I use the 24-105 and 70-200 lenses on my 5D Mkiii and shopping for a variable ND filter for shooting video. Was wondering which brand to buy. Local camera shop showed me a ProMaster variable 11 stop filter for a decent price compared to the Sing Rays I see online. Also curious about the so-called vignette problem with these filters. For a 77 diameter lens, is it best to go with an 82 and use step-up ring or is that not necessary?