I just ran a quick vignetting test with a new 16-35 F/4L IS with Lee Filter setup attached just now.Method:
Results with the CPL ring on a two-slot Lee holder but NO CPL in place:
- Used a FF rig, a 5D3 in my case.
- Shot at F/9 perhaps 18" away from a large white wall. Confirmed focus once at beginning of series and then switched to MF for all shots so the lens wouldn't hunt on the bare white wall.
- Used a Lee 77 wide angle adaptor ring directly on the lens' filter threads
- Attached a 2-slot Foundation Kit holder with the 105mm CPL ring screwed on the front.
- Ran a series with nothing in the Holder, and then ran it again with the 105 CPL attached.
- Walked the FL from 16-24 in small manual increments (the gap in the ring is relatively small in between 16 - 20 and 20 - 24, so it was not an exact science.)
- Cable release, tripod, LV, etc.
- The CPL was a 105mm B+W Kaesemann filter (BWKCPMC105 at B&H)
- The camera did not have peripheral illumination enabled, but I don't think it would have mattered as (a) there is no lens profile recognized by my 5D3 and (b) the type of vignetting this issue creates is a black and white hard obstruction.
- Pulled Focal Length value from the EXIF from a Mac OS Command-I (info) pull. No idea if there is a more exact way to get the value.
16mm - 24mm: Clean. No vignetting. Results with the CPL ring on a two-slot Lee holder and the CPL was in place:
16mm: Vignettes considerably. Expected.
18mm: Vignettes slightly. Cloning/editing this out is only a small chore at this point.
19mm: Vignettes the smallest possible amount. A trivial fix in PS or whatever you use.
20mm - 24mm: Clean. No vignetting.
Pleasantly surprised. I thought I'd need to painstakingly disassemble my Holder down to a 'two options' variety (one slot with CPL, two slots with no CPL ring, etc.) to use this holder at all with the 16-35. But as my holder stands, I can shoot with 2 slots (no CPL) at 16mm and all three starting at 20mm. I love it.
That the only wildcards here that might differ on your FF rig with this lens are (definitely) the thickness of your CPL's front ring and (possibly) the version of your Lee Holder. Lee is known to have snuck in weird versions over the years that are ever-so-slightly different.
Hope this is helpful to you!