[daft_biker]

Originally Posted by Eastree

Would that be two CPL or two linear polarizers? And how exactly does that work, when cross polarization so often blocks a great deal more light, or can even block light completely (at a perfect 90 degree cross)?

Cross-polarised (UK spelling ) flash is a method that is sometimes used to control highlights on reflective surfaces. I use Lee polarising sheet on the flashes and a circular polariser on the lens.

Here's an article discussing the technique.

I suspect if you want to use AF then you need a circular polariser on the lens as normal and it you are focussing with available light the kind of polariser on the flashes is irrelevant. Not that I've ever tried to use AF but even with the polarising sheet covering the modelling lights on my macro twin-flash so I can preview the effect through the viefinder it never goes completely dark and I can manually focus.....haven't a clue about the physics of why it works...it just does