Circular polarizer with GNDs

[Jim Gronau]

I'm about to buy some GNDs for the first time.  I expect I'll get Lees, 4x6s, with the Lee holder and the wide adaptor ring.  My question is about using a circular polarizer with them.  I know this can be done, but I'm unclear exactly how, so I'm not sure what I need to do.  I have a Tiffen 82 mm cp (got it for a Canon 16-35 f/2., but I gather this won't work with the GND holder?  The reason I ask is that Lee has a 105 mm cp that (somehow) works with the GNDs and the holder, but it's deadly expensive.  Is this really the only way to get a cp working with the Lee system, or is there a workable work-around?

I know some people hand-hold GNDs, but I'd scratch them for sure (klutz).  And I know some people prefer to do the work of GNDs in post, but so far my experience with software of this type hasn't always been happy, so I'm going for the filters.

For what it's worth, I'm using a Canon 5DmII, with the 24-70 f/2.8, 70-200 f/2.8, and the above-mentioned 16-35 f/2.8 (which I plan to trade soon for a new 17-40 f/4).

Any guidance would be much appreciated. 

[RFPhotography]

The way the Lee CPL works is that the Lee filter holder system is modular and you can attach different numbers of filter slots/adapters.  There's an adaptor that screws onto the front of the filter holder that you screw the polarizer onto. 

Sigma makes a 105mm CPL that's much more reasonably priced than others available.  Major retailers carry them.  Adorama has it for about $170.

[Jim Gronau]

Thanks, BobFisher, that's good to know.  Anyone have info about vignetting with the Sigma?