Just to add to the discussion re Linear vs Circular polarisers, I just came across this, in the UK "Landscape Photography" magazine:
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Polariser Choice: Linear or Circular?
There are two types of polarising filter on the market - Linear and Circular. Only the Circular will work properly with your Digital SLR. Although both varieties are physically circular and similar in appearance, the Linear variety will affect the accuracy of your camera's metering system.
This is because digital SLRs polarise some light inside the camera. If this light has already been polarised by a Linear polariser, a false meter reading is given. Circular polarisers are constructed with a wave-retardation plate, allowing the light waves passing through to rotate and appear unpolarised to the camera's metering system.
Buy Circular.
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Don't know how accurate or legitimate all that is, but I just thought I'd throw it into the mix....