Astigmatism is a refractive error that comes in many types and categories. Your eye doctor determines what kind of astigmatism you have in order to properly correct your vision.

Reviewing the key types of astigmatism can help you understand its effect on vision and how you and your eye doctor can team up to correct it.

Astigmatism explained

Astigmatism usually results from flaws in the cornea, a clear, window-like covering on the front surface of the eye. A normal cornea is rounded like the lens of a camera, and it performs much the same function. Working in unison with the eye’s internal (or intraocular) lens, the cornea uses refraction to bring objects into visual focus.    

Though other parts of the eye, such as the intraocular lens, may contribute to the development of astigmatism, a diagnosis depends primarily on identifying three categories of refractive errors in the cornea: myopic, hyperopic and mixed astigmatism.

Eye doctors scan the cornea for signs of these imperfections. If they find them, they then use lines called meridians to divide the arc of the cornea into sections. These lines in the cornea function much like the north-to-south and east-to-west meridians on a map or globe.

When assessing astigmatism, eye doctors view meridians like the hands on a traditional clock. One meridian, for example, connects the 6 (bottom) to the 12 (top). Another meridian goes from 9 (left) to 3 (right).

A normal cornea has a consistent arc. With astigmatism, the arc is inconsistent: too steep in one meridian and too flat in another. Diagnosing astigmatism usually requires detecting principal meridians — the steepest and flattest parts of the cornea — and figuring out how they blur or distort vision.