Normally, a polarizer will remove 1 stop (half the light when the light is randomly polarized). Measured on a dark sky, the change in light depending on rotation varied from -0.5 to -2, so the polarizer could be used to lighten or darken the sky with respect to the rest of the picture.
Here are some examples of the polarizer used on the sky, taken in
the Bay of Fundy National Park in Canada on Fuji Velvia, first without
and then with the polarizer:
Note that the road surface is polarizing, as well as the ocean and
clear sky, while the foreground guard rail, grass, concrete wall, and
clouds are not. The polarizing surfaces turn blue or yellow as the
filter is rotated, whereas the non-polarizing surfaces just look
purplish regardless of rotation. Ignore the glare in the bottom
right, which was caused by my holding the filter up in front of a
digital camera without a mount.
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Terran Melconian.
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