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What cameras need a Circular Polarizer?
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Apr 16, 2019 17:15:33   #
John_F Loc: Minneapolis, MN
 
To avoid this mirror difficulty is one more reason to go mirrorless. Now what about polarization at lens surfaces?


larryepage wrote:
You are correct...there is a lot of misunderstanding around this topic. I'll try to explain based on the physics of the situation.

When light reflects off a surface at an angle, it is partially polarized at an angle parallel to the surface. The shallower the angle, the greater the polarization. This includes light reflecting at 45 degrees off the mirror in a SLR...film or digital. A linear polarizer may be at such an angle that it is "crossed" with the angle of polarization due to the mirror reflection, reducing the light that gets through to the metering sensor. When the mirror raises and the shutter opens, the light hitting the film or sensor gets no cross-polarization effect, resulting in overexposure...sometimes a little, sometimes a lot.

So the answer to your question is that a linear polarizer shouldn't be used anytime on a SLR/DSLR with through the lens metering.

To a much lesser extent, it may reduce light levels enough to interfere with autofocus operation, but that is not the primary reason.
You are correct...there is a lot of misunderstandi... (show quote)

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Apr 16, 2019 17:43:40   #
DirtFarmer Loc: Escaped from the NYC area, back to MA
 
John_F wrote:
Circular polarizer is used very loosely on these UHH threads and means the filter rotates. In the theory of optics there is technical circular polarization. Most physics of optics text books treat the subject.


That is not what circular polarizer means. Even the linear polarizer filters rotated.
The circular polarizer filters are in the technical sense producing circularly polarized light.

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Apr 16, 2019 18:03:23   #
larryepage Loc: North Texas area
 
John_F wrote:
To avoid this mirror difficulty is one more reason to go mirrorless. Now what about polarization at lens surfaces?


Theoretically there could be some polarization at lens surfaces. But for any significant polarization to occur, light has to strike the reflective surface at a shallow angle...45 degrees or less. And the shallower the angle, the greater the polarization. So don't worry about lens surfaces polarizing incoming light to the camera.

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Apr 16, 2019 18:44:53   #
nadelewitz Loc: Ithaca NY
 
John_F wrote:
Circular polarizer is used very loosely on these UHH threads and means the filter rotates. In the theory of optics there is technical circular polarization. Most physics of optics text books treat the subject.


WHAT?? "Circular polarizer" does NOT mean the filter rotates. It is describing how the filter "arranges" light. A circular polarizer does not have to be itself round. It could be a square filter that goes into a rotating holder.

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Apr 17, 2019 12:50:05   #
speters Loc: Grangeville/Idaho
 
speters wrote:
I just realized that I in fact wrote linear polarizers in my reply, I apologize, I certainly meant circular polarizers!


Please disregard my last comment, had one of those dead brain moments!

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Apr 17, 2019 13:05:08   #
Bill P
 
JohnFrim wrote:
Seriously???


It's a late April Fools Day post.

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Apr 17, 2019 14:42:28   #
jtbdal
 
Do you have any recommendations for brand of circular polarizing filter?

Thanks.

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Apr 17, 2019 14:47:25   #
JohnFrim Loc: Somewhere in the Great White North.
 
My recommendation for a polarizer is not based on brand (quality) but rather on physical depth. If the filter is "thick" it will tend to vignette the corners on wide angle lenses, especially if you are in the habit of keeping a protective UV filter on the front and then simply adding the polarizer (to avoid actually swapping filters). A thin filter (thin rings) works better from that standpoint, but is also harder to add and remove because there is less ring to grab. Trying it before you buy is the best. BTW, be sure to stop down the aperture when checking for the vignetting; it gets worse at higher-numbered f-stops.

Another recommendation is to buy a step-up ring (cheaper) for the smaller diameter lenses and get a larger diameter polarizer (more expensive) that will fit your telephoto lenses. The vignetting is not present on longer focal length lenses, and this way one filter (overall savings) will cover more than one lens.

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Apr 17, 2019 22:10:04   #
Keen
 
A CPL is pretty much needed on any digital camera, or on any camera-film, or digital-which has AF / TTL. Brand wise, Hoya is good. Your camera manufacturer (Nikon, Canon, etc) is another good choice for filters to use with the manufacturer's camera, and lenses.

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Apr 17, 2019 22:38:20   #
Keen
 
JohnFrim wrote:
That is a different question, and one that I have wondered about... will linear polarizers make a comeback, and will the price of a comparable quality linear polarizer be cheaper than its circular counterpart? Optically speaking the linear is simpler, with less "material" in the optical path, so it should actually provide less degradation in the image.


I doubt it. A Linear Polarizer is cheaper, because it has less material....a single piece of polarized glass. It is cheaper, and easier to use, but can cause problems when the light gets re-polarized / cross polarized. A Circular Polarizer is more expensive, and takes a bit more effort to use, but causes fewer problems. It has a second piece of glass-a wave plate-behind the linear polarizer in front. The Linear polarizer in front of the wave plate of the CPL stays stationary. The wave plate behind can rotate. The CPL gives better results, with fewer problems, in a wider variety of circumstances, with a wider variety of cameras. Why would anyone want an inferior product, even at a lower price?

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Apr 17, 2019 23:17:33   #
larryepage Loc: North Texas area
 
Keen wrote:
I doubt it. A Linear Polarizer is cheaper, because it has less material....a single piece of polarized glass. It is cheaper, and easier to use, but can cause problems when the light gets re-polarized / cross polarized. A Circular Polarizer is more expensive, and takes a bit more effort to use, but causes fewer problems. It has a second piece of glass-a wave plate-behind the linear polarizer in front. The Linear polarizer in front of the wave plate of the CPL stays stationary. The wave plate behind can rotate. The CPL gives better results, with fewer problems, in a wider variety of circumstances, with a wider variety of cameras. Why would anyone want an inferior product, even at a lower price?
I doubt it. A Linear Polarizer is cheaper, because... (show quote)

Actually, polarizing filters are not normally made of glass. The technique normally used is to sandwich a sheet of polarizing film between two thin pieces of glass or hard plastic.

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Apr 18, 2019 00:16:27   #
JohnFrim Loc: Somewhere in the Great White North.
 
Keen wrote:
I doubt it. A Linear Polarizer is cheaper, because it has less material....a single piece of polarized glass. It is cheaper, and easier to use, but can cause problems when the light gets re-polarized / cross polarized. A Circular Polarizer is more expensive, and takes a bit more effort to use, but causes fewer problems. It has a second piece of glass-a wave plate-behind the linear polarizer in front. The Linear polarizer in front of the wave plate of the CPL stays stationary. The wave plate behind can rotate. The CPL gives better results, with fewer problems, in a wider variety of circumstances, with a wider variety of cameras. Why would anyone want an inferior product, even at a lower price?
I doubt it. A Linear Polarizer is cheaper, because... (show quote)

Very little of what you state is true. As Larry already mentioned, the polarizing component is NOT glass. And the linear component AND the quarter-wave plate BOTH rotate together when you turn the filter. The polarization axis of the linear element MUST REMAIN at 45 degrees to the slow and fast axes of the quarter-wave plate at all times.

CPLs are NOT superior to linear polarizers in general because of the extra component in the light path; they DO PERFORM better on cameras that use mirrors to implement auto focus and metering. But as has now been stated several times, MIRRORLESS cameras do their focusing and metering right off the sensor without a mirror in the optical path, so there is technically NO REQUIREMENT for a CIRCULAR polarizer with these cameras.

Whether linear polarizers will again become popular as the mirrorless market expands is a marketing issue, not a technical issue.

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Apr 18, 2019 11:45:48   #
John_F Loc: Minneapolis, MN
 
From Wikipedia:

“In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electric field of the wave has a constant magnitude but its direction rotates with time at a steady rate in a plane perpendicular to the direction of the wave.”

Thus, as time passes the wave front moves forward and the electric vector describes a spiral. When the light wave enters the polarizing filter the electric vector will be in a specific position and if parallel to the filter’s plane of extinction, will be blocked. As the wave passes through the filter it will have rotated millions of times and so extinction can not be 100%.

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Apr 18, 2019 11:55:18   #
JohnFrim Loc: Somewhere in the Great White North.
 
John_F wrote:
From Wikipedia:

“In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electric field of the wave has a constant magnitude but its direction rotates with time at a steady rate in a plane perpendicular to the direction of the wave.”

Thus, as time passes the wave front moves forward and the electric vector describes a spiral. When the light wave enters the polarizing filter the electric vector will be in a specific position and if parallel to the filter’s plane of extinction, will be blocked. As the wave passes through the filter it will have rotated millions of times and so extinction can not be 100%.
From Wikipedia: br br “In electrodynamics, circul... (show quote)
This is correct. But remember that in a CPL the “circular” element comes after the linear element, and that restores the capability of the autofocus and metering to work. The unwanted glare is removed by the linear element.

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Apr 18, 2019 12:38:41   #
scsdesphotography Loc: Southeastern Michigan
 
JohnFrim wrote:
This is correct. But remember that in a CPL the “circular” element comes after the linear element, and that restores the capability of the autofocus and metering to work. The unwanted glare is removed by the linear element.


Thanks John and John, I did not have a clear understanding as to how a CPL actually rotated the light. So when I'm rotating the front element, it's the linear filter that's moving while the CP behind it remains fixed?

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