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Astigmatism occurs when the cornea is irregularly shaped preventing light from converging on the retina. Parts of the image may focus on the retina, while others focus behind it or in front of it; think of the vertical and horizontal lines of a cross ... the vertical and horizontal lines will appear in sharp focus at two different distances. Astigmatism can occur alone or in addition to either nearsightedness or farsightedness.

There are two distinct forms of astigmatism. The first is a third-order aberration, which occurs for objects (or parts of objects) away from the optical axis. This form of aberration occurs even when the optical system is perfectly symmetrical. This is often referred to as a "monochromatic aberration", because it occurs even for light of a single wavelength. The second form of astigmatism occurs when the optical system is not symmetric about the optical axis. This form of astigmatism is extremely important in ophthalmology, since the human eye often exhibits this aberration due to imperfections in the shape of the cornea or the lens.

Although astigmatism can complicate refractive surgery, it can be effectively corrected with LASIK, PRK or LASEK.