Circular polarising filters are really helpful in certain situations, but can also ruin your photos if used at the wrong times. So here is a quick guide to what they are, and when you should (and shouldn't) use them.
What is a CPL?
A circular polariser, as its name suggests, is a polarising filter that is circular, so it can screw onto the threads at the end of your camera lens. Note that many entry level (point and click or bridge cameras) may not have threads on the end of their lenses, and so you won't be able to use a CPL. All interchangeable lens cameras will allow you to use one.
What does a CPL do?
If you've ever worn polaroid glasses, you will already know what a CPL filter does; it essentially cuts out glare. Reflected light off water (bodies of water or even wet leaves in a forest) are polarised; this means that all of the light is oriented in one direction and this is the cause of glare. CPL filters (and Polaroid glasses) essentially significantly reduces this glare. The beauty of a circular polarising filter, is that you are able to rotate it and therefore change how much or how little you want to reduce it.
Where to use a CPL?
There are a small number of specific occasions where I will always dig out my CPL:
When I am photographing a body of water and I want to be able to see under the water surface (e.g. rocks under the waters edge); this can be done using a CPL to reduce the surface glare.
When I am photographing waterfalls and want to manage the excessive glare that occurs on the white water parts of the scene (and could therefore be over exposed in the final image).
If I am taking photos in woodland after rain, when the leaves are wet.
When NOT to use a CPL?
Although CPL's are invaluable in the situations listed above, it's important NOT to leave the CPL on your camera all of the time, as there are many situations where it can cause problems, or even ruin an image. These include:
As well as cutting out polarised light, a CPL will also cut the overall amount of light entering your camera. So when light is low, it will mean you will have to use higher ISO or longer shutter speeds to get a well exposed image.
In some situations, it's the glare of the sun that helps to make a photo, e.g. when the setting sun leaves a beam of light reflected in the sea, so why would you want to remove this?
If you are taking a photo of a clear blue sky, especially with a wider lens, a CPL can cause uneven colour tones across the sky, which look very odd and unnatural and are almost impossible to correct in post processing.
So, a CPL is an important bit of kit for any landscape photographer, and can be picked up relatively cheaply. Ensure you get one the correct diameter for your lens.
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