Just like a camera, the eye works by focusing light rays onto a film. Light entering the eye first passes through a transparent layer called the cornea which acts as a lens by focusing the light. Located behind the cornea is another lens, known as the crystalline lens. This lens further focuses the light to make a clear image on the 'film', known as the retina, at the back of the eye. Finally, the information is transmitted to the brain.
   
Just as a camera cannot produce a clear photograph if the incoming light is not focused precisely onto the film, so the eye cannot produce clear vision if the cornea and crystalline lens do not focus the light precisely onto the retina.
  
The most common problem with vision is the inability to focus incoming light precisely onto the retina. The result is blurred vision.

There are four types of focusing errors:

		Nearsightedness Nearsighted people see near objects better than those farther away. In nearsightedness (also known as "myopia"), light rays from distant objects are focused not onto the retina but in front of the retina. Nearsightedness occurs because the cornea and the crystalline lens together have too much focusing power for the length of the eye. If the cornea and the crystalline lens had less combined focusing power, or if the eye were shorter, then the light rays would be focused precisely onto the retina.
		Farsightedness Farsighted people see far away objects better than those that are near. Farsightedness (also known as "hyperopia") results when the cornea and the crystalline lens together have too little focusing power for the length of the eye. Light rays from distant objects are focused not onto the retina but behind the retina. If the cornea and the crystalline lens had more combined focusing power, or if the eye were longer, then the light rays would be focused precisely onto the retina.
		Astigmatism People whose eyes focus light rays unevenly have astigmatism. Astigmatism occurs when the cornea has an irregular shape. The cornea should be round and symmetrical like a basketball, but in cases of astigmatism it is shaped more like a football. People with astigmatism see both near and far objects out of focus. Astigmatism frequently accompanies nearsightedness or farsightedness.

Presbyopia
Presbyopia (which comes from the Greek for "old vision") refers to the gradual loss of the eye's ability to adjust the focus. A normal part of the aging process, presbyopia usually begins to occur between the ages of 40 and 50. It is corrected by the use of reading glasses or bifocals.

Back to About Lasik