Refractive Errors

Nearsightedness (Myopia)

More than 70 million people in North America alone are nearsighted. Myopia is the medical term for nearsightedness. Myopia usually starts in childhood and gets progressively worse through adolescence. It usually decreases the severity of change by the late teens, but it can sometimes continue to get worse into the mid-twenties. Myopia occurs when an eye is too long or too strong for the light-bending ability of the cornea's curvature. Light rays entering the eye do not come to a sharp focus on the retina at the back of the eye. Instead, they focus farther forward, producing a blurred image. The term nearsighted means that myopic individuals can see "near" objects clearly without glasses, but objects further in the distance are blurred. The more myopic, the more blurred distant objects appear, the higher your eyeglass prescription and the thicker your glasses needed for correction. Of all myopic individuals, about 90% have corrections less than -6.00 diopters.

Myopia can be corrected by any method that reduces the total refractive power of the eye. 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 and Intacs procedures decrease eye legth by flattening the central part of the cornea.

Farsightedness (Hyperopia)

Hyperopia is the medical term for farsightedness. This occurs when an eye is too short for the light-bending ability of the cornea's curvature. Light rays entering the eye do not come to a sharp focus on the retina at the back of the eye. Instead, they focus farther backward, producing a blurred image. Farsighted individuals, however, can use their focusing muscles to "pull" the image forward onto the retina. In young persons with high degrees of hyperopia or in individuals over the age of 45, this compensation ability may be inadequate to produce clear images and vision will be blurred. Farsighted eyes are too short and need a "plus" power to help them focus.


Astigmatism

Astigmatism is the most common refractive (focusing) error. Astigmatism creates distorted and ghosting vision by a "warpage" in the optics of the eye. It occurs when the front of the cornea is somewhat oval-shaped (like a football) rather than round (like a baseball). The two different curves in such a corneal surface each bend light rays to a separate focus point. Astigmatism correction makes all the rays of light focus at the same distance so that they all fall correctly on the retina. Small degrees of astigmatism do not impact vision significantly, but with larger amounts, distortion and blur occur. Astigmatism can occur alone as the sole optical error, or may occur together with either myopia or hyperopia. The categories of severity for astigmatism are:

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