neuro, BTW, what ND filters do YOU use (if any), I think I've read somewhere on the forum from you about it and it's just now that I get interested too.
I don't (yet) have a grad ND system, but if/when I do it will most likely be the Schneider Optics one. For standard ND filters, I have all B+W screw-in filters. I've got 3-stop (#103/0.9) and 10-stop (#110/3.0) in 77mm, and a 3-stop in 72mm that's great for outdoor portraits (35L/85L II/135L all use 72mm filters). I've got B+W KÃ¤semann CPLs in 77mm and 82mm, and I also have 67→77mm and 72→77mm adapter rings so I can use the 77mm filters on other lenses.
Actually, one of my main reasons for being interested in the rectangular filters was for a 10-stop ND filter to use on my EF 16-35mm f/2.8L
II and TS-E 24mm f/3.5L
II, which both take 82mm filters. For quite some time, the only 82mm ND filters available were 1- and 2-stops, or the Vari-ND/Fader-ND versions which start showing a maltese cross artifact at 24mm and wider (derived from the uneven polarization you get with CPLs on UWA lenses, because a variable ND filter is essential a pair or stacked polarizers).
So, my only real option was a filter holder and probably the Lee Big Stopper. But, replying to this post (thanks for the question!!
) spurred me to have another look, and sometime very recently, Schneider released an 82mm 10-stop ND filter! Happy happy joy joy. I'm off to order one...