Lens Aperture and Aberrations

Lens Aberrations
"Aberrations are departures of the performance of an optical system from the predictions of paraxial optics."
-http://en.wikipedia.org/wiki/Lens_aberrations

Spherical Aberration
"A type of lens aberration which causes blurriness, particularly away from the centre of the lens"
- http://en.wiktionary.org/wiki/spherical_aberration

Chromatic Aberration
"an optical aberration in which the image has colored fringes."
- http://wordnetweb.princeton.edu/perl/webwn?s=chromatic%20aberration

Astigmatism
"A defect of a lens such that light rays coming from a point do not meet at a focal point so that the image is blurred.."
- http://en.wiktionary.org/wiki/astigmatism