Astigmatism is a condition where the lens or the cornea develops an irregularity, changing its shape from the normal curve. Part of the surface becomes flatter, causing light to focus irregularly on the retina. This creates blurry vision, and requires a special type of corrective lens.
Astigmatism is a common condition. Few corneas and lenses are perfectly round, but in many cases the irregularity is small enough that it does not affect the person's vision in any significant way.
Optometrists measure astigmatism by looking for the principal meridians of the eye. If the eye were a clock face, a meridian would be a line connecting 12 and 6, or 1 and 7 and so on. The principal meridians are the meridian with the steepest curve and the meridian with the flattest curve. Each can produce a nearsighted effect or a farsighted effect. In some cases, one meridian may cause nearsightedness, while the other causes farsightedness.
Eyeglasses can correct astigmatism, as can contact lenses. Contacts for astigmatism are designed with thicker edges, to ensure the lens remains oriented properly against the eye. This allows the lens to correct the refraction errors for different areas of the cornea simultaneously, bringing the person's vision into focus.