An irregular corneal curvature causes astigmatism. A normal cornea has a regular curvature with no steep or flat axis where an astigmatic cornea is bended, having a steep and a flat axis. The sight of astigmatic patients resemble to the images at funny mirrors in amusement parks. Small astigmatic diopters are frequently seen and do not need to be corrected. 
A linear light beam entering the eye are seperated into different meridians on retina, if the corneal curvature is irregular, making the image bend according to these meridians. 

There are few astigmatism types:

- simple hypermetropic astigmatism: one meridian on, one behind the retina
- simple myopic astigmatism: one meridian on, one in front of the retina
- compound hypermetropic astigmatism: both of the main meridians behind the retina
- compound myopic astigmatism: both of the main meridians in front of the retina
- mixed astigmatism: one meridian in front of, one behind the retina

Blurry sight, difficulty in reading, eye strain and headache can be seen in astigmatism. It can be measured with autorefractometer, keratometer, corneal topography that measures the corneal curvature and retinoscopy. 
It can be corrected with cylindric lenses, toric contact lenses and refractive laser surgery like LASIK, LASEK, PRK.