The one I use on the 18 mm f3.5 is a B+W Slim mount c-pol and have noticed no
problems with vignetting, however I have not done a scientific test trying
every f stop. Just put it on and went about my business capturing whatever
images I wanted to with this lens. Got that one off ebay at about half what
a new one runs. I use the Hoya super HMC Pro1 UV which is also in a slim
mount. The only time I really have had a problem with vignetting is when I
wasn't paying attention and I stacked a polarizer on top of another filter
and used the lens hood with my 24 mm f2.8. The results were predictable.
Sure glad that was the only time I had ever made a mistake with my
photography. . . I'm sure everyone believes that one. Bill Barber
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