aberration, optical

Any of a number of defects that impair the image in an optical instrument. Aberration occurs because of minute variations in lenses and mirrors, because no lens or mirror can perfectly focus all light rays entering the instrument at different angles, and (with lenses) because different parts of the light spectrum are refracted by varying amounts.

In chromatic aberration the image is surrounded by coloured fringes, because light of different wavelengths is brought to different focal points by a lens. In spherical aberration the image is blurred because different parts of a spherical lens or mirror have different focal lengths. In astigmatism the image appears elliptical or cross-shaped because of an irregularity in the curvature of the lens. In coma the images appear progressively elongated towards the edge of the field of view. Elaborate computer programs are now used to design lenses in which the aberrations are minimized.