A defect of a lens or an optical system in which the focal length along one diameter differs from that along another. Typically, as the focus is approached, the image of a star appears first as a short straight line; then, at the best focus, as a small circle (the least circle of confusion or focal circle); then, once the best focus is passed, as a line perpendicular to the first. Astigmatism may be caused by distortion of an optical component, often as a result of mechanical stress; or it may be inherent in an optical design, in which case its effects increase with distance from a system's optical axis. It can occur in the human eye as well as in optical instruments.