Why circular polarizers?

Started May 11, 2014 | Questions thread
Wheatfield7 Senior Member • Posts: 5,936
Re: Actually, it's the metering

Leonard Migliore wrote:

JimCo wrote:

Okay. I think I got it now. It all has to do with the auto-focus mechanism and not the sensor. That had never occurred to me. Many thanks for all your nice replies.

I don't see how polarization effects can do much to a phase-detect autofocus; there might be some slight birefringence effects but those should be minimal. Phase-detect depends on comparing the position of two features, which is not affected by polarization.

What does get bothered a lot is the metering system, and autofocus is the reason for that.

A phase-detect autofocus needs to see some light when the mirror's down. So the reflex mirror in an autofocus SLR is partially transparent; the light that gets through the mirror is sent to the autofocus. The metering is up in the pentaprism.

Now, the transmission of a partially-reflective mirror depends on the polarization of the light that's hitting it. It transmits a lot more light with P-polarization than with S-polarization (google those terms for some pictures). But if more light goes through the mirror than it should, the meter calibration becomes incorrect. So a linear polarizer can give you exposure errors.

Mostly correct. The partially silvered main mirror in AF SLR cameras allows some light to pass to a sub mirror that then reflects light to whatever sensors reside in the bottom of the mirror box. The AF sensor invariably resides there, in some cameras, there might be a metering sensor as well.

The partially silvered mirror polarizes the light that passes through it, and if there is a linear rather than circular polarizer on the lens, it is possible to get a cross polarization which can blind the sensor(s) receiving light from the sub mirror. The circular polarizer eliminates this potential problem.

Many SLRs still meter off the focusing screen, and so it's fine to use a linear polarizer with them, as the light hitting the screen is not polarized by anything inside the camera.

I generally just use linear polarizing filters as I don't find using a polarizer lends itself to using auto focus anyway, and I bought very high quality polarizing filters long before auto focus was a reality.

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