Astigmatism is caused by an irregular shape of eye. Think about the difference between a round basketball to an oval American football. A round or regular shaped eye has the same prescription all around while an oval or irregular shaped eye has a different prescription across the two directions that are shaped differently. As well, the "football" shape can be oriented in different directions. This complex shape causes a complex focussing error that can blur objects at far, middle and near distances. To correct astigmatism, the Optometrist prescribes eyeglass lenses with more prescription in one meridian, and less in another, all in the proper amount and orientation to compensate for the complex shape of the patients eye. Patients with astigmatism usually wear their glasses full time when their amount of astigmatism is great enough to cause full time blur or full time eyestrain.
A More Detailed Description of Astigmatism:
People without any astigmatism at all have the same amount of nearsightedness or farsightedness all over the eye, in all meridians of the eye. People with astigmatism have different amounts of nearsightedness or farsightedness in different meridians of the eye. To understand how that impacts vision, think about the lines that make up the letters of the alphabet.
Letters are made up of lines, oriented in different directions. While some are curved, you'll understand astigmatism more easily if you think about letters that are made up of straight lines. First think of a few letters that have lines oriented vertically (These letters also have lines oriented horizontally): L T H I E
As it turns out, it is quite common for patients to see the vertical lines more clearly than the horizontal lines. As a rule, fewer patients see the horizontal lines more clearly than the vertical lines. Some patients see angled lines less clearly. The blurry line can be angled in any orientation. Depending which angle is blurry, some of the following letters might be difficult to see: V W A Y N Z X
That's Astigmatism. It's a pretty simple concept with a really bad reputation. Unfortunately, that reputation might be deserved when it comes to contact lenses. If a patient has astigmatism affecting lines (for instance) in exactly the orientation of the angled line in the letter Z, then the Optometrist can add a little bit more minus or plus lens power, depending on the patients needs, in exactly that meridian. In eyeglasses, the frame around the lens rests on the patients nose and ears, and keeps everything lined up with the horizon so the astigmatism lines up precisely, making the letter Z sharp. Hurray! In contact lenses, there's nothing to stop the lens from spinning and rotating each time the patient blinks. Thus, the astigmatism would only rarely be corrected and the patients vision would only rarely be clear. You can learn how we overcome this contact lens fitting problem here.