...but the punchline is to try to stay in a generic 4x4 / 4x6 filter ecosystem as your options are far more plentiful and affordable than if you chase the
ultra ultra-wide FLs, in which case it's time to buy the huge/pricey filters needed to cover such wide FLs without interrupting your FOV.
For me with my Lee 100mm system, that floor is 16mm FF. Wider than that, you typically have to step up to huge filters (sometimes on outrigger-based holders that don't use the filter ring) or you have to 'unblacken' some corners of your shots in post, which can be difficult depending on what you are shooting.
Some of the newer systems (we've seen some kickstarters on this forum) promise a little better performance than the Lee from a stacking/FL perspective -- one inventor came on the forum and I believe promised stacking three 4x6 / 4x4 filters with zero vignetting at 16mm and told us to trust him that it will work :
-- but I haven't seen a comprehensive vetting of each companies' vignetting tables (like Lee's in my last post). Remember, since the filter ring to front element distance is slightly variable with each lens, it's a lens specific phenomenon you need to try yourself.
- A