I have some basic questions about polarizing and IR filters! I want to buy both, but the question of what to consider confuses me. 1 - What's the difference between a linear and a circular polarizer? Pros/Cons? 2 - Will the polarizer/IR filter I buy fit/work on ANY dSLR lens as long as the openings are the same circumference? [say, 55mm] 3 - For the IR filter, how will factors, such as aperture/exposure time/ISO affect the tones of the picture that results?