Eye Basics – How does the eye see?

The eye is like a camera that has two lenses. One of these lenses is the crystalline lens inside the eye. The other lens is the cornea, which is positioned in the eye like a watch crystal but actually provides 2/3 of the eye’s focusing power. When the light rays that enter the eye are perfectly focused on the retina, then the eye is neither nearsighted nor farsighted. This condition is called “emmetropia.”


Emmetropia focuses light on the retina.


What are refractive errors of the eye?


Myopia, or nearsightedness, occurs when the eye has too much focusing power. Therefore, objects at a distance are blurry, whereas objects that are close can be clearly seen (unless astigmatism is present). Typically, the myopic eye is slightly longer or has a steeper cornea compared to the normal eye.

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Myopia focuses light in front of the retina.


Hyperopia, or farsightedness, occurs when the eye lacks sufficient focusing power. This usually causes objects at a distance to appear blurry, and near objects are seen even less clearly. Typically, the farsighted eye is slightly shorter or has a flatter cornea than the normal eye.

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Hyperopia focuses light behind the retina.


Astigmatism is a condition in which light rays in different orientations are brought to different points of focus. A useful analogy is the comparison of a basketball to a football. The surface of a basketball has equal curvature in all directions, whereas a football is flatter in one orientation and steeper in the other. The surface of the football represents the situation that is present in the cornea when the eye has astigmatism.  This surface focuses light obliquely, so no true image forms on the retina.

Usually, the astigmatism occurs in the cornea. This often can be corrected with glasses or contact lenses, or in many instances, treated with refractive surgery.

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Astigmatism focuses light in an oblique fashion, so no true image forms on the retina.


Presbyopia is a natural condition that typically becomes noticeable around age 45. In children and young adults, the crystalline lens inside the eye can easily look at a distant object and then increase its focus to look at a near object. As we get older, the lens inside of the eye loses this capacity to focus in and out. As this occurs, individuals find that they need to hold reading material at greater distances from them. Ultimately, reading glasses are needed, typically by the mid-40s.

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Presbyopia focuses light behind the retina at near distances only.   This is because the eye doesn’t have the strength to “autofocus” at reading distances.


What is refractive surgery?


Refractive surgery involves the use of lasers or other devices to treat the refractive errors of the eye. Radial Keratotomy has been practiced in the United States for nearly 20 years, but it has now been surpassed by the use of the excimer laser for treating nearsightedness, farsightedness and astigmatism. A new laser, called the holmium:YAG laser, is now being studied for treatment of farsightedness. If you are nearsighted, with or without astigmatism, you may be a candidate for refractive surgery.

LASIK (laser in situ keratomileusis):

LASIK is an excimer laser surgical procedure that treats the middle layers of the cornea to correct nearsightedness, farsightedness, and/or astigmatism.  LASIK is performed using numbing drops to help prevent pain during the procedure. A vacuum ring secures the eye while a thin layer of the cornea is formed into a flap. This is done with a microkeratome, which is a specialized scalpel that makes the corneal flap. This flap is folded over, and the laser surgery is performed in the bed of the cornea that is exposed. The laser treatment reshapes the cornea to treat the refractive condition. The actual laser portion of the procedure lasts 10 to 90 seconds. The flap is then folded back into its original position and observed for 2-3 minutes to insure that it is fully adherent.

Advantages of LASIK include: 1) little or no discomfort following the surgery, 2) recovery of vision often within 1-2 days, 3) little or no risk of developing haze following treatment, and 4) ease of doing additional laser treatment should this be necessary. As with any surgical procedure, complications can occur, including those that could result in loss of vision. Fortunately, severe complications are extremely rare.

PRK (photorefractive keratectomy):

PRK uses the excimer laser to reshape the front surface of the cornea to change its focusing power. Surface cells of the cornea (the epithelium) are gently removed from the central portion of the cornea to expose the region of the cornea that is treated with the laser. The actual laser time is similar to that with LASIK.

The surface cells require two or more days to heal, and a soft contact lens is placed over the eye during this period to serve as a bandage. PRK has been approved for treatment of nearsightedness up to 12 diopters with the VISX Star laser.

LTK (laser thermal keratoplasty):

LTK uses light from the holmium laser to heat the cornea in order to change its shape. A typical treatment consists of two rings, each containing eight spots, which are placed around the central zone of the cornea. By gently warming the cornea, these spots increase the focusing power of the cornea to treat the farsightedness. This laser has recently been FDA approved, and the only one available is the Sunrise Hyperion.

Intracorneal Ring Segments (Intacs):

Intracorneal Ring Segments use small polymethyl-methacrylate (PMMA, a type of clear plastic) to alter the front curvature of the cornea.  These ring segments are placed within the substance of the cornea, and cause the central part of the cornea to change its shape.  By flattening the central cornea, Intracorneal Ring Segments can be used to treat nearsightedness.  They are limited for use in only patients with mild nearsightedness, and without significant astigmatism.