Astigmatism (optical systems)

Astigmatism: The image of point G is at point BM for rays in plane M, and at point BS for rays in plane S.

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".[1]

  1. ^ Harper, Douglas (2001). "Online Etymology Dictionary". Retrieved 2007-12-29.