A defect of mirrors and lenses in which rays of light parallel to but far from the optical axis are brought to a different focus from those close to the axis. Spherical aberration is seen in the images formed by spheroidal mirrors and by some lenses and eyepieces. A star image suffering from spherical aberration has no unique focus but instead has a least circle of confusion where the image is at its smallest. Beyond this point the image is a bright spot surrounded by a disk; inside it, the image is a ring darkening towards the centre. If rays from the edge of the mirror or lens come to a focus closer than those from its centre, the aberration is called undercorrected; in the opposite case it is overcorrected.

spherical aberration:

Light incident at the edge of a lens (left) or spherical mirror (right) is brought to a different focus from light incident closer to the centre.