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04-17-2011, 07:38 AM   #16
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Circular Vs. Linear Polarizers
There are two types of polarizing filters available - linear or circular. Linear polarizers are more effective and less expensive than circular ones. But circular polarizers are needed with just about any camera that has a through-the-lens metering system, or autofocus.

The reason for this is that both of these systems use semi-silvered mirrors to siphon off some of the light coming though the lens. If that light is linearly polarized it renders either the metering or the autofocus ineffective. This means that you're going to have to buy circular polarizers unless you're shooting with a pre-1970's camera, or a view camera.

I use a slim Sigma CPL for my 12-24 and I payed around 120 for it. It produces nice polarization (deep blue skies, white clouds and enhances the contrast through the whole picture) and like some said - don't go cheap.

Some mildly edited examples of mine shot with the 12-24 and Sigma CPL: <- uneven sky (but that would be visible even without a polarizing filter)

04-17-2011, 08:00 AM   #17
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QuoteOriginally posted by Wheatfield Quote
If you look at the optical path, the light comes in through the lens and is directed both up to the focus screen and also down to the AF sensor (via a partially silvered section of the mirror and a sub mirror).
Because of the way the partially silvered mirror section works, that light is partially polarized.
However the light going to the screen is not polarized.

So, when using a linear polarizer, it is possible to "blind" the AF sensor, however in practice I have rarely found this to be an issue, and in even more general terms, polarizer use and AF aren't conducive to each other anyway.
Usually, when I am using a polarizer, I am on a tripod shooting manual focus anyway.
The concern people have with linear polarizers is exposure inaccuracy, but Pentax still takes it's light readings off the focusing screen, which is not receiving polarized light, so the possibility of cross polarization affecting light readings doesn't exist.
I could see a person's needs being different if they were shooting a brand other than Pentax which has the metering component under the mirror being fed by a pellicle mirror section, but for the Pentax brand, there is no pressing reason to use a CPL, and my experience has shown that there are some very good performance reasons to not use them.
Thanks for that explanation. A price-conscious friend just came back from the camera store with a linear polarizer for her K100d Super that had been recommended to her by an employee there whom I respect. Now I understand why. I, on the other hand, had followed the conventional line and bought an expensive B&W CPL when I went digital.
04-18-2011, 12:47 AM   #18
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QuoteOriginally posted by Wheatfield Quote
This is incorrect in relation to Pentax cameras. People should do more research both before writing this sort of stuff and before posting it.
I acknowledge that, but you can't find a low profile, multi coated linear pol filter. Which is needed here.
Hoya only makes circular filters in the Pro1 series.
The only one I found is the Heliopan Slim-linear Polfilter SH-PMC.

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