optical Aberration

Aberration, optical, failure of a lens to form a perfect image of an object. The 2 types are chromatic aberration, where dispersion causes colored fringes to appear around the image, and spherical aberration, where blurring occurs because light from the outer parts of the lens is brought to a focus at a shorter distance from the lens than that passing through the center. Chromatic aberration can be reduced by using an achromatic lens; spherical aberration can be reduced by separating the elements of a compound lens.

See also: Lens.