Q: What exactly is astigmatism?

A: We see an object clearly when light from it or reflecting off it enters the eye and forms a focused image on the retina. Consider the simplest example of this, a point source of light like a star. If there is no astigmatism present, light entering the eye from the star will be focused forming an image that is also a point. (If the focused image forms in front of the retina, the eye is myopic or nearsighted. If it forms in back of the retina, the eye is hyperopic or farsighted. If it forms on the retina, there is no refractive error and the eye is said to be emmetropic.)

If there is astigmatism present, the light from the star will not focus as a point but instead will be distorted. (Actually, the image of the star focuses as two lines, one in front of the other and separated by some distance, with one line oriented 90 degrees from the other.) Don't bother trying to understand what astigmatism is. Just remember it distorts vision and can be corrected with special lenses called cylinders. The cornea having a shape that resembles a football instead of the normal case, a basketball, usually causes astigmatism. In most cases astigmatism occurs with either myopia or hyperopia. Some people have astigmatism only but not many. Astigmatism can be easily corrected with glasses, with rigid glass permeable contact lenses or with so-called "toric" soft contact lenses. "Toric" refers to the shape of one of the lens surfaces. It is the toric surface that is responsible for correcting the astigmatism.

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