And Ash is quite right, with a couple extensions:
* When shooting B&W, a Red B&W filter increases dynamic range slightly -- as well as reducing light by a couple stops.
* When shooting glaring neon lights at night, a Yellow B&W filter gives interesting (psychedelic?) colour effects.
* When shooting B&W, a Blue B&W or a violet CC (color correction) filter replicates early photo emulsions, which only saw 'actinic' (UV-violet-blue) light.
* IR filters block much visible light and pass invisible IR spectra. Shooting B&W with a 720nm (#72) partial IR filter reduces light by several f-stops.
Yes, stacking filters reduces contrast and should be avoided for best image quality. But for anyone who likes to experiment with tones and effects, filters are fun to play with.
Last edited by RioRico; 08-12-2011 at 05:01 PM.