Ever wonder what causes vignetting, and what you can do about it? The linked article is one of the best I have seen describing the causes and potential remedies for vignetting. It's probably not for the technically faint of heart, but has illustrative figures and is well presented.
If you only want the Reader's Digest version, here it is: Optical vignetting (the kind you can do something about) is due to the dimensions of the lens, and the fact that off-axis light is blocked by the lens barrel when using large apertures (see the diagram in the article if this doesn't make sense to you). You can mitigate this vignetting by stopping down (closing the aperture diaphragm on the lens).
Here's the link to the full article:
"Vignetting" by Paul van Walree
