This one is not for the feint hearted. It is quite an in depth look at the optical properties of camera lenses and the reasoning behind diffraction and aberration. It is very informative, but it very detailed. it even talks about bokeh!!

Here is a collection of illustrated articles on the chief causes of image degradation in photography. Currently available pages can be accessed via hyperlinks. The reader should keep in mind that the articles are about principles of photographic optics and not about tests of specific lenses. Also, most of the phenomena have been captured on film. Since a film acts as no more than a light-sensitive medium, the examples equally apply to a digital recorder array. In the few cases where the nature of the recording medium is important, this will be emphasised.

via Photographic optics.