Clear vision requires the cornea and lens of the eye to focus light perfectly on the retina. For this to happen, the cornea and lens must be symmetrical in shape.
Astigmatism occurs when the cornea or lens (or both) are not perfectly symmetrical. Instead of having an equal shape across their entire surface, they are shaped more like a football or the back of a spoon. Astigmatism is the loss of focus (or Refractive error) caused by this irregular shape.
In astigmatism, the difference in curvature in different meridians of the eye (due to the irregular shape of the cornea and/or lens) causes a difference in the power of the eye along each meridian.
The steepest and flattest meridians of an eye are called its principal meridians. The principal meridians are perpendicular to each other (i.e. separated by 90 degrees). There are five types of astigmatism based on the amount of nearsightedness or farsightedness in each of the principal meridians:
In simple myopic astigmatism, one principal meridian is free of nearsightedness or farsightedness, but the other meridian is nearsighted.
In compound myopic astigmatism, both meridians are nearsighted, but one is more nearsighted than the other.
In simple hyperopic astigmatism, one meridian is free of nearsightedness or farsightedness, but the other is farsighted.
In compound hyperopic astigmatism, both meridians are farsighted, but one is more farsighted than the other.
In mixed astigmatism, one meridian is nearsighted and the other is farsighted.