A polarising filter will cost you two stops, But only in maximum position So, if you focus AND lock and then set the polariser, you will under-expose your image.
I'm thinking about buying a polarizer for the 18-55 kit lens, has anyone done this. I know the front element of the lens spins when focusing, so is the work around that simply focusing first, and then rotating the polarizer? Or does that affect metering?
BTW, here's a nice software filter site: http://www.opanda.com/en/pf/index.html
As for whether its worth it. It certainly can be. Its main job is to eliminate glare. The reflections from glass and water that cannot be captured by your camera, but do act to blow out the picture.
They also have the side effect of adding saturation to the image. Your sky will be deeper, your forest will look more "foresty." :lol:
So, there is a trade off. But a useful gadget to have around.
Back when I was using my mini Rollei rangefinder with a polarizing filter, I compensated for the change in stops by setting the dial which indicates theASA value of the film to be a certain amount less than it actually was. Doing this, Iwas able to use the camera's light meter as usual, setting shutter speed and fstop so that the light meter said things were perfectly exposed. This worked fine and I got perfectly exposed pictures. I can only imagine the same can be done with a DSLR somehow, by using a programmed mode and setting the +- exposure setting a few tics in the right direction. This should then give you the compensation you need for the polarizer in M,S,A and P modes.
I used the polarizer with my Rollei specifically to darken skies, and have some incredible photos of white buildings in the Cyclades in Greece with deep blue skies.