In the course of doing the SFX200 test with assorted filters,
http://www.pbase.com/dw_thomas/sfx200_test
an observation was made. There are relatively inexpensive Bay 1 filters out there
with plastic frames, as well as top notch units from makers like Rollei. It was noted
during the test that corners were vignetting with some filters, namely the plastic
frames, but not the others. Note that the metal framed Rollei filters have almost
5 mm additional clear diameter. Note also in the profile view, the plastic frames
are longer/higher; both conditions serve to increase the likelihood of vignetting.
Not to impugn the sellers too badly, The inexpensive filters are OK optically and
do save some money, but be aware they may clip the corners of that special shot.