To understand why and how laser vision correction works, it's best if you first understand how your eyes work, and what types of vision problems affect us.
How Your Eye Works Just like a camera, the eye works by focusing light rays.
Light entering the eye first passes through a transparent layer called the cornea.
The cornea acts as a lens by focusing the light.
Located behind the cornea is another lens, known as the crystalline lens, that further focuses the light to make a clear image on the retina at the back of the eye.
Finally, the image is transmitted to the brain by the optic nerve.
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 vision problem is the inability to focus incoming light precisely onto the retina.
The result is blurred vision.
There are four types of focusing errors:

• Nearsightedness
• Farsightedness
• Astigmatism
• Presbyopia

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 faraway 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 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, as we age, of the eye's ability to adjust the focus from far to near.
Presbyopia is a normal part of the aging process, affecting each and every person, and usually begins to cause a problem with near vision between the ages of forty and fifty.
It is corrected by the use of reading glasses or bifocals.
Presbyopia occurs because the crystalline lens no longer adequately adjusts its shape to focus clearly on close-up objects.
Though presbyopia is referred to as "farsightedness" by many people, it should not be confused with true farsightedness (hyperopia).
Presbyopia is caused by difficulty in changing the focus from far to up close - a problem with the crystalline lens.
True farsightedness is caused by too little focusing power in the eye a combination of the cornea and the crystalline lens.
Both presbyopia and hyperopia cause blurring of the close-up vision, though hyperopia can blur the distance vision as well.
Whereas presbyopia is an aging effect and begins to be experienced sometime after thirty-five years of age, hyperopia affects people of all ages, even young children.
Presbyopia can be present by itself or in combination with nearsightedness, farsightedness, or astigmatism.