2x cpl as nd fader?

[Marsu42]

I recently bought a 10 stop nd filter and a cpl filter. Since the 10 stop is very nice for the usual people removal, water/sky blur as predicted by some it has to be accompanied by a brighter nd filter to complete the setup.

Question: I read that the nd faders are nothing more than two polarizers together, so if I add another b+w xs-pro mrc2 cpl to my existing b+w slim mrc that should do the trick?

This is my current research concerning the "stacked" solution, it even seems to have advantages...

(-) vignetting on ultrawide if stacking the same size
(-) more hassle to adjust, no scale
(-) iq loss due to more space between filters(?)
(-) uv leakage in addition to ir (like in non hot-mirror nd filters)

(+) another cpl is cheaper than a vari-nd
(+) cleanable since the two glass layers can be removed
(+) both filters usable independently
(+) it's possible to adjust the rear polarizer angle, important for the usual cpl shots with added nd effect

[neuroanatomist]

You'll want a linear polarizer (cheaper, too) to stack with the CPL to make a homemade variable ND.

[Marsu42]

You'll want a linear polarizer (cheaper, too) to stack with the CPL to make a homemade variable ND.

You're saying that 2x cpl doesn't work as a vari nd or just that cpl+linear is cheaper?

[neuroanatomist]

You're saying that 2x cpl doesn't work as a vari nd or just that cpl+linear is cheaper?

Two stacked CPLs won't work effectively as a variable ND.  A CPL is a linear polarizer with a 1/4-wave plate behind it, that 1/4-wave plate in a sense 'un-polarizes' the light, because AF systems cannot use linearly polarized light.  A varible ND cuts down the light as the orientation of a pair of linear polarizers approaches orthogonal (90° to one another would be 'black' if the polarizers were 100% efficient, which they aren't).  The commercial variND filters have just one 1/4-wave plate at the back to circularize the the output so AF works.  If you stack two CPLs, the light is already circularized when it hits the second linear polarizer (the 'front' of the second CPL), meaning you can't approach the orthogonal orientation that results in progressive light blocking. 

Therefore, two stacked CPL's in 'normal' orientation (i.e. if you don't modify one of them) won't yield very much darkening.   If you pull the glass from one of the filters, flip it, and put it back in the mount, and put that modified CPL in front of the unmodified CPL, then it will work as a variable ND filter - but that's because by flipping it, you've put the 1/4-wave plate in front, and actually converted your circular polarizer into a linear polarizer.  Easier to just buy a linear polarizer...

Bottom line, the way to make a DIY variable ND filter is with a linear polarizer stacked in front of a CPL (not the reverse).