Spherical Aberration
A defect caused by grinding the surface of a lens or mirror to a spherical rather than a parabolic shape.
Negative Aberration
When rays of light passing through the curved surface of a lens near its edge converge at a point closer to the lens than those passing through its centre.
Positive Aberration
Wwhen the outer zone has a longer focal length than the centre.
See also: Aberration, Aspherical.
