I mentioned in an earlier post something about a filter. What is a filter and why would you use one? The answer is: to focus your attention on the subject. A filter is a colored piece of glass or plastic, that fits onto the lens of your camera, and changes the color cast of your photo to the color of the filter (when you use color film).
But to fully understand the effect of filters, let’s use them with black and white (b/w) photography examples, since the effect is more pronounced in b/w photographs, when using color filters. Some filters are used just for b/w photography, and some just for color photography. And some are used for both.
One way to focus attention on your subject in b/w photography is to make it stand out by using a filter that makes the subject contrast with the background. The main goals are to make a dark sky stand out, or to make one color contrast with another. Filters transmit light of their own color, and hold back light from other colors. So, to accentuate a yellow rose in a b/w photo, use a yellow filter. This will make the rose appear as light gray, while the leaves a dark gray, thus providing contrast to the scene and making the rose stand out.
If you use a red filter for a sunset shot with color film, you will highlight the red in the light of the setting sun. To cut down on haze in a scene, use a yellow, green or red filter in b/w photography, since haze is caused by the scattering of the blue light from the sky.
A polarizing filter may be used in either b/w or color photography, and it is used to reduce glare from glass or the surface of water, for example. It may also be used to darken the sky in a scene. To get the most effect from the polarizing filter, the light must be coming at right angles to the camera. In other words, the light is coming from overhead or from the side when you are taking a picture.
There is an excellent example of how a polarizing filter works here.