Astigmatism occurs when the curvature of the cornea is asymmetric.
The normal cornea is shaped spherically, similar to the lens of a camera. Therefore, horizontal as well as vertical lines are are refracted to form a focused image. A dot-like source of light such as a street light will produce a dot-like image. If the curvature of the cornea is not spherical, but rather elliptic, the light rays are focused at different points and the image will be distorted. A point, for example, will not produce the image of a point but a small line. In our example the street light will not produce the image of a bright dot but a small line.