Astigmatism is a condition where the optical power of the eye varies depending on the angle of light passing through it. Astigmatism produces blurred vision at all distances. It is usually due to the shape of the cornea (the front surface of the eye). If the curvature of the cornea is not the same in all directions (like the side of an Australian football) it will bend the light passing through it by different amounts depending on the direction of the light, producing astigmatism.
Astigmatism is a focusing error which causes asymmetric blur. Some directions in an image are more out of focus than others. This can be contrasted with short-sightedness (myopia) where all directions are uniformly blurred.
Astigmatism causes different amounts of blur in different directions. This causes images to appear distorted, or sometimes even double. Certain letters may be more difficult to read than others, depending on the orientation of the lines within them. An example of the effect of astigmatism on vision would be looking at a letter T. A person with astigmatism might see the vertical line of the T in focus, but the horizontal line blurry.
Most astigmatism is caused by the shape of the front surface of the eye (the cornea). It can also be caused by slight tilting of the lens inside the eye. It may be an inherited characteristic or a normal variation accompanying growth.
A magnifying glass focuses the sun to a point image because its two surfaces are spherical, each like the surface of a basketball. Now imagine a transparent surface shaped like the side of an Australian Rules or rugby football. It has two different curvatures. These result in light focusing at two different locations. The image does not focus to a point and so is blurred.
Objects at all distances are indistinct or blurred and the eye cannot focus. Even slight degrees may cause headaches and visual fatigue, and reduce concentration. This is because the eyes may try, without success, to correct the blur, and because there is a tendency to screw up the eyes to try to see better, producing discomfort in the muscles of the eyelid and face.
Spectacles and contact lenses can correct astigmatism. Sometimes correction of astigmatism can cause change in the apparent size and shape of objects and may affect judgement of distance. A patient may feel taller or shorter, or walls may appear to slope and floors curve.
In most cases, adjustment to these side effects takes only a week or so. Astigmatism correction may involve a compromise between optimal clarity and visual discomfort.
High levels of astigmatism are best corrected with rigid contact lenses. This is because if a soft lens rotates on the eye just a small amount, the vision decreases dramatically. Mild to moderate levels of astigmatism can be effectively treated with soft contact lenses and orthokeratology (overnight corneal reshaping lenses).
Some people notice blur themselves. Only a proper eye examination will determine for certain if you have astigmatism. Astigmatism is not an eye disease and any changes are generally gradual and not necessarily for the worse. Most people have at least very slight astigmatism.
Courtesy of OAA (http://www.optometrists.asn.au/)