 Astigmatism (optical systems)

In an optical system, astigmatism occurs when light rays associated with two perpendicular meridians focus differently. For an off-axis point in a rotationally symmetric system, tangential and sagittal bundles can form line-like images at different axial positions rather than one common point image.

Move a screen through focus while imaging an off-axis point: the blur can pass from a line in one direction through a more compact patch to a line in the perpendicular direction. This differs from Petzval field curvature, which concerns the shape of the best-focus surface. A detector placed at one compromise focus does not remove the underlying astigmatism.

Astigmatism is a common optical imperfection that occurs in lenses and optical systems. It arises when a lens cannot focus light equally in all directions, leading to a distortion in the image produced. This is often due to the curvature of the lens surfaces not being perfectly spherical or, in the case of mirrors, not being perfectly parabolic. In a typical situation where astigmatism is present, light rays that enter the optical system at different angles will converge at different focal points.