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I have the same question as Geraint below. I am new to Contax 645 and the forum and so far I have discovered that Hoya doesn't make a Super Muti-Coated filter in 95mm only a Standard Coated one. If vignetting is an issue does this mean trying to find a slimline filter in 95mm? Similar to say Leica's for the later Tri-Elmar lens. Over to the more experienced. Cheers Alan.
I use a Contax 95mm skylight for my C645 35m, 45-90mm zoom, and the N17-35 f/2.8 without any problem.
I also use Contax 95mm circular polarizer for the above lenses without any problem. You use one filter at a time though. The Contax 95mm POL does not have front thread, so it is thinner for polarizer. You can't put a lens cap on it.
I use the standard Hoya skylight (1B) on my C645 35mm distagon and haven't had problems. This filter is 8 mm high at the rim. However, I use it for landscapes with an aperture of at least f/16. I also have a B+W 95E Slim CPol, which, of course, dosen't vignette on the C645 or N 17-35mm zoom. I don't stack filters.