Spherical Aberration

A type of optical aberration resulting from failure of a lens (or optical system) to form a perfect image of a monochromatic, on-axis point source object. Spherical aberration is a form of ‘higher-order’ aberration. A lens can be perfectly spherical in curvature, but that does not mean that parallel light rays entering near the center of the lens will intersect the lens axis at the same point behind the lens as rays entering from more peripheral points. The differences in where these rays intersect determines the amount of spherical aberration. When rays from a point on the axis passing through the outer lens zones are focused closer to the lens than rays passing the central zones, the lens is said to have negative spherical aberration; if the outer zones have a longer focal length than the inner zones, the lens is said to have positive spherical aberration. Spherical aberration can be corrected by lenses with aspheric design.