1. An aberration arising from the fact that rays which are initially at different distances from the optical axis come to a focus at different distances along the axis when they are reflected from a spherical mirror or refracted by a lens with spherical surfaces.
2. A fault in a lens or curved mirror in which light passing through the edge has a different focal point from light passing through the center, resulting in blurred images.
3. The blurring of an image that occurs when light from the margin of a lens or mirror with a spherical surface comes to a shorter focus than light from the central portion.

The changing focal length is caused by deviations in the lens or mirror surface from a true sphere.