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09-16-2011, 08:22 AM   #1
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Does the K-5 need a circular polarizer, or will linear do?

I just read an article that recommended circular polarizers for newer SLRs, because the newer models tend to have partially-reflective mirrors, and this affects the metering. But he didn't say anything about DSLRs...seems like this wouldn't be a big deal for a DSLR, even if it was throwing the metering off a bit. After all, the only difference between circular and linear is to correct for that ... right?

If I was going to get a polarizer, what are your recommendations? I currently sport a Tamron 28-75 macro (67mm width) and I'm looking into a DA-15 or a Sigma 10-20 - I don't suppose either of those are 67mm as well?

Thanks!

09-16-2011, 09:31 AM   #2
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You MUST use a circular polarizer for any digital sensor else the autofocus will not work. It's not the metering.

You need a circular polarizer (I don't think they even make linear polarizers anymore).

Check the lens database for filter diameters for each of your lenses. The 10-20 is a 77mm. The 28-75 is a 67mm. The DA 15 is a 49mm.

Brands I recommend are Hoya and B+W. You'll need a pricey thin filter for that 10-20mm or you'll vignette.
09-16-2011, 10:05 AM   #3
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QuoteOriginally posted by Aristophanes Quote
You MUST use a circular polarizer for any digital sensor else the autofocus will not work. It's not the metering.

You need a circular polarizer (I don't think they even make linear polarizers anymore).

Check the lens database for filter diameters for each of your lenses. The 10-20 is a 77mm. The 28-75 is a 67mm. The DA 15 is a 49mm.

Brands I recommend are Hoya and B+W. You'll need a pricey thin filter for that 10-20mm or you'll vignette.
I saw a post on either the Sigma 10-20 lens thread or ? that a linear polarizer could be used for our dlsrs in most situations. And they are far cheaper than the circular variety. Especially for the 77mm size. Written by someone that was using them for Los Angeles sunny weather. I've never tried them on my dslr so can't recommend either way myself.

For the OP, instead of getting several sets of filters (polarizers and ND are useful) for each lens, think about getting a stepup adapter to reduce the number you need to carry.
09-16-2011, 10:10 AM   #4
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Good call, thanks! And let me know if anyone discovers conclusively this circular vs. linear on the K-5 business. Would certainly save me some.

09-16-2011, 10:11 AM   #5
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I have the Sigma 10-20 and have just purchased a few Conkin P series pieces including the circular polarizer to fit the Conkin P holder.....I understand there may be some edge distortion when shooting at 10mm........unfortunately, I haven't yet had a chance to use them.............
09-16-2011, 10:18 AM   #6
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Don't know about the rest, but the DA 15mm Limited doesn't need a CPL, because it shoots as if it had one built in. In fact, I find my 49mm Hoya CPL does much more harm than good on the 15mm. Any time you can get away with not adding extra glass in front of a fine lens, you are better off.
09-16-2011, 10:19 AM   #7
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QuoteOriginally posted by Aristophanes Quote
You MUST use a circular polarizer for any digital sensor else the autofocus will not work. It's not the metering.

You need a circular polarizer (I don't think they even make linear polarizers anymore).

Check the lens database for filter diameters for each of your lenses. The 10-20 is a 77mm. The 28-75 is a 67mm. The DA 15 is a 49mm.

Brands I recommend are Hoya and B+W. You'll need a pricey thin filter for that 10-20mm or you'll vignette.
From what I've read, it's more an exposure issue (for some cameras, not all) than an autofocus issue.

See All about Polarizers - Linear and Circular

The mirror in the DSLR splits the incoming light into two paths and linearly polarized light will be split with different intensities depending on the polarization. Thus, I guess it's possible that very little light gets to the AF sensor which stops it from working but as long as enough light gets to the AF sensor (which would be true most of the time) focusing functions should work.

However, different intensities of light (depending on orientation of the polarizer) hitting the metering sensor will definitely impact exposure.

I seem to recall reading somewhere that Pentax DSLRs may not be affected on exposure because it reads from a more direct light path than a split path, however I can't find that reference right now.
09-16-2011, 10:27 AM   #8
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QuoteOriginally posted by kyc Quote
I seem to recall reading somewhere that Pentax DSLRs may not be affected on exposure because it reads from a more direct light path than a split path, however I can't find that reference right now.
According to this person Checked with Pentax support: Pentax SLR Talk Forum: Digital Photography Review Pentax support state that exposure is not affected but AF may be affected. That same thread also has contributors having tested linear polarizers with no effect on AF (although one user had some focusing inaccuracy).

09-16-2011, 11:09 AM   #9
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The effect on a PDAF is that part of the sample may not get through (not in phase) and therefor AF will often be inaccurate.

I made the mistake of using a LPL on my K200D years ago and all shots were OOF. That filter went back in the film toolkit.
09-16-2011, 11:29 AM   #10
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QuoteOriginally posted by Aristophanes Quote
The effect on a PDAF is that part of the sample may not get through (not in phase) and therefor AF will often be inaccurate.

I made the mistake of using a LPL on my K200D years ago and all shots were OOF. That filter went back in the film toolkit.
Agreed, but isn't this an inherent risk with any polarizer (CPL or linear)? Polarizers remove essentially all light in a certain phase, so detail that might help AF could disappear before the split beam. The linear polarizer would add possibly another phase that gets filtered out at the split. So I would think AF could be affected by any polarizer but that LPLs increase the chance of AF being affected.
09-16-2011, 12:21 PM   #11
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QuoteOriginally posted by Aristophanes Quote
You MUST use a circular polarizer for any digital sensor else the autofocus will not work. It's not the metering.

You need a circular polarizer (I don't think they even make linear polarizers anymore).

Check the lens database for filter diameters for each of your lenses. The 10-20 is a 77mm. The 28-75 is a 67mm. The DA 15 is a 49mm.

Brands I recommend are Hoya and B+W. You'll need a pricey thin filter for that 10-20mm or you'll vignette.
Re phrase this, the sensor is irrelevant, you need to have a circular polarizer for the AF on ANY SLR digital or film, for the AF to work,
09-16-2011, 01:01 PM   #12
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QuoteQuote:
Re phrase this, the sensor is irrelevant, you need to have a circular polarizer for the AF on ANY SLR digital or film, for the AF to work,
I think to clarify further add " if you are opting to use a polariser" it has to be a CPL not an LPL. Correct????
09-16-2011, 02:05 PM   #13
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I've played around with both linear and circular polarizers. My personal experience has been that the exposure is affected more than the focus. It isn't consistent so its not like you can put in an EV to compensate. But I see that to a lesser extent with a CP, too. As far as AF goes, my own experience has been that the camera will not be able to lock focus as well as without a polarizer, but that it will be otherwise accurate. I didn't think that the linear polarizer was all that much different than a CP in this regard. My conclusion was that I wouldn't buy an LPL, but that I wouldn't hesitate to use the ones I already have, as long as I chimped most shots.
09-16-2011, 03:38 PM   #14
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I just tried a linear polarizer on a k-x; it could focus regardless of the polarizer's orientation in a dim lighting situation.
09-16-2011, 03:55 PM   #15
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When i had my camera gear stolen some time ago, the camera store acting for the insurer 'accidentally' replaced my cokin CPL with a LPL. I also thought it was useless on digital having read various articles, so I purchased a CPL and have used it exclusively since then.

As a result of this thread I have just conducted a quick n dirty experiment with the LPL mounted on the Fa 77 and K-5. The focus and exposure look ok to me. Nothing scientific about it. Just put the LPL on the lens, focused on something nearby, pressed the green button for exposure (shooting in manual) and took a couple of exposures.

One thing I noticed is there is not a lot of difference when rotating the filter through 90deg's and taking another shot. Maybe the disadvantage is that the polarisation is not as nuanced as with a CPL? Someone much more educated than I may have the answer.

Last edited by wizofoz; 10-03-2011 at 04:26 PM.
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