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I have just purchased a Canon SX30 IS and though extremely happy with it I will need to use a polarizing filter due to the fact that I am travelling to Indonesia to do volcano photography/landscape and wildlife photography also. In the past I used to take my bulky Panasonic camera that was fitted with a circular polarizer and have had amazing success and great photos with it but had to sell it due to weight and size and lack of zoom power.

Hence my question, I have been reading that you do not need a circular polarizer for the SX30 and that a linear will work just as well....but is this true? and how do linear polarizers give you the same effects that you can manually achieve with a CPF just by rotating the filter to get desired effect?

Sorry to sound so novice but never used a linear polarizer and it is confusing to me...

Look forward to your advice and help. Cheers

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See also "What is the difference between a linear and a circular polarizer?" photo.stackexchange.com/questions/1368 — but none of the answers there directly address your confusion. –  mattdm Mar 6 '11 at 15:43
    
Wow... Thanks guys. Amazing answers and I thank you all for taking the time to enlighten me. Looks like I will be out buying new filter this weekend with a much better understanding. Along that line, apart from Hoya which I have used before would anyone care to comment on Marumi filters? I have read some good things about them and they are so much cheaper. Up to the $50 mark what would be a great circular polarizer... Thanks once again, you guys are a real help. –  user4185 Mar 7 '11 at 6:05
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3 Answers

You have an understandable misconception which is clouding the issue for you. The "circular" in "circular polarization" doesn't refer to the way the light is polarized in a different plane as you rotate the filter. It refers to the way the waves of light themselves are aligned. Check out this (public domain) illustration from Wikipedia:

illustration of circular polarization

Trippy, huh? But really, this part doesn't matter. The problem is that linearly polarized light might interfere with metering in an SLR camera. That's because the half-mirror used to direct light to an SLRs' metering and autofocus system also has a polarizing effect. The concern is that certain alignments of incoming linearly-polarized light might get canceled out, affecting the metering and focus. Using circularly-polarized light bypasses this.

But, really, it's not this wacky radial polarization that's useful. It's the linear effect that's really desired — that's what keeps out light that's aligned inconveniently in your scene. So, why does a circular polarizer work anyway? Another picture (again, public domain from Wikipedia) makes this clear:

Construction of a circular polarizer

You can see that the "normal" light coming in from the right side is first restricted to being linearly polarized, and only after transformed into the convenient-for-metering circularly-polarized light.

That was long, so here's the summary answer in bold: In either a simple linear polarizer or a more expensive and complicated circular one, it's rotating the orientation of this linear polarization that gives the important effect.

Your point & shoot camera doesn't use a half-transparent mirror to direct light to a dedicated set of metering and autofocus sensors, so there's no need to do an additional transformation to prevent interference. So you can just go ahead with the linear polarizer and it will work the same way.

(All of this said, I've seen pretty good anecdotal evidence that linear polarizers don't significantly interfere with metering or focus in most SLRs either. Presumably the polarization effect of the half-mirror isn't as strong as assumed by the circular-is-necessary theory.)

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I once tested linear polarizers with my Nikon D90, and I can confirm that the effects really are minor. When setting to auto mode, the brightness of images taken with the polarizer held horizontally and vertically differed by about 1/3 f-stop (estimated). –  Simon A. Eugster Mar 6 '11 at 17:43
    
A lot depends on what you're shooting. On a digital camera, you normally have enough latitude that the error from a linear polarizer would almost never matter. Speaking from experience, that's not true when you shoot Velvia though. –  Jerry Coffin Mar 6 '11 at 18:19
    
it also depends on the specific camera (make and model). Linear polarisers were fine with older SLRs up into the 1990s for example, when different materials and sensors were starting to get into SLRs requiring circular polarisers (I've tested it years ago, and did see a difference with my Nikon F80, when an old Minolta X300 could use linears with no difference for example). –  jwenting Mar 7 '11 at 7:33
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Circular polarizers are only necessary because of the type of autofocus detection that DSLRs (and autofucus film-based SLRs) use. The polarizer part is the same.

A circular polarizer is just a linear polarizer with a "depolarizing" filter behind it -- light has to be available to the phase-detection autofocus sensors in the polarization that they're oriented to. If you use a normal, linear polarizer, then the camera would only be able to focus when the filter is oriented close to vertical or close to horizontal (depending on the sensor orientation -- "cross-type" sensors would focus in either orientation). Anything too far from the sensor orientation, and there's no light left for the autofocus system to work with.

Contrast-detection systems (as in most point-and-shoot cameras) work differently, so they don't need the light to be depolarized after it passes through the polarizer layer.

If you know how to use a circular polarizer, you know how to use a linear polarizer -- the polarizer element is identical. The difference between them is in what happens after the polarizer has done its job.

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The circularly-polarized light isn't _de_polarized. It's still polarized but in a different way. And, I don't think it's right that the autofocus sensors themselves are oriented to a specific polarization — it's just a matter of how light is diverted to them. The net effect is that the conclusion of your answer is right, but the background details you've given are questionable. –  mattdm Mar 6 '11 at 15:48
    
All light is linearly polarized, all of the time. That is, every photon of light has a polarity (an orientation of the E and H fields perpendicular to its direction of travel). A linear polarizer merely blocks light that is not polarized within a small tolerance of its line orientation. A CP twists the light to be tangentially rather than sagitally polarized at back of the filter, but the result is that at the sensor plane the light is effecively unpolarized (or, rather, more randomly polarized than it would have been without the LP element). –  user2719 Mar 7 '11 at 0:30
    
Saying that every photon has polarity is not the same as saying that all light is polarized. I'll agree that circularly-polarized light is effectively same as unpolarized, for the purposes of mitigating any polarizing effect of the beam-splitter. But I've never heard that phase-detect autofocus sensors themselves are sensitive to polarization. Do you have a source for this? –  mattdm Mar 7 '11 at 13:30
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I don't agree that you can set «unpolarized» and «circularly polarized» equal. 3D video in cinemas works with circularly polarized light, left hand side polarized for the left eye, right hand side for the right eye (or also swapped), and a «reversed» polarizer converts the circularly polarized light back to linear polarized light and either blocks or passes the light. –  Simon A. Eugster Mar 7 '11 at 17:00
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To put it simply, the polarizing effect of a linear polarizer messes with the phase-detect autofocus systems used on DSLRs which uses polarization itself.

Compact cameras and mirrorless cameras almost always use contrast-detect autofocus, where it doesn't matter how the light hitting it is polarized.

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What about phase-detect autofocus as implemented in the Fujifilm F300 EXR? –  mattdm Mar 7 '11 at 3:10
    
I've added the words "almost always" –  thomasrutter Mar 7 '11 at 4:54
    
So, as I understand it, it's not the phase-detect sensors themselves that are sensitive to polarization, but the beam-splitter used in autofocus SLRs to divert light down to the AF module at the bottom of the mirror-box while sending most of it up to the viewfinder. The phase-detect sensors in the Fujifilm F300 EXR are directly embedded in the sensor, so there's no beam-splitting. –  mattdm Mar 7 '11 at 13:35
    
mattdm I've wondered about that before actually. I'm guessing you are right in that with no beam-splitting polarization shouldn't be a problem. –  thomasrutter Mar 7 '11 at 15:19
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