Originally posted by Alex645
Bob,
Interesting. Wouldnʻt you get different results if the light reflecting thru the filter be at different angles? In other words a real situation where there the polarizer is reducing a lot of glare from 90 degree sources?
Yes. That's true for a scene in which some of the light is polarized by angled surfaces (and the sky), but the change in exposure seen there is just the camera readjusting to get what it thinks is a correct exposure.
For example, if you place a 0.3 neutral density filter over the lens, you would see the camera bump the exposure by 1 f-stop to compensate.
Same with the polarizer (actually two bumps since the polarizer is a bit of an ND filter as well as a polarizer). In some cases, one might need to crank in some exposure compensation to get darker skies since the camera could fight that polarizer effect by increasing the exposure and bringing the skies back toward where they were before the polarizer is adjusted for sky darkening.
To test the exposure system's sensitivity to changing the rotation of the polarizer, I shot a diffuse wall which reflects very little polarized light. The light getting to the camera metering system is polarized by the filter only and rotates as the filter is rotated so if it has any change in sensitivity as the "sense" of the polarized light changes, it would show up as a differing exposure as the filter is turned. Assuming that produces no exposure change, the changes seen in a real life shot are due to actual darkening of parts of the scene due to the filter not passing as much light for those portions and the metering system would respond to that.
Hope that's not too long winded.