An optical system with astigmatism's one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism's 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". Forms of astigmatism There're two distinct forms of astigmatism. The first's a thir… (More on Astigmatism)