I think you are "vignetting" because you use a polarizer. The angle of view is so large that different portions of the sky (and the rest of the view) are differently affected by the polarizer -- or at least that's what I was always told. I don't have a 12mm lens, but I have the Contax 18mm Distagon, and with a polarizer you can create some strange (i.e. unnatural-looking) images.
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