Comatic aberration is in some ways similar to spherical aberration but is caused by the difference in distance of the incident light from the optical axis and a difference in magnification. Spherical aberration is caused by a difference in focused point.

Comatic aberration, or coma, causes off-axis rays of light in the object plane to create a trailing "comet-like" blur trailing directly away from the optical axis. It is this comet-like effect that gives rise to the name "coma". The aberration is most noticeable when a lens system is misaligned. A lens with severe coma typically produces a sharp image at the centre of the image field with decreasing sharpness and image quality toward the periphery.

The images on the right simulate the effects of severe coma.