I haven't tried any of this before, so forgive my ignorance: but suppose one could keep the adjustable elements close to the lens, i.e. a 4" nd Grad and a 4 "CPL, and then hand hold a large ND in front of that assembly, would that work? how often would one need a three-element setup that included a 10 stop ND?
ND Grad needs independent rotation control.
CPL needs independent rotation control.
Straight NDs do not
need rotation control.
So your proposed option will not
work in a standard Lee 2 slot setup as you'll be fighting the CPL rotation versus the ND grad rotation. Assuming your ND Grad is king for rotation, you can 'clock' your 4x4 CPL 90 degrees in the holder, but you won't be able to dial-in the degree of CPL effect -- that's a very crude adjustment.
I'd also going to rule out hand-holding either the ND Grad or 4x4 CPL to the front of the apparatus if the Big Stopper type ND is in use
. You can hand-hold an ND grad for a few seconds, but not for the 30 seconds, 90 seconds, 120 second exposures needed for that.
So your options to use all three simultaneously are (and I welcome others, there all kinds of tricks here it seems):
1) Get a 105 CPL ring (from Lee), screw it into your holder, and pony up the bucks for the 105 CPL.
It goes in front of your 2 slot Lee holder. Your ND and your ND Grad (or possibly just two ND grads) go in the slots. You use the outrigger/holder rotation to set your ND grad rotation, and you independently use your CPL.
Pro: Cleanest and easiest method. Oodles of flexibility.
Con: Pricey. Likely vignettes around 24mm (depends. Could be 21, could be 23, could be 25mm, depends on the lens filter thread height, if you are using a WA ring, the CPL rim height, etc.).
2) Get a tandem Lee holders that allow 4x4/4x6 filters to be independently rotated to one another.
One gets the 4x4 CPL and the other gets the ND Grad. (I'd imagine the straight ND could go onto either plank of this as its not rotation dependent.)
Pro: No need for the 105 ring and CPL cost.
Con: Pretty thick? Still a vignetting on WA still a problem? I've never seen numbers on this.
3) Screw a standard CPL right on to the lens and then hook the Lee ring and holder on to that
Pro: Super cheap as you likely already have a CPL for your lens.
Con: Each time you rotate your ND Grad, you are rotating your CPL and have to fidget to deal with that. That sounds a horrible pain in the butt. Also: Vignetting on WA is still in play as you have effectively made the lens' filter ring 'taller' by stacking a CPL on it -- you can't exactly lose those filter threads with a thin version (or this idea won't work), and the value of the Lee WA ring is lost here as well.
4) This one's a leap, but... Somehow retro fit the Lee SW150 system to work on a non-Nikon 14-24 lens?
Provided you could find a way to mount it (a foam donut?), you'd get the width of the 150mm, and since it doesn't touch the front of your lens, you could possibly leave a standard CPL on it.
Pro: Possibly slays the vignetting beast on WA (no idea about that -- you may still have to aggressively index the 'lens barrel grabbing end' of this monster towards the front element to pull this off, and the on-lens CPL opportunity may be lost.
Con: Where do you start: cost, time, trouble, larger hardware, bigger filters, etc.
There is another option that is eluding me, and that might be a wiseass one, like giving up on a CPL at 16mm as it will render such inconsistent polarization and reflection control over such a wide FOV. Perhaps at FL under 24 you pull one slot out of your holder
and leave one tool (the CPL, the ND or the ND Grad) in your bag...