A circular polariser is without doubt the one filter that every photographer shooting landscapes needs to own
iT can be argued that, with advancements in sensor technology, digital techniques, and editing software, the effects of some filters such as nd grads and, to an extent, nd filters themselves, can be replaced during post- processing. bracketing and exposure blending can simulate graduated nd filters, whilst multiple exposures over time can create an effect remarkably similar to that of using an nd filter to achieve a long exposure. However there’s one filter that simply cannot be replicated – the circular polariser ( CPL).
In order to fully understand how a polarising filter works, we need to explain a little bit about how light works, and the different types of polarising filters available. Light consists of both electric and magnetic waves and, in unpolarised light, these waves are travelling in chaos, oscillating on multiple planes. Direct light waves that are reflected off nonmetallic surfaces, such as glass, water or foliage, are partially polarised and reflected largely across one plane. a CPL filter blocks these light waves from reaching your sensor at certain angles, depending on the filter orientation, effectively sorting out the chaotic and perpendicular waves, allowing only light waves on a single plane to pass through. Why does all of this matter? because, in doing so, a polarising filter can be used to block glare and reflections, to cut through haze and to saturate the colours in your images. not to mention the two- stop light loss offers added creative benefits.
There are two types of polarising filter – a linear polariser and a circular polariser. When it comes to digital cameras, only circular polarisers should be used; using a linear polariser can play havoc with metering and autofocus systems. a circular polariser comprises two filter rings that can be rotated independently of one another – the front ring contains the polarising filter, whilst the rear- most ring holds a quarter- wave plate. Rotating the polarising filter allows you to control the direction of polarised light, effectively like an on- off polarisation dial. The quarter- wave plate then converts this linear light into circular polarised light, allowing your camera’s autofocus and metering functions to operate properly.
Above: A circular polariser is the perfect tool for the discerning outdoor photographer, whatever the weather.