Most of the focusing power in the eye occurs along the front surface of the eye, at the cornea (the clear 'window' in the front of the eyeball), or within the lens, which sits behind the cornea inside the eye. The ideal cornea has a symmetrically curved surface. Astigmatism is caused by a cornea or a lens that is not symmetrical.
The cornea is the transparent window in front of the colored part of the eye. It bends (refracts) light rays to focus the light onto the retina in the back of the eye. When the cornea is oblong shaped (like an oval rather than a round teaspoon), it is not possible for the light rays to be focused at a single point. As a result, people with significant astigmatism may have distorted or blurry vision.
Astigmatism is measured in diopters. More than 1 diopter typically requires correction with glasses or soft contact lenses.
Astigmatism can run in families and often occurs in combination with other refractive problems such as nearsightedness or farsightedness.
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