Putting filters on ultra-wide-angle lenses isn’t usually an easy or inexpensive task. Sure, it’s possible, but you need to get some pretty large filter holders and filters to match. The trend amongst some lens manufacturers lately has been to support rear “gel” filters. Filters that sit behind the lens that you attach before you mount it to your camera.
Aurora Aperture has recently announced their “next-generation” of rear mount filters. And best of all, they actually work with lenses that don’t have built-in mounts as the system uses its own magnetic mounts, tailored specifically to your lenses. Now, they’re available on Kickstarter and they’ve already doubled their goal.