aberration

(in optics) a defect in the image formed by a lens. In chromatic aberration the image formed by a lens has coloured fringes as a result of the different extent to which light of different colours is refracted by glass. It is corrected by using an ACHROMATIC LENSES . In spherical aberration, the image is blurred because rays from the object come to a focus in slightly different positions as a result of the curvature of the lens: the rays passing more peripherally through the lens are bent more than those passing through centrally. This occurs even with monochromatic light.