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What is the (technical?) difference between a normal polarising filter and a circular polariser?

Nov 5th, 2008 03:36
Asrar Khan, John Clarke, Andrew Chilton, http://www.sunspotphoto.com/ssp/lenses/polarizers.php


If you've ever played with two pairs of polarising sunglasses you'll
have seen what happens when two linear polarising filters are not
aligned at the same angle - some of the light is blocked.  When the two
polarisers are aligned at 90 degrees almost all light is blocked.
SLR cameras have a beam splitter which sends some of the incoming light
to the meter.  A side effect of the beam splitter is to polarise the
light that reaches the meter.
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A linear polarising filter and the beam splitter interact in much the
same way as the two pairs of sunglasses.  The amount of light hitting
the meter is dependent upon the relative angle of polarisation of the
filter and the beam splitter, and so varies according to the rotation of
the filter.  This results in less light reaching the meter and hence
incorrect meter readings.
A circular polariser fixes that by changing the polarisation of the
incoming light so that it's not linearly polarised when it reaches the
beam splitter, and hence the right amount of light reaches the meter.