Astigmatism

Astigmatism is a condition where the optical power of the eye varies depending on the angle of light passing through it. Astigmatism produces blurred vision at all distances.

It is usually due to the shape of the cornea (the front surface of the eye).If the curvature of the cornea is not the same in all directions (like the side of an Australian football) it will bend the light passing through it by different amounts depending on the direction of the light, producing astigmatism.

Astigmatism is a focusing error which causes asymmetric blur. Some directions in an image are more out of focus than others.

This can be contrasted with short-sightedness (myopia) where all directions are uniformly blurred.