SPHERICAL ABERRATION The spherical aberration is a defect of mirrors and lenses in which light rays that affect the optical axis parallel to but some distance from it, are taken to a different focus the rays next to it; Spherical aberration is monochromatic aberration of third order type that affects differently each wavelength. This effect is proportional to the fourth power of the diameter of the lens or mirror and inversely proportional to the cube of the length focal being much more pronounced in short-focus optics, as in the lens of a microscope. In telescopes, optical instruments once used focal length to reduce the effect of spherical aberration .