Jon is on the money with the CIRCULAR polarizer.
This is a polarising filter fitted to a rotating lens thread.
It allows for the best reduction of glare and colour saturation depending on where the sun is coming from.
Think of it as polarized sunnies for your camera.
You look through the view finder and adjust the filter to get the result you are trying to capture.
The standard ones are ND or Neutral Density which is a dark grey colour.
I use these though I have been wanting a "Moose" Peterson Circular Polar to best capture the browns and greens when out around reef and for intense sunsets......it's the closest thing to my Maui Jim sunnies that I have found without needing extra time processing the image in Photoshop.
This filter is more bronze/brown in colour, which is more in line with the sunnies I like to wear and therefore, the image colour that I am after.
as for the effect and shutter speed loss, Patrick has covered that to a tee.....
Aaron.
