Astigmatism is present in nearly 100% of the population to some degree.
Astigmatism is usually caused by an abnormal corneal curvature that resembles the shape of a football rather than that of a basketball. Light rays focus at different places depending on their orientation, and objects at all distances appear blurred. Astigmatism may be accompanied by myopia or hyperopia.
For example, this picture demonstrates that the vertical light beams are focused in front of the retina, while light beams oriented 90 degrees away (horizontal) are focused at or near the retina. The result is two different focal planes 90 degrees apart, instead of a sharp focal point.
A special type of lens is needed to correct astigmatism. Soft contact lens are not able to correct astigmatism unless they are a special type known as toric lens. Large amounts of astigmatism often can only be treated by rigid contact lenses.
Most commonly, moderate astigmatism (1 to 4 diopters) is corrected using a diamond scalpel by making accurate incisions across the steep meridian of the cornea. This flattens the steep areas of the cornea, giving it a more spherical shape. Currently, the excimer lasers approved in the United States do not correct astigmatism. However, software which allows the ablation to be oval instead of circular, is being tested and refined in Europe and South America and will soon be available in the United States.