Following on from last week's post on UV filters, this week I'm going to delve into the other filters that I use.
I have a bag of filters that's pretty much permanently attached to whichever camera bag I'm using at the time. In it I've a couple of different sized circular polarisers (though I don't remember which particular brand I'm on at the moment as I seem to go through these things like they're ten a penny), a Singh- Ray Variable Neutral Density Filter, a B+W 77mm #110 Solid Neutral Density Filter and assorted LEE Filters 4x6" Graduated NDs.
Polarisers are extremely useful for cutting down on glare and reflections, and adding some saturation to skies or foliage. This site provides quite a good example of the effects of a polariser with an article title "Demonstration of what can be done using a polarizer". You'll see as you mouse on and off the image that the foreground grass shows more colour, the sea less reflection, and the sky has more definition. For any respecting photographer, a polariser is essential. The effects of a polariser, particularly the reflection reduction are extremely difficult to re-create after the fact in Photoshop. Not impossible, but painstakingly time consuming, and for the sake of a few seconds experimentation at the time, and the price of a case of a good red it's a no-brainer.
I also use the two circular neutral density filters quite a lot, though obviously not together! I find the Singh-Ray variable ND filter very useful for adding just a little adjustable density to cut out enough ambient light just to slow my shutter a stop or two, though I do often find that it throws a rather bluey colourcast when dialed all the way in. This would be when I use the B+W 110. It's great fun to use and artistically opens up a lot of options. The downside is that once it's on, you can't see diddley through the viewfinder so focus and composition have to be set up before screwing on the filter. Then it's some calculated guess work to get a ball park shutter speed.
Finally, graduated neutral density filters are exactly that. The graduation can be either hard or soft. I would use a hard one, if there was clear separation between the dark and light zones in my scene; an ocean horizon for example. Or I'd use a soft if there were elements from my foreground that extended into the lighter zone; buildings or trees spring to mind.
I've written a couple of posts previously on using neutral density filters so to save going over old ground, there's "The key to long exposures", "Silky water shots - it's not that hard" and "Variable neutral density filters".
You can also get reverse grads, gold and blue polarisers, infra-red filters.... the list is almost endless, but the only one of those that I've ever really had my eye on is a Gold 'n' Blue polariser simply because the effects it can create can be stunning.