Polarising filters are used in both colour and black and white photography and the function of a polarising filter is to filter out light that is polarised perpendicularly to the filters axis. Polarisation through the use of a polarising filter thus allows light waves to oscillate, or move, in only one direction and using a polarising filter in photography has a major effect on the resulting photographs as it both reduces reflections created by most surfaces and darkens certain images, like the sky.
There are two types of polarising filters used in photography: the linear polarising filter and the circular polarising filter. Both the circular polarising filter and the linear polarising filter have the same effect but the autofocus and metering mechanisms of some cameras do not work well with linear polarising filters. This has the result that most photographers prefer to use circular polarising filters in their cameras.
Circular polarising filters come equipped with small linear polarising filters on the front but instead of splitting the light beams during the polarisation process – which is what a pure linear filter does – circular polarising filters select one direction of light beam while rejecting the others. The circular polarising filter will then use a retarder, or quarter wave plate, to change the polarisation of the light beams traveling through it. The light beams polarisation as initially selected by the linear polarising filter is thus changed inside the camera by the quarter wave plate into circularly polarised light. It is thus this alteration of the lights selected polarisation into circular polarisation which gives the circular polarising filter its name.
Circular polarising filters should be used when photographs are being taken of objects with non-metallic surfaces, like water, as they greatly reduce the glare caused by these surfaces reflecting the available light.