When a polarising filter is fitted on the end of a lens, it prevents light from some angles from passing through to the camera. This reduces reflections on shiny surfaces and boosts colour saturation. It can be especially noticeable with blue skies, as it can turn them almost black while bringing out the detail of any clouds.
How to use a polarising filter
Companies like Hoya, Kenko and B+W make polarising filters in a range of sizes to suit all sorts of lenses. If you’re not sure which size you need, you can usually find out the diameter of your lens by looking at the inside of the lens cap.
A polarising filter has its maximum impact when the camera is perpendicular to the sun. A really easy way to work out where the peak effect will be seen is to point your index finger at the sun, while holding your thumb out at 90 degrees. Keep your finger pointing at the sun as you twist your hand — this will enable you to see the directions that the camera can point in order to get the strongest polarising effect.
The filters are made with two rings: one that screws into the filter thread on your lens and the other that holds the filter material. This second ring can be rotated to change the angle of the polariser, and to adjust the degree of polarising effect.
If you look through the viewfinder or at the camera screen in live view mode, you’ll see the brightness of the image change and/or reflections and saturation vary as you rotate the filter. The degree of polarisation builds from zero to maximum and back to zero through every 90 degrees of rotation. It’s up to you to decide how much impact you want.
As a polariser cuts out some light, you should measure and set the exposure after fitting the filter and setting the level of effect. Be careful of using a filter with a very wide-angle lens, as the polarising effect can vary across the frame.
While they are best known for their impact with blue skies, polarising filters are useful with a wide variety of subjects. They can cut down the glare on vegetation, for example, making the colours more intense. They can also dramatically reduce reflection in windows and water, making them useful for landscape, riverside, lake and coastal images, as well as architectural photography.