The other day I have met someone who has just started getting into photography. When he bought the camera the sales persons not only sold him a cheap UV filter as protection for the camera but as well a polarizer. First of all I would like to tell you that you you don’t need to buy a UV filter. Certainly not a cheap one. Why do you buy an expensive lens and put a 10 dollar filter in front of it? Only in certain situations where you would like to protect your camera from the environment (salt water, sandstorm, etc…) it is useful. Otherwise use your lens hood all the time. It not only makes your images more contrasty by blocking unwanted light but although protects your camera.
But I wanted to talk about the polarizer. The photographer I have met had the filter on his camera but had no idea what he could do with it or how to use it. So, I have showed him that by turning the circular polarizer you can remove certain reflections from a scene. I opened his eyes when I showed him the effect on a window where we reduced the amount of reflections to a limit.
I mainly use the polarizer in landscape photography. The effect is most visible in the sky (it becomes more blue) and water (it becomes clearer). In general your images will have more contrast. It is very important that you always try to have the sun on of your sides (optimal is a 90º). If it is in front or behind you it won’t work. The disadvantage is that the polarizer takes some light away. You probably have to overexpose 1-2 stops.
Only use the polarizer (and other filters) when you know what you are using it for. You should not leave them on the lens all the time.
Above I have posted two unedited images straight out of camera. The first one with the polarizer and the second one without it. As you can see the sky has a darker richer blue and the water is much clearer on the first one. Go and try it.