Circular Polarising Filters Explained | What is a Circular Polariser?

So what's the deal with circular polarising filters? They are a great tool for landscape photography but they have a whole load of uses. For one, they help to darken the sky, make clouds pop and bring out colours in your photo. They also help to cut through reflections if you want to photograph through water or through glass. Let's talk about it.

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[shaggygoat]

An appropriately tilted linear polariser was traditionally used to improve the sky-cloud contrast because blue sky is partially linearly polarised. This comes at the cost of transmitting only 70.71% (down 1 stop) of unpolarised light and by transmitting polarised light, interferes with the camera’s glass-or-silica-coated mirror (which often partially polarises light horizontally, yielding unpredictable light reduction). A true circular polariser, if one could be constructed, would not yield the effects you describe, since it is rotation-invariant. As johnflatcher1036 says, a typical circular polariser is actually a linear polariser with a quarter-wave plate behind it (with their slow and fast axes 45° to the linear polarisation), yielding circularly polarised light which in turn yields a consistent dimming (1 more stop) upon reflection (and H-polarisation) by the mirror.

[johnfletcher]

You never explained what the polariser is. It is two filters glued together, the front filter is an ordinary polarising filter, followed by a quarter wave length filter. This quarter wavelength filter causes the polarisation of the light to rotate its polarisation which enables the optic sensors in the camera to evaluate the light as though it was not polarised by the front filter. The camera sensors will not work with plane polarised light, so such functions as focusing will not work reliably.