What is aberration

aberration n. (in optics) a defect in the image formed by a lens. In chromatic aber¬ration 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 lens. 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.