Why circular polarizers?

Started May 11, 2014 | Questions
JimCo Regular Member • Posts: 483
Why circular polarizers?

I understand the principles of polarized light and circular and linear polarizers, but I don't understand why only circular polarizers are acceptable for digital cameras? I had a linear polarizer for my old film camera, and was flummoxed when told that I needed circular polarizers for my first digital cameras. Can anyone clarify this for me in a not-too-complicated way? Thanks, and please excuse my naivety.

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MediaArchivist
MediaArchivist Senior Member • Posts: 3,919
PDAF
1

Phase detect auto focus does not work (or at least not reliably) with a linear polarizer. A CPL uses a slightly different technique that yields the same polarization effect, but allows PDAF to work.

The Wikipedia article gets into the gory details:

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SkiHound Senior Member • Posts: 2,919
Re: Why circular polarizers?

Not a technical answer because I don't know the technical answer. It's not the digital sensor, it's the pdaf system on dslrs. I have a linear polarizer that works fine on an Oly E-M5.

Aberaeron Senior Member • Posts: 5,949
Re: Why circular polarizers?

JimCo wrote:

I understand the principles of polarized light and circular and linear polarizers, but I don't understand why only circular polarizers are acceptable for digital cameras? I had a linear polarizer for my old film camera, and was flummoxed when told that I needed circular polarizers for my first digital cameras. Can anyone clarify this for me in a not-too-complicated way? Thanks, and please excuse my naivety.

I just thought that it was so the polariser could be rotated to optimise the effect wanted by the cameraman. Maybe there's more to it than that? As far as my eyes can see, contrast lines are not altered in a scene by a polariser, only the light is altered. But what do I know? Very little, I admit.

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dlevitt Regular Member • Posts: 425
Re: Why circular polarizers?
1

Echoing some other replies - its the 'phase detect' auto focus that needs circular polarizers.

My old manual focus FL & FD series cameras only needed linear polarizers [At the time, I don't think circular ones were commonly available].

When I want to auto focus [Canon EOS Elan & RT bodies] I need circular polarizers - I splurged an got Canon branded ones.

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OP JimCo Regular Member • Posts: 483
Re: Why circular polarizers?

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.

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Leonard Migliore
Leonard Migliore Forum Pro • Posts: 14,816
Actually, it's the metering
2

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.

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Wheatfield7
Wheatfield7 Senior Member • Posts: 4,111
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.

Mike_PEAT Forum Pro • Posts: 12,868
Cameras that beamsplitter or semi-silvered mirror....
2

The real answer is cameras that use a beamsplitter or semi-silvered mirror require a circular polarizer. That's dSLRs and late SLRs. A circular polarizer is a linear polarizer with a 1/4 wave plate that recirularizes the light after going through the linear polarizer. This eliminates the issue polarizers have.

So if you use a mirrorless camera even with phase detect auto focus you DON'T need a circular polarizer...you can get away with using a cheaper linear polarizer.

Wheatfield7
Wheatfield7 Senior Member • Posts: 4,111
Re: Cameras that beamsplitter or semi-silvered mirror....

Mike_PEAT wrote:

The real answer is cameras that use a beamsplitter or semi-silvered mirror require a circular polarizer. That's dSLRs and late SLRs. A circular polarizer is a linear polarizer with a 1/4 wave plate that recirularizes the light after going through the linear polarizer. This eliminates the issue polarizers have.

So if you use a mirrorless camera even with phase detect auto focus you DON'T need a circular polarizer...you can get away with using a cheaper linear polarizer.

Technically, you don't need a circular polarizer with a camera with a beam splitter either, as long as the camera is metering off the focus screen and you can tolerate having to manual focus with the polarizer in place from time to time.

sillette Regular Member • Posts: 381
Re: Why circular polarizers?

I believe circular polarisers are needed when used with a SLR or DSLR which uses a half silvered mirror to enable viewing, focusing and exposure control at the same time. I am not sure they are necessary when used on a mirrorless digital camera. My circular and linear polarisers both work well on my EOS M.

Gerry Winterbourne Forum Pro • Posts: 13,191
Re: Why circular polarizers?

Aberaeron wrote:

JimCo wrote:

I understand the principles of polarized light and circular and linear polarizers, but I don't understand why only circular polarizers are acceptable for digital cameras?

I just thought that it was so the polariser could be rotated to optimise the effect wanted by the cameraman. Maybe there's more to it than that?

There is. The Wikipedia article linked in an earlier reply explains in detail, but in outline polarisation itself comes in two flavours - linear and circular. Linear polarisation is when the light waves move transversely in a plane; circular polarisation is when the waves move round, rather like a helix.

With either kind you need to rotate the filter on the lens for best effect, but that rotation is nothing to do with the circularity of polarisation

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Joseph S Wisniewski Forum Pro • Posts: 34,130
"Real" late SLRs?
1

Mike_PEAT wrote:

The real answer is cameras that use a beamsplitter or semi-silvered mirror require a circular polarizer. That's dSLRs and late SLRs.

My Nikon FA needed one. That's a manual focus SLR from 1/3 century ago. Not sure it qualifies as "late".

A circular polarizer is a linear polarizer with a 1/4 wave plate that recirularizes the light after going through the linear polarizer. This eliminates the issue polarizers have.

So if you use a mirrorless camera even with phase detect auto focus you DON'T need a circular polarizer...you can get away with using a cheaper linear polarizer.

Cheaper, and often better. Two bonds instead of three, so less chance of delamination.

Wow. I just got to wondering where the FA fell in the "early/late" scale. If you call the Nikon F the "beginning" of SLRs in 1959 and the D3 the end of film SLRs in 2007 (it marked the end of production of the Nikon F6, which was built largely from repurposed D2X parts) then the FA is a year on the "early" side.

I feel so old...

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bosjohn21
bosjohn21 Forum Pro • Posts: 18,561
Re: Why circular polarizers?

this is a quote from the Luminous Landscape in answer to the question why a circular Polarizer is necessary for almost all slr and dslr cameras.

"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."

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Ellie John New Member • Posts: 5
Re: Why circular polarizers?

hello everyone!  To see how a polarizer functions, it serves to first comprehend a tad bit about how light functions. Light is embodied two sets of swaying waves – electric and attractive – that are turned at right plot to one another. Unpolarized light is clamorous, on the grounds that the light waves originate from electric charges that vibrate on more than one plane. Spellbound light, then again, goes in a solitary, foreseeable heading.

BorisK1 Veteran Member • Posts: 5,638
Re: "Real" late SLRs?

Joseph S Wisniewski wrote:

Mike_PEAT wrote:

The real answer is cameras that use a beamsplitter or semi-silvered mirror require a circular polarizer. That's dSLRs and late SLRs.

My Nikon FA needed one. That's a manual focus SLR from 1/3 century ago. Not sure it qualifies as "late".

Late, as in the late Arthur Dent.

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MaxTux Regular Member • Posts: 449
Actually, "linear" polarizers may be preferred if...

Circular polarizers are required to preserve auto-focus functionality of DSLR cameras. They might also make exposure metering more accurate.

In practice however, polarizing filters are used primarily in landscape photography. There, focus is typically set at infinity, and exposure is best fine-tuned by inspection of the histogram of a test photo. If such instances, "ordinary", linear polarizers would have the advantage of cost, less thickness and slightly better optical performance.

MaxTux

Wheatfield7
Wheatfield7 Senior Member • Posts: 4,111
Re: Why circular polarizers?
1

bosjohn21 wrote:

this is a quote from the Luminous Landscape in answer to the question why a circular Polarizer is necessary for almost all slr and dslr cameras.

"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."

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John aka bosjohn21

Amazing how they package partial misinformation disguised as actual information.

bosjohn21
bosjohn21 Forum Pro • Posts: 18,561
Re: Why circular polarizers?

if this is miss information I am not aware of it as I have seen this explanation elsewhere it would be more helpful if you  told me where its not correct then I would learn something

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Wheatfield7
Wheatfield7 Senior Member • Posts: 4,111
Re: Why circular polarizers?

bosjohn21 wrote:

if this is miss information I am not aware of it as I have seen this explanation elsewhere it would be more helpful if you told me where its not correct then I would learn something

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John aka bosjohn21

A linear polarizer MAY cause a problem with the exposure, depending on where the exposure sensor is. Check your individual camera. If the exposure sensor is looking at the focusing screen and not buried in the bottom of the mirror box, it will not be affected by a linear polarizer.

The AF sensor is in the bottom of the mirror box, and depending on it's sensitivity, the maximum aperture of the lens in use the degree of polarization dialed in and the efficiency of the filter, it MAY affect the AF. All the linear polarizer does in this regard is reduce the amount of light getting to whatever sensor is buried in the bottom of the mirror box. If the AF sensor is sensitive enough to not be blinded, AF will be unaffected.

Luminous Landscape is speaking in absolutes, and in this they are wrong. Having said that, since it's well nigh impossible to buy a linear polarizer these days, the point is rather moot, but if you have a camera that meters off the focusing screen and tend to use a polarizer in situations that lens themselves to manual focus, and have a good linear polarizer sitting around, then you might as well use it rather than buying a circular polarizer.

Also, if you are using a mirrorless camera, there is absolutely no need to use a circular polarizer, other than that might be all that is available to you. I still shoot with linear polarizers that I bought for my view cameras and manual focus SLRS.

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