Astigmatism (optical systems)

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.