Just a quick add-on:
the rokinon/bowen/samyang cannot use circular filters...but, actually it can.
by accident/experiment with my IR camera - canon 40D, I was playing with different WA and settings to get diffeent results, when I decided to hanheld a filter just in front, just by the hood. Well, happen to be a 82mm filter, and for my surprise - and a small adjustment, was able to remain by itself , grasped by the lens hood.
I am sure some difraction and reflections issues may become very visible on a regular camera, but as last resource, it might be a 'trick" worth to consider.
I think it may even take the 77mm and maybe 72's
Other big advantage of this lens, is not having such a heavy "comma' effect as the Canon's 14mm counterpart