Illustration courtesy Ciba Vision®.
Astigmatism is a lens or mirror defect in which the size and shape of an image vary for different points of focus. Light passing through different parts of an astigmatic lens, for example, is focused at different distances beyond the lens, so that the image of a point can appear variously as a short horizontal or vertical line or an ellipse. The best focus is a small circle known as the circle of least confusion.
Astigmatism of the eye
Astigmatism in human vision happens when the front part of the eye – the cornea – (or sometimes the eye lens itself) is not perfectly regular and symmetrical in shape. If the cornea is more oval than round (like the back of a spoon), then light passing through it isn't focused properly on the retina at the back of the eye. This results in a blurring of vision at all distances.
Astigmatism is very common and is usually present from birth. Most people have a slight astigmatism because it's rare for the cornea to have developed in a perfectly symmetrical way. But in mild cases, the eye can adjust to focus the light adequately.
In many cases of astigmatism the person also has other visual problems such as nearsightedness (myopia) or farsightedness (hyperopia). Two-thirds of people with nearsightedness also have significant astigmatism.
In most cases, astigmatism can be corrected simply by wearing properly fitted spectacles or contact lenses. In some cases, astigmatism can be corrected by an operation which reshapes the cornea. The most common type of surgery used to correct astigmatism is LASIK (laser in situ keratomileusis).