I shot a B&W circular polarizer a few times with the 21, once I shot on a very sunny clear day and I found that with my angle to the sun an sky a area of the sky was extremely dark and I had a better image with out it. The lens is just so wide and I had so much sky in the shot that there was just always some part of the sky that be too dark for my taste, The filter was a thin type with out front threads and I did not see a problem vignetting. I tried a standard Hoya and did have problems with it.
