Can someone explain this one to me?\n\nI have two polarising filters, both screw-ons with std filter threads and\nrotating mounts.\n\nOne is a Hoya PL-CIR.\nThe other, Suparon PL.\n\nBoth, individually, do the usual "darken the sky" and "reduce the reflected\nglare" stuff; quite satisfactorily. (I could post pics, but take my word...)\n\nOk, so today I held both of them in line and rotated them to observe the\n"darken to black" effect we all know and love when we cross two polarised\nfilters. I did this with the filters off the lens - just held in my hand.\n\nNowt!\n\nWell, at least, very little effect. The colour balance altered\nsignificantly, from bluish (not quite 80A blue, but blue at least) to yellow\n(tungsten yellow). No significant darkening at all.\n\nAhhh... but it gets better.\n\nWhen I flipped the closest-to-my-eye (Hoya) filter so that it was now\nthreads-facing-me, the two filters polarised perfectly, from almost clear to\nsolid black.\n\nFlipping the Suparon had no effect.\nFlipping the Hoya altered completely its properties. By itself, it fully\npolarises the LCD screen I'm reading now. Flipped back to front, it just\nalters the colour balance.\n\nIs this a property of Hoya filters that's well-known? How does it work? I\nwould have thought that a polariser would not be dependent on direction of\nthe light source.\n\nAnyone?