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Trying to decide which type of polarization filter I should get.

Some information I've read suggests that there is no difference and that a circular polarizer is just an ordinary linear polarizer, but a circular piece of film that can be rotated in order to adjust the phase of the filter.

Others seem to suggest that it is the result of two linear filters, aligned 90° out of phase. Like this Hyperphysics page, located on the Georgia State University network:

If light is composed of two plane waves of equal amplitude but differing in phase by 90°, then the light is said to be circularly polarized. 

It goes on to say:

Circularly polarized light may be produced by passing linearly polarized light through a quarter-wave plate at an angle of 45° to the optic axis of the plate.

I've probably misunderstood something about the fundamentals of how it all works, or got something backwards. But was hoping someone could demonstrate the difference it makes in terms of the result it has on a photograph, rather than trying to explain with diagrams. Thanks.

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  • @ths It's not the same. Please check before suggesting possible duplicates so that questions aren't closed unnecessarily, for having similar titles.
    – voices
    Dec 19, 2017 at 10:28
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    @tjt263 There's nothing to compare. The camera (film or digital) records light from a linear polarizer and light from a circular polarizer exactly the same. Cameras do not record which direction the light was polarized when it strikes the sensor.
    – Michael C
    Dec 19, 2017 at 12:20
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    Not sure what you mean, but the answer is no. Cameras do no record polarization, and the photographic effect of a circular and linear polarizer is exactly the same. There is no difference.
    – Aram Hăvărneanu
    Dec 19, 2017 at 12:49
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    I think it is the same question — the earlier one just needs better answers.
    – mattdm
    Dec 19, 2017 at 14:00
  • 1
    Maybe we could merge them.
    – voices
    Dec 19, 2017 at 14:23

3 Answers 3

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A 'circular' polarizer is just a linear polarizer with a quarter-wave plate behind it that repolarizes the light in a circular kind of way. But the polarized light removed by passing through the linear polarizer is gone, and thus is not reintroduced by the "re-polarization" of the remaining light.

A quarter-wave plate is not another linear polarizer. It is arranged 45° with respect to the linear axis of the polarizer. As the polarizer is rotated to alter the effect of the filter with regard to light from a particular direction, the quarter-wave plate is attached (bonded) to the linear polarizer and rotated as well. The quarter-wave plate is always turned 45° with respect to the polarizer's linear axis.

The polarization of the light by the quarter-wave plate allows phase detection autofocus (PDAF) systems and light meters to function properly. PDAF systems often fail when trying to focus light that has passed through a linear polarizer alone. Light meters placed past the viewscreen in reflex¹ cameras are also affected by linearly polarized light.

There is no effect of the quarter-wave plate on the appearance of the photo compared to a linear polarizer without a quarter-wave plate. Neither film nor digital cameras record in which direction light is polarized when it strikes the sensor or film. The linear polarizer allows light polarized in one direction to pass through it. The light that is not allowed to pass through is gone. The quarter-wave plate can only act on the light that has been allowed to pass through the linear polarizer in front of it. It can't recreate the light that the linear polarizer blocked. So the light striking the film or sensor is the exact same light either with or without a quarter-wave plate behind the linear polarizer (other than the minimal amount of light lost due to transmission through a refractive medium as happens with every lens element in the optical path).

Two linear polarizer filters arranged 90° out-of-phase with one another will (theoretically) block all light from passing through (with or without a quarter-wave plate behind the second one). So-called 'Variable Neutral Density' filters are actually two stacked polarizers. As one is rotated with respect to the other, the varying angle between them blocks more or less of the light striking them.

¹ 'Reflex' is the 'R' in SLR and DSLR and refers to the reflex mirror used to divert light from the lens to the viewfinder.

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  • "Two linear polarizer filters arranged 90° out-of-phase with one another will (theoretically) block all light from passing through..". Thats what I thought. One of the reasons I found it confusing.
    – voices
    Dec 19, 2017 at 10:51
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    @tjt263 But a quarter-wave plate is not another linear polarizer. It is also arranged 45° with respect to the linear axis of the polarizer. As the polarizer is rotated to alter the effect of the filter with regard to light from a particular direction, the quarter-wave plate is attached (bonded) to the linear polarizer and rotated as well. The quarter-wave plate is always turned 45° with respect to the polarizers linear axis.
    – Michael C
    Dec 19, 2017 at 10:56
  • So the quarter wave plate is like the rifling of a gun barrel that causes a bullet/linear-projectile to spin on its axis; like a football spiral?
    – voices
    Dec 19, 2017 at 11:26
  • The effect is similar in concept. The mechanism is different. A quarter-wave plate acts as a retarder to the light passing through it. As the light entering oscillates on a single axis the degree of the wave's oscillation determines at what angle it is oscillating when exiting the plate. On the other hand, if the light entering a quarter wave plate is circularly polarized it will oscillate on a single axis when it exits the plate.
    – Michael C
    Dec 19, 2017 at 12:09
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I think the most valuable filter a photographer can own is a polarizing filter. 1. Darkens Blue Sky 2. Mitigates Reflections from non-conducting surfaces 3. Increases Color Saturation 4. Cuts Haze.

They come in two flavors: Linear and Circular. The Linear was king and then came cameras with automation. Most of these depend on semi-silvered mirrors and perhaps internal polarizers. Such strategies are impaired if a linear polarizer is mounted. What happens is underexposure and interference with autofocus mechanisms.

The remedy is a Circular Polarizer. These do the same job as a Linier Polarizer and they work with older cameras as well. The problem is these carry a higher price tag. Let me add that I have often used the old style on modern cameras with little or no ill effects.

The Circular Polarizing filter is actually two filters sandwiched together. A Linear Polarizing filter is upfront, and does the deed. A retarder filter is set just behind the Linear. The retarder acts to de-polarize the image forming rays which allows the automation to function unhampered. Keep in mind that if the Circular is mounted backwards, it fails.

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  • Thanks. My main interest is in attenuating direct surface reflections when taking (sometimes extreme) macro photos. Why do people want to darken the sky?
    – voices
    Dec 19, 2017 at 19:08
  • @ tj263 --- For copy and macro work, the maximum effect to mitigate glare is to set the lamps at and angle of 57 degrees and find tune the filter by rotating it until the reflections are at a minimum. For water its 53 degrees, that's why we fine tune by adjusting both lamp angle and filter rotation. As to darkening sky, often clouds do not standout against a mundane sky. The polarizing filter darkens the sky more than it darkens the clouds. Thus the contrast of sky vs. cloud is elevated.
    – Alan Marcus
    Dec 19, 2017 at 19:15
  • I'm thinking of adding polarizing filters to my two flash tubes; out of phase with the main one over the lens.
    – voices
    Dec 19, 2017 at 19:26
  • @tjt263 -- Not out-of-phase as this will induce cancelation. You compose with filters on the flash, no filter on the camera and rotate for effect. When the maximum effect is achieved, mount the camera filter and rotate for effect. This is a trial and error adjustment. Better if you use continuous lamps as you can visualize the effect of the filters on the lamps.
    – Alan Marcus
    Dec 19, 2017 at 19:31
  • I thought cancellation was the goal. Because I want to cancel out the harsh reflections.
    – voices
    Dec 19, 2017 at 19:47
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You should always get a circular polarizer.

There is no difference in the appearance of you final photograph between a linear and a circular polarizer.

Linear vs. circular polarizer has nothing to do with the shape of the filter, they both rotate. Linear vs. circular polarizer has to do with the polarization of light.

Back in the day we used to use easy to make linear polarizers for photography. Unfortunately they interfere with autofocus, so we now use the more difficult to make, and possibly more expensive circular polarizers. I don't know why they still make linear polarizers for photographic applications, you should always use circular polarizers these days.

For photography, anything a linear polarizer can do, a circular one can do also, but not the other way around.

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  • So, the only reason we use circular polarizers (instead of linear polarizers), is to accommodate the auto focus? And it has no effect on the glare and/or reflections recorded by the camera? I find that hard to believe, so I just wanted to confirm I understood you correctly.
    – voices
    Dec 19, 2017 at 11:02
  • Why do you find that hard to believe?
    – Aram Hăvărneanu
    Dec 19, 2017 at 11:25
  • Well, for various reasons. But I'm not a professional optical engineer or light physicist. So it's difficult for me to articulate and I'm not confident in my reasoning. But then again, I'm not saying you're wrong; just trying to confirm I understood you correctly. I'm skeptical, and it seems unlikely. But if you insist that's the case, then I'll take your word for it.
    – voices
    Dec 19, 2017 at 11:38
  • So did I understand you correctly or not?
    – voices
    Dec 19, 2017 at 14:18
  • @tjt263: Yes: the only reason to use circular polarizers is for the auto focus to work correctly. As explained elsewhere, the thing that is called a circular polarizer in photography is actually a linear polarizer + a quarter-wave plate. The light that has passed through the linear polarizer has linear polarization; the quarter-wave plate changes the polarization to linear. The linear polarizer works the same wether or not there is that circulazer behind it, so effects on glare and reflections are exactly the same.
    – Roel Schroeven
    Dec 19, 2017 at 14:40

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