How polarising filters work
When rays of light pass through the lens of your EOS DSLR camera, most of them are reflected up to the viewfinder by the reflex mirror. However, the centre of this mirror is semi-transparent. Some of the rays of light pass through and hit another, smaller, mirror (often called the ‘sub-mirror’). Here, they are reflected down to the autofocusing sensor in the base of the camera.
The light rays are polarised as they are reflected from the sub-mirror. However, if the light is already polarised before it reaches the mirror, it won't be reflected. This means that if you use the simplest type of polarising filter on the lens – the linear polariser – the amount of light reaching the autofocusing sensor will be reduced. If this happens, the autofocusing system may not work, or at best will give unreliable or inconsistent results.
There can be a similar problem with the exposure sensors in the viewfinder area. The light reaching the sensors will be reduced, which means that the exposure readings may not be accurate.
These problems can be resolved by using a circular polarising filter. This is a linear filter with a ‘quarter-wave’ plate bonded to it. In simple terms, this plate ‘stirs up’ the light after it has passed through the linear filter, in effect giving it a circular motion.
This means that you gain the effect of polarised light, without the limitation of all the rays oscillating in the samedirection. A few of the rays will still be lost when they are reflected from the mirror, but the majority will get through to do their job. The autofocus and exposure systems shouldgive reliable results.
Most polarising filters you buy today will be the circular type. If you are buying second-hand look for ‘Circular’, ‘CPL’, ‘PL-CIR’ or similar engraved on the filter mount.