Ultrafast wide angles

[Viggo]

Please rent a 24 L II and use it for two days, then all your questions will be answered. My most used lens for everything.

[Lawliet]

I wonder how big the filter threads would need to be?

Depends on your definition of need. With long lenses you have a hard lower limit on the filter size, not so with the wide ones. Here you want the front element to be as bulbous as possible to avoid a shallow angle of incidence.
Compare the Nikkor 17-35/2.8,16-35/4, Canon EF 16-35/2.8I and 17-40/4, as counterpart we have the Tokina 16-28/2,8 which has got really good corners.

[neuroanatomist]

I wonder how big the filter threads would need to be?

Depends on your definition of need. With long lenses you have a hard lower limit on the filter size, not so with the wide ones. Here you want the front element to be as bulbous as possible to avoid a shallow angle of incidence.

The original 16-35mm f/2.8L used 77mm filters.  The MkII version uses 82mm filters.  I'd expect an f/2 version of such a lens to be designed for at least 86mm filters, perhaps 95mm filters - getting further into the area of hard-to-find filters.  Even with my 16-35mm f/2.8L II, though I have a B+W Käsemann CPL in 82mm (which also fits my TS-E 42mm f.3.5L II), I'd really like a dark ND filter (e.g. 10-stops), but they aren't available in round/threaded.