And here are the shots with the CPL...
And the verdict is in.
Using this apparatus without a CPL shows a kiss of vignetting right at 24mm but nearly immediately disappears above that FL. So the Lee 105mm adaptor ring will slightly vignette on the 24-70 F/4L IS at 24mm.
However, with the B+W CPL I linked higher up in this thread, we see stronger vignetting at 24mm, but it seems to be gone by 28mm. So the CPL does in fact make it worse (based on the intensity of the vignetting) by I'd guess 2-3mm.
Limitations of this comically unscientific study:
1) I'm told not all lenses are their exact FL, so your mileage may vary with different lenses in this setup.
2) Note that the vignetting is strnger on the right hand side (the forum view seems to crop a bit, please open the images as downloads). I would say that this could be a little tolerance or play in the 4x6 filter in the holder but it wasn't using that -- everything keys off the adaptor ring and very snuggly attached holder with the Lee two nubs + brass guillotine latch. No idea why the vignetting is asymmetric.
So now I have a quandary for my upcoming Hawaii trip. This setup is A+ for my 28 prime or my 24-70 zoomed in the slightest amount. But if I want to rent an ultrawide (and I do), this is much more complicated than just taking off the CPL for those shots -- the adaptor ring is also in the way, and as said before, I wouldn't perform that surgery in the field.
So I've got some strategizing to do on this front. Options as I see them:
1) Eliminate one 4x6 holder slot and shoot whole days of the trip that way. Hope I don't need an exotic ND grad combination or the need to use an ND grad and the Big Stopper simultaneously.
2) Use the 24-70 for the whole trip and shoot 24mm without the CPL or 28mm with the CPL. Give up on ultrawide with filters.
3) Just give up on ultrawide CPL use. Return the 105 CPL and adaptor ring to B+H and rent that ultrawide lens and shoot with it. When I need a CPL, forego the Lee setup and just switch to the 24-70 with a traditional 77mm CPL attached.
4) The nuclear spendy option would be to have two Lee setups, one with one slot dedicated to ultrawide use, and another like shown in this thread for 24-28mm minimum use.
I'll just say now that #4 is out. That's just wasteful for such a niche need.
Ideas? Your thoughts?