Polariser - for stunning blue skies with puffy white clouds - BUT - only works at certain angles to the sun, and requires 1 1/2 - 2 stops exposure compensation
Just like to contend the exposure compensation thing on polarisers Colin... Might have been true when cameras didn't have TTL meters and everyone was toting a hand-held meter, but these days most modern SLR TTL meters should deal with a polariser without the user having to set any exposure compensation on camera. What is true of polarisers with moderns SLRs is that they will reduced the EV by 1 1/2 to 2 stops, effectively meaning a slower shutter speed or wider aperture.
[PEDANT MODE OFF]
One other thing to think about if anyone's contemplating a polariser - they come in linear or circular forms. Make sure you buy the one that's right for your camera's metering system, as the wrong one will wreck the exposure setting calculated by the camera's meter - most modern AF cameras need a circular polariser, which sadly are usually a bit more expensive than linear polarisers.
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