I've been giving thought to buying a copy of the Zeiss 15mm/2.8 Distagon in the ZF.2 flavor. It's specs say it accepts a 95mm screw-in filter.
I have to set aside the debate about whether or not to use a protective filter because I'll be subjecting the lens to hostile conditions (blowing dust and sand) that are likely damage the front element without one. So I'm looking for recommendations for an ultra-thin protective filter that's suitable for a 15mm ultra-wide angle lens. They are not easy to find.
I normally use B+W filters on my Zeiss lenses. They used to use the name "XS-Pro" for the extra-slim filter rings, but B+W keeps changing the names of their filter lineup and that nomenclature is gone now. Today, B+W sells 3 lines of filter rings, with any grade of filter you want stuffed into them:
BASIC -- thicker rings with cross-hatch knurled edges
MASTER -- thinner rings with less aggressive edges (formerly XS-Pro)
T-PRO -- titanium finish with smooth edges
B+W specs the "Master" and "T-Pro" ring footprints as being suitable for 17mm lenses on full frame, which isn't quite wide enough not to cause vignetting on the Zeiss 15mm.
Can anyone recommend a suitable high quality ultra-slim filter for this application? I'm looking for something that's equivalent to this, but slim enough for 15mm:
