Did you ask this in another section? I thought this was just asked and answered.
In any case, the basic polarizer is the "linear" polarizer -- but most modern cameras requrie a "circular" polarizer.
A "circular" polarizer is a really a liinear polarizer... with a quarter-wave plate just behind the linear polarizing layer. This has the effect of twisting the polarity of the light AFTER it passes through the polarizer. Without this, the auto-focus and metering systems on DSLRs will struggle to accurately focus and meter the shot.
If you have a DSLR, you must use a "circular" polarizer -- your camera will have issues attempting to use a simple "linear" polarizer.
Tim Campbell
5D III, 5D IV, 60Da
