Originally posted by photoptimist
The issue with UWA lenses and polarizing filters is that the sky has different amounts and directions of polarization across the sky. Thus, there's no setting of the polarizer than gets nice uniform results. In a UWA image with a polarizer, some parts of the sky will be darkened but some will not. The result is a strange look with dark blue sky in one part of the image and light blue sky in the other.
Correct, but why is this so? Lemme tell ya:
The polarized light in the sky comes from scattered sun light. The polarization effect on scattered light is strongest for light scattered at a 90 degree angle from the source (= the sun). So, the sky viewed roughly towards the sun shows very little polarization (as does looking in the opposite direction from the sun). If you look at the sky that is 90 degrees away from the sun, there will be the maximum polarization, and the maximum effect due to any polarizing filter.
So, if the sun happens to be nearly overhead, the sky along the horizon - in all directions - will show a substantial polarization effect. If your wide angle lens covers from the horizon to the zenith in this case, the polarization can vary considerably over the frame, depending on the angle the polarizer is set to.
This is physics - there's no way around it!!