Originally posted by ChrisPlatt My manual focus film camera have no problem with linear polarizers.
I choose them for their more pronounced effect.
In a pinch a linear polarizer can be useful when you don't have a neutral density filter.
I don't suppose a circular polarizer would be much use for this purpose.
Chris
You can stack two linear polarizers or stack a linear polarizer in front of a circular one to create a variable ND filter. You can stack two circular ones but the front one must be reversed for it to work.
The results are extremely sensitive to the quality of BOTH filters and usually there's a nasty color cast.
If you have a bunch of polarizers of unknown quality, you try various paired stacks (rotated to maximum darkness) to work out which are decent and which are lower quality. The darkest pair are your best two polarizers. It's a bit of a logic puzzle because if a pair can't produce a dark color-free filtering, you don't know whether the front or back polarizer is the cause unless other pairings prove one of them to be good.
Another quick test is to look through the polarizer at a high-quality LCD panel such as are found on laptops, monitors, and smartphones. To get good blacks on an LCD, the screen needs a very high quality polarizer. Rotating a good photographic polarizer will make the screen go to almost black whereas a bad polarizer will let more light through or have a color cast.