On Mon, 1 Feb 1999 07:40:39 +1300 [EMAIL PROTECTED] (nikon-digest) wrote: > > Could someone please tell me what the difference between circular and >linear polerizers? I think it is related to auto-focus, but would >appreciate an explanation. I have done a bit of RTFM on this, but my >books don't cover the distinction. In AF cameras the mirror not really a mirror - it is a beam-splitter. This means that some of the light passes through the "mirror," while most of the light is directed upwards towards the viewfinder. The light passing through the beam-splitter is dircted downwards to the AF sensors by a small sub-mirror that hangs off the beam-splitter. This beam-splitter is sensitive to the polarization of light. If all of the light is polarised the same way, then metering/AF errors may result. Circular polarisers overcome this problem by un-polarising the light after it has polarised it. It can do this because it is really two filters in one: there is a linear polarised at the front, and a quarter wave plate behind it. The polariser polarises the light, so all the waves have the same orientation. The quarter wave plate rotates the angle of polarisation of some of the waves, so that the light waves have different orientations.
