In this article, we look at the polarising filter, providing a guide to what it is for, and how it works
Light entering the lens of your camera is composed of electro-magnetic waves that oscillate in an infinite number of planes. The polarising filter allows only light waves through that oscillate in one plane, or linearly polarised light.
So why is this beneficial? The polarising filter gets rid of glare, which is often a bright concentration of light that has been linearly polarised naturally. Examples include reflections of sunlight and other bright-light sources on the flat surfaces of pools, ponds, windows, and so on. It can block this light if you rotate it until the linear axis matches that of the glare. Once the polarised glare is removed, you are left with a more natural view of the scene that has better contrast and colour because your eyes and your camera aren’t being dazzled by the unwanted polarised light that is now blocked.
However, the polarising filter does present a few problems. It darkens the view and polarised light can play havoc with the systems in a modern digital camera sensitive to the polarisation of light. There is no way to claim back lost brightness, but the other problems can be fixed through the use of a ‘circular’ polarising filter. This is a normal linear polarising filter on one side, but the light exiting it is circularly polarised.
On the camera side of the filter, an additional layer of a material slows down one of the two components that make up the light’s electro-magnetic wave. By slowing down that component by a quarter of a wavelength, the polarisation rotates 360° along the passage of each wave. Circularly polarised light is less likely to upset SLR camera exposure and autofocus sensors and even some digital imaging sensors.
Use a polarising filter with care because it can cause uneven density in skies, due to changes in the sun’s light polarisation by the atmosphere, which means it’s best left in the bag when shooting stitched panoramas.