Polarise filter

 - Polarizing Filters -


Q1: What does a polarizing filter do?

A1: In order to answer this question, you'll first have to know something about
   light. Light can be regarded as a flow of particles (called photons), or as
   an electromagnetic wave. I'll try to stick to the wave theory. So, light
   can be regarded as a waveform, oscillating in an arbitrary direction
   perpendicular to its direction of motion. There will be waves oscillating
   up/down, oscillating left/right and all that's in between those two.
   A polaroid filter only allows to pass the component of the oscillation that
   is directed in the polarizing direction of the filter.
   So only the waves that are oscillating in the polarizing direction can pass
   the filter unattenuated. All other waves will attenuate according to the
   formula:

   Amplitude after polarizing = Amplitude before polarizing * cos (theta),

   where theta is the angle between the oscillating direction of the wave and
   the polarizing direction of the filter.
   The overall effect is blocking out half of the available light, and
   'directing' the other half.


Q2: What is the difference between a linear and a circular polarizer?

A2: The main points are:

   (1) If you use a standard linear polarizer with cameras that use auto-focus
       and/or auto-exposure, you may have problems.
       This filter may also cause trouble with manual cameras, if you're using
       TTL light measurement. A little dependent on the camera, your meter
       might go crazy.
       (It is not said you _will_ get problems, only that you might get them.)