In article <31C37B8C.7F05@us.oracle.com>, sji@us.oracle.com says...

>I shot two pictures with both my UV and polarizing filters on. It
>turned out that the four corners of both pictures are dark. The
>pictures are fine if only the UV filter is on. Anybody has any
>idea why?

It is called "vignetting". Lens thread sizes are often chosen to
be the minimum that will not cause vignetting when one filter of
normal rim thickness (plus, usually, the correct shade for the
lens) is used (though some companies design the lens to fit a
given filter size). Using filters with excessive rim thickness
(due to bad design, or the mechanical needs of the filter) will
cut the corners of the image (as would shooting through a hollow
cylinder, the interior walls of which would be imaged out of focus,
but would block the imaging of the subject in a circular way).
BTW, it is never a good idea to stack filters, if maximum
sharpness and freedom from ghost images is desired.
Hope This Helps