Subject: How does a circular polarisation filter works?

Thanks for your site,
I fully understand linear polarisation. I want to stick to electromagnetic 
waves: a magnetic field vibrating perpendicular to an electric field according 
to Maxwells equations. I'm sure, circular polarisation does not mean that the E-
vector of one single wave (photon) rotates around the poynting-vector. That 
means that circular polarisation must be the inteference of two waves with the 
same amplitude, same frequency, but perpendicular plane of the E-field and a 
phase difference of pi/2. If I'm right, how can a material let through only one 
plane of E-field and also the perpendicular direction but with a phase 
difference of pi/2? Why is this type of filtered light usefull in photography 
if perpendicular light waves come through (reflected light, diffracted light)? 
What is the advantage of circular polarised light concerning auto focus cameras 
and are there any disadvantages?
Sorry for the broken english!

Re: How does a circular polarisation filter works?