[Leica] Re: Polarizers

["Peter Su" <psu@jprc.com>]

>If someone told you that circ. polarizers were for autofocus lenses, I
>guess it could be true. Perhaps one of the camera makers uses a polarized
>beam splitter to extract focus information. I've never heard of this
>though. Leica publishes the reason for circular polarizers.

My understanding is that most AF SLR cameras have a beam splitter in the
mirror that directs part of the light from the middle of the mirror into a
module near the bottom of the mirror box where the AF hardware lives. The AF
hardware then splits the beam further (like a rangefinder, actually) and
determines focus by reading contrast information measured by an array of CCD
sensors.

You therefore need to use a circular polarizer on such camera in order for
the beam splitters to work correctly.

On my Nikon, the in-camera spot meter also uses the light from the beam
splitter to make its measurements, while the other meters are in the
viewfinder. I think.

Pete