical impulses are sent to the brain through the cable-like optic nerve. The eye takes the picture, and the brain develops it into the image you actually see.

The natural clarity by which some people see distance images without glasses or contact lenses indicates that the image is being focused on the retina. If the image is focused either in front or behind the retina, the image is blurry.

A major factor determining the success of the image focused on the retina is the length of the eyeball:

• If the eyeball is too long in length (shaped like an egg), then the focused image will be in front of the retina. This condition is called myopia (or nearsightedness).

• If the eyeball is too short in length (shaped like a Ping-Pong ball), the focused image will be behind the retina. This condition is called hyperopia (or farsightedness).

Both nearsightedness and farsightedness cause blurred vision.

Another condition that can blur vision is astigmatism, which indicates that the cornea is misshapen (like a football). Astigmatism can occur in combination with nearsightedness as well as with farsightedness. The ideal corneal shape is round (like a basketball).

Presbyopia, when the lens inside the eye can no longer focus on closer reading material (books, menus, shopping tags, etc.), is the reason most people over 40 must use bifocals and reading glasses. Presbyopia, to some degree, eventually happens to everyone.

Your Prescription

Eye doctors write prescriptions for glasses and contact lenses-usually a single number or three numbers-that indicate the condition of the eye:

• A minus sign (-) in front of the first number indicates nearsightedness
• A plus sign (+) in front of the first number indicates farsightedness
• The number itself indicates the degree of nearsightedness or farsightedness.

Mild to moderate nearsightedness shows in the range of -1 to -6 diopters (the units of measurement). Mild to moderate farsightedness generally falls within the range of +0.75 to +4.00 diopters.

The absence of a second or third number indicates there is no astigmatism. A second number verifies both the presence of astigmatism and the degree of it. Astigmatism may have either a minus sign or a plus sign.

The third number, called axis, indicates the direction of astigmatism: right and left eye is designated by OD and OS, respectively.

Two sample prescriptions for right eyes are:

OD -4.50
OD +2.25 -1.00 x 165