Spherical aberration
Spherical aberration is the phenomenon of imperfect lens performance arising from lens elements being made with spherical surfaces. A spherical surface is easy to make (because the glass element can move around on the grinding tool surface while still fitting closely to it), and is often a very close approximation to the ideal shape of the lens. However, the difference between the ideal surface and the spherical approximation to it leads to different areas of the lens surface focusing their image at different distances (each circular zone of the lens surface having a different focal length). Sharp detail is slightly blurred. The problem is overcome by using a small aperture (because less zones of the lens surface are used).
Coma (where point objects away from the lens axis are stretched in the image, and rendered as comet-like shapes) is essentially the same problem when affecting off-axis rays.
