B+W PL Filter: Standard or Kaesemann?

[neuroanatomist]

My understanding is that it only affects autofocus systems that use birefringent lenses between the secondary mirror and the AF sensor. I think that's only the old ones (or maybe also the cheap ones?).

I believe it can affect both. I'd phrase it that a linear polarizer will affect autoexposure, and might affect autofocus.


dSLRs use a 'half-silvered' reflex mirror which reflects some light up to the viewfinder and metering system, and transmits some light which is then reflected downwardto the AF systemby the secondary mirror. 'Half-silvered' isn't really correct - in most cameras, ~75% of the light is reflected up for the VF and metering, and ~25% is transmitted for AF. Linear polarization alters the ratio of light that is reflected by the reflex mirror, whereas circular polarization does not. That actually means that a linear polarizer is somewhat more polarizing, in a way, which is one reason Daniel prefers them. The variance in the ratio of reflection by the reflex mirror means a linear polarizer willcertainly affect metering (because the metering sensor will receive a greater or lesser amount of light at a given intensity, depending on the polarization angle), but it also means that the light reaching the AF system may be reduced to the point where the AF system cannot achieve a focus lock, depending on intensity and polarization angle.

[Daniel Browning]

Aha. Thanks for the correction and clarification, John. I may not have noticed it because I tend to use LPL only in bright light.