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Customer Discussions > Photography forum

Difference between Linear Polarizer and Circular Polarizer Filter

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Showing 1-14 of 14 posts in this discussion
Initial post: Aug 28, 2011 6:01:35 PM PDT
I've been shopping for better quality, multi-coated filters than the inexpensive ones I first purchased with my camera about 3 years ago. I've seen two types of Polarizing filters listed in catalogs with no real explanations as to the difference. One thing is for sure, the Circular Polarizer's are definitely more expensive. Can someone "in the know" please explain the difference?

Posted on Aug 28, 2011 6:33:38 PM PDT
Last edited by the author on Aug 28, 2011 6:35:26 PM PDT
Floored says:
I stole this from another website.

A linear polarizer has a single plate for polarizing the incoming light. When this plate, which is attached to the filter, is rotated, the polarized light will change "phase/angle."

Some cameras, especially those SLR/DSLR bodies, have a beam-splitter to split the incoming light to the viewfinder, AF system and metering. Each of these components requires some minimal amount of light to work properly. For example, the AF system usually requires an aperture of F5.6 or larger. Thus, this splitting of light should follow a constant ratio, say 80% to viewfinder, 10% to light meter, and 10% to the AF system (just an example).

If the incoming light is NOT polarized, the phase/angle of the incoming light may be in all directions and evenly distributed to each of the three components. If the incoming light is polarized, depending on the way the incoming light is polarized, the phase/angle of the polarized light may be biased. In other word, since some portion of the incoming light is missing due to polarization, the light diverted to the three components may not be constant. For example, 70% to viewfinder, 5% to light meter, and 8% to the AF system. If the light is polarized significantly, the portion of the incoming polarized light that can reach the AF system may be too low to allow the AF system work properly. Or, if the light that reaches the light meter is lower than the expect level, the meter will "think" the scene illumination is insufficient and force the camera to use a larger aperture and/or a slower shutter speed. As a result, you might get over- or under- exposed images.

To overcome this problem, a second plate is added to a polarizer to "repolarize" the polarized light. The polarizers that have one plate are the "linear" ones, and the polarizers that have two plates are "circular" ones.

Therefore, if your camera does not have a beam splitter, you can use linear polarizers. Virtually all consumer level digicams do not beam splitter, and all SLR/DSLR bodies have beam splitters.
Hope this answers the question well."

In reply to an earlier post on Aug 28, 2011 6:35:56 PM PDT
Phase-detect auto-focus (at the least, though in some systems the exposure metering too) are affected by polarization angle. This is because the focus sensors are behind a reflective surface that, itself, causes some polarization.

If the focus system is registering horizontal waves, and you attach a polarizer that passes vertical... nothing gets into the focus system.

Circular polarizers are linear on the subject side, but on the camera side they have a coating that "unpolarizes" the light that came through. This is probably a frequency specific coating -- longer wavelengths get rotated by a different amount from shorter ones. As a result, the focus sensors will "see" some light regardless of the input polarization.

Posted on Aug 28, 2011 8:37:48 PM PDT
Wow, thanks guys - "Tallgrass" and Dennis Bieber - for the info. I can see that this is a highly Technical subject. It would seem that using the wrong Polarizer filter with a DSLR could cause some issues. It seems odd though that the filter would "repolarize" some of the light when that is what you are trying to keep from happening in the first place. I would think that the camera manufacturese would recommend the "circular polarizer" for their DSLR's, but I don't ever remember reading that. I'll go back and check the manuel...

Posted on Aug 28, 2011 9:06:49 PM PDT
Neo Lee says:
Without getting into the technical, circular polarizing filter is comprised of a linear polarizing filter in the front and a quarter wave plate at the back. Yep, the only difference is the addition of a quarter wave plate.

What is that quarter wave plate for?

Modern SLRs have autofocusing sensors that are less sensitive to linear polarized light. The quarter wave plate is there to transform the linear polarized light into circular polarized light so that the AF sensors will function properly.

In reply to an earlier post on Aug 30, 2011 1:20:24 AM PDT
Last edited by the author on Aug 30, 2011 1:22:53 AM PDT
" It seems odd though that the filter would "repolarize" some of the light when that is what you are trying to keep from happening in the first place."

Circular polarization is a two step process. First, there's a linear polarizing filter that rejects light that is orientated the wrong way. This gives you the polarizing effect of cutting out reflections or darkening skies. Light from this process is almost entirely orientated in the same direction. Then the light is rotated according to its wavelength (or color) so that the resulting light covers all orientations again. This rotation process is why its called "circular". The resulting light isn't really randomly orientated like natural light but its close enough for DSLRs to function properly.

In reply to an earlier post on Aug 30, 2011 1:34:39 AM PDT
Last edited by the author on Aug 30, 2011 2:01:47 AM PDT
Neo Lee says:
"It seems odd though that the filter would "repolarize" some of the light when that is what you are trying to keep from happening in the first place."

In personal and commercial photography, the point of using a light polarizer is to reduce reflections and glares. The goal is not about polarizing the light in certain angle. A polarizer in classical physics does not polarize all light coming through, it only rejects the light whose polarization is not aligned with the polarizer's axis.

Polarized light is a special kind of light. Here's why:

Jaws drop at the end of the video. Quantum physics is witchcraft.

In reply to an earlier post on Jul 23, 2012 1:33:26 AM PDT
Thank you for posting this. I have been wondering, too. i have a nice Tiffen polarizer from the early 80's and am glad to know I can still use it.


Posted on Jul 23, 2012 7:20:43 AM PDT
T. Campbell says:
Incidentally... if you're looking at a polarizer and don't know what type it is, there's a way to tell.

Hold the polarizer in front of your eyes and look through, twist the polarizer and note whether you're seeing the polarization effect (works especially well in front of a lot of computer monitors.) Now FLIP the polarizer around and do the same thing. If the polarizer works regardless of which way you look through it, you've got a linear polarizer. If the polarizer works correctly when looked through one way, but when flipped it doesn't work nearly as well (it may appear to shift colors between a gold & blue) then you've got a circular polarizer.

Polarizers work best when the light is coming from the side (not directly ahead or behind you).

In reply to an earlier post on Jul 23, 2012 10:08:31 AM PDT
Polarizers work best when the light is coming from the side (not directly ahead or behind you).

That applies when using the polarizer to darken a clear sky... But if trying to cut down glare off water, for example, the glare is going to be coming from "in front" (of the camera) {The source of the light may be to the side, but the ripple that reflects it to produce the glare is in front}.

Using a mirror is an easy test for linear vs circular. If you look through a polarizer at its image in a mirror, a linear will show "transparent" from either side. A circular polarizer will show black from one side, but transparent from the other.

In "correct orientation", the light coming through the back of the CP is rotated by wavelength, then polarized. The light then reflects off the mirror straight back (so polarization is not changed), passes back through the polarizer phase (no effect) and rotated by wavelength before reaching the eye.

When flipped, the light is first polarized, then rotated and bounced off the mirror. The return light gets some rotation and gets repolarized -- making the reflection darker than the first pass.

Posted on Jul 25, 2012 3:55:04 PM PDT
JCUKNZ says:
If you are starting out without any filters and want a strong Neutral Density filter it is worth knowing that a CPL and LPL mounted together and rotated against each other give you a variable ND filter ... with the drawback that you have added two glass elements to you camera lens.
As far as prices there are a huge number of filters offered these days and one needs to research the quality of each so that you are comparing apples with apples.

In reply to an earlier post on Jul 27, 2012 2:17:12 PM PDT
Tom Martin says:
This is what I did.
Just a note the LP has to be on the outside of the CP.

Posted on Jul 27, 2012 2:32:18 PM PDT
JCUKNZ says:
Thankyou Tom .. I did not know that wrinkle .. as yet Linears have been on the expensive side and yet to purchase one/any :-)

Posted on Jul 27, 2012 6:48:02 PM PDT
Last edited by the author on Jul 27, 2012 6:53:52 PM PDT
Tom Martin says:
The CP is actually a sandwich of a LP with a quarter wave plate on the camera side, this removes the linear polarization of the light after it has already been filtered, and allows the cameras autofocus to work.

So, by adding the LP to the outside, what you actually have is LP/LP/quarter wave plate/lens.

The the nice thing about the variable ND, is you can still autofocus, then rotate the outer LP, to darken it.
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Discussion in:  Photography forum
Participants:  9
Total posts:  14
Initial post:  Aug 28, 2011
Latest post:  Jul 27, 2012

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