Deviation of light rays by lens es or mirrors which causes the images to be blurred.
Spherical aberration occurs because curvature in a lens or mirror causes rays falling on the outer edges to be brought to a focus at a different point from those from the middle. This makes the images formed appear blurred. Chromatic aberration, which occurs in lenses but not mirrors, is the failure of a lens to focus all colours ( wavelength s) of light in the same plane; the image appears blurred and shows rainbow-coloured fringes around the edges. See also astigmatism .