The cornea and the lens of the eye focus light like a camera lens to form an image on the retina at the back of the eye. The cornea, where the light first enters the front of the eye, provides about two thirds of the eye's focusing power, and the lens inside the eye provides the other third.

Myopia (click for a larger images)
Myopic Eye / Profile	Eye following Excimer Laser Correction of Myopia / Profile
	Excimer Laser Correction of Myopia - Ablation Depth

Some eyes focus, or refract, the light too much, so the images of distant objects are formed in front of the retina, and the image is blurred. This condition is called nearsightedness, or myopia. Myopia usually starts in childhood and gets progressively worse through adolescence. It usually stops changing by the late teens, but it can sometimes get worse into the mid-twenties. In astigmatism the image is not evenly focused to a single point in front of the retina but the light rays are divided into two parts that focus along two lines with opposite orientations that are different distances from the retina.

Nearsightedness - myopia

Laser Vision Correction for Nearsightedness with Astigmatism (click for a larger images)
The astigmatic eye (front view).	The eye after laser vision correction (front view).
The eye during laser vision correction for nearsightedness with astigmatism.	The eye after laser vision correction for nearsightedness with astigmatism.

Nearsightedness can be corrected by any method that reduces the total refractive power of the eye. Astigmatism correction makes all of the rays of light focus at the same distance so that they all fall right on the retina. Eyeglasses and contact lenses do this by putting in front of the eye "negative" lenses that are thicker at the edge than in the center. LASIK corrects nearsightedness by flattening the central part of the cornea, and it corrects astigmatism by flattening the central cornea by different amounts at different orientations to correct for the uneven focus of the rays of light.

Farsightedness - hyperopia

In farsightedness (hyperopia), the image focuses beyond the retina. In our youth, the innate accommodating (focusing) power of the eye often compensates for farsightedness. But, as we age, our eyes become less able to accommodate. For this reason, farsightedness most commonly becomes a problem later in life. Many farsighted eyes do not need correction until the individuals reach their forties or fifties.

Hyperopia (click for a larger images)
Hyperopic Eye/Profile	Excimer laser correction of Hyperopia - Ablation Depth

Farsightedness can be corrected by any method that increases the total refractive power of the eye. Eyeglasses and contact lenses do this by putting in front of the eye "positive" lenses that are thicker in the center than at the edge. LASIK does it by making the central part of the cornea more steeply curved.

During a regular eye examination, your doctor uses lenses to measure your nearsightedness, astigmatism, or farsightedness in units called "diopters" (D). The VISX STAR™ Excimer Laser System is approved for photorefractive keratectomy (PRK) for the reduction or elimination of myopia (nearsightedness) between 0 and -12.0 D with up to -4.0 D of astigmatism, hyperopia (farsightedness) between +1.0 and +6.0 D with no more than 1.0 D refractive astigmatism, and hyperopia (farsightedness) between +0.5 and +5.0 D of sphere at the spectacle plane with refractive astigmatism from +0.5 to +4.0 D with a maximum manifest refraction spherical equivalent (MRSF.) of +6.0 D. The VISX STAR Excimer Laser System is also approved for laser assisted in situ keratomileusis (LASIK) for the reduction or elimination of nearsightedness (0 to -14.0 D) with or without astigmatism ranging from -0.5 D to -5.0 D. Safety and effectiveness data has not been established beyond these limits. PRK and LASIK are elective procedures with alternatives including eyeglasses, contact lenses, and other refractive surgeries.