I constantly get mud, water and dog slobber on my lenses
Hi, I don't see Murami as a filter manufacturer so think you probably meant Marumi. Is the difference in cleaning attributable to differences in between B+W, Marumi, and Breakthrough's nanocoatings or did the B+W and Marumi filters used for comparison lack nanocoating? If the latter, Aurora, B+W, Haida, NiSi, and probably some others I'm not thinking of offhand offer nanocoated NDs. (Marumi---and Hoya---don't currently make nano NDs, though they do offer nano protectors, UVs, and polarizers.)
Their thinness helps prevent vignetting against the thicker aluminum ones which also seem to stick at times on removal verses the brass BT ones.
To be more specific, a thin filter helps avoid vignetting from mechanical obstruction. I'm only aware of
one native mount lens where this is much of an issue. Whether a fixed ND exhibits optical vignetting and or color shifts depends on the angle of view over which its designers were able to maintain reasonably constant transmission. No manufacturer I'm aware of mentions this and it's atypical for ND filters to be tested or reviewed with ultrawides. The amount of vignetting (or shift) depends upon how the ND is constructed and ranges from basically zero to reports of three stops. Given some of their lower cost competitors manage it, I'd expect Breakthrough to maintain near zero to a 15mm equivalent focal length but haven't come across confirming measurements.
Breakthrough doesn't appear to specify stack heights for their filters but, from their image comparing the X2 to a Hoya (probably chosen to make the X2 look thinner as Hoya makes some of the thickest filters I know of) I'm guessing it's a bit over 3 mm. Good chance that's thicker than Aurora and it's almost certainly thicker than Haida. I have B+W and Marumi filters around 3mm and, whilst those aren't NDs, their NDs may be similar.
Besides Breakthrough, B+W and Heliopan and a few others use brass rings for their NDs. I haven't, however, been able to locate evidence supporting claims for brass over aluminium. Whilst brass's 10-20 MJ/m² stacking fault energy makes binding from galling less likely than the ~135-180 MJ/m² thresholds of aluminium, bearing surfaces in lens and filter threads are (almost always) anodized. Effects of anodization on threads do not appear to be well studied but the lack of issues in practice suggests it's effective in preventing trapped particles from initiating galling in aluminium-aluminium thread contacts (
Muzyk 2018,
Shkatulyak 2013,
Shih 2014,
Shih 2016 have some background). The galvanic potentials of aluminium (~-0.9V) and brass (0.4V) suggest opportunity for bimetallic corrosion when combining the two but this doesn't appear to be a problem either, consistent with anodization providing an oxide barrier but probably more due to lack of a circuit. Perhaps there's another mechanism, maybe the marketing for brass is just disconnected from reality.
). It's well built in that there's nothing loose and the frame is solid, however the antireflective coating is unimpressive. It's better than a cheap Tiffen filter, but there is still a very distinct reflection when looking at the surface unlike my B+W, Haida, or Kenko filters (which have almost none, the B+W being the best).
