aberration, optical

The main defects, or aberrations, of optical systems. Chromatic aberration, or coloured fringes around images, arises because light of different colours is focused at different points by a lens, causing a blurred image. Spherical aberration arises because light that passes through the centre of the lens is focused at a different point from light passing through the edge of the lens. Astigmatism arises if a lens has different curvatures in the vertical and horizontal directions. Coma arises because light passing directly through a lens is focused at a different point to light entering the lens from an angle.

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.