Years back I wanted to get an UV filter for my digital camera and ended up getting a 3-pack that also contained a polarising filter (and a macro filter). However, until recently it stayed home most of the time: I knew that it is useful for darkening the sky and making colours more vivid, but hadn’t really found it that useful.
Then last spring I was playing around with it again and finally noticed where it really shines: controlling the highlights in a photo to look more into the shadows. To illustrate what this means here’s a short series of shots with and without the filter. All of them are straight off the camera with no post-processing.
First two shots of a tower. Even if the difference is not that big here I do prefer the one with the filter (it’s more vivid and well-balanced between the sky and the grass), yet I’m not sure I would bother putting on and adjusting the filter for a photo like this: much of this effect could also be done in post-processing and sometimes you actually overshoot the colour.
However things start to change with the next two shots of a summer sky where the effect of the filter is more pronounced: to me there’s quite a difference between the two shots, not least in how the clouds stand out. While this could still be done in post-processing, it is starting to get more involved — and my end result would probably be pretty close to the shot with the filter.
As a final example have a look at these two shots of my girlfriend by a lake. While the one without the filter is flat and boring, the filter reveals an interesting play in the shadows and gives a very different photo. Of course this could still be fixed in post-processing but at this point it definitely makes sense for me to put on the filter and get a way better offset.
Note that it also becomes clear what the filter does: by taking away the reflections from the water it allows the camera to get a better overall exposure, as the light is now more evenly distributed through-out the frame. In this particular case the light coming from the lake is reduced so much that the shutter speed goes from 1/500 to 1/200 of a second, and hence more light from the darker areas (the trees and my girlfriend) is captured.
In summary, the polarising filter allows one to capture more details from the sky and from the shadow areas. On top of that it makes colours more vivid.