Normally, the cornea is spherically curved, similar to a contact lens. This shape, together with the lens of the eye, ensures that the incident light is focused at one point on the retina, resulting in a sharp image.
Astigmatism means something like "pointlessness". In the astigmatic eye, a focal line is created instead of a focal point, as horizontally and vertically incident rays of light are refracted differently. Themost common form of astigmatism isastigmatism of the cornea. Less common is an astigmatic curvature of the lens of the eye; very rare is a deformation of the back of the eye (including the retina).
Astigmatism is theGerman technical term for astigmatism. It refers to the effect of defective vision: those affected see point-like objects distorted and more as strips or bars.
Astigmatism is themost common form of astigmatism, which is why the two terms are often used interchangeably. Astigmatism is present when the cornea is not evenly spherically curved.
A distinction is made betweenregular and irregularastigmatism. Withregular astigmatism, incident light rays are imaged on perpendicular focal lines ("rod"). Regular astigmatism is the most common form of astigmatism.
In the case ofirregular astigmatism, the cornea is curved to varying degrees at different points. In extreme forms (for example due to scarring), the light is refracted in many different directions so that there are hardly any focal lines left. Correcting irregular astigmatism is therefore more difficult.