How to physically tell the difference between circular and linear polarizing filters?

[Blake Gibson]

Hi guys, sorry this might be a silly question but I've spent a while googling and can't find an answer. Maybe I'm getting the terminology wrong.

Is there a way to tell the difference between circular and linear polarizing filters if you have 2 with no indication of what they are physically?

 

[jussi rovanpera]

circular polarizer can be rotated to different angles, so the difference is just the frame of the filter.

 

[Brody_B]

circular polarizer can be rotated to different angles, so the difference is just the frame of the filter.

I don't think so. It has something to do with whats in the actual filter. I don't remember the science, but the linear polars will mess up auto focus on dslr's. The image quality is supposed to be the same between both.

 

[jussi rovanpera]

I don't think so. It has something to do with whats in the actual filter. I don't remember the science, but the linear polars will mess up auto focus on dslr's. The image quality is supposed to be the same between both.

You're right, the linear and circular filters are different.

 

[Les Hillis]

Hi guys, sorry this might be a silly question but I've spent a while googling and can't find an answer. Maybe I'm getting the terminology wrong.

Is there a way to tell the difference between circular and linear polarizing filters if you have 2 with no indication of what they are physically?

"...You can also easily tell if a polarizer is linear or circular even if it is not marked. Hold the polarizer about 2 or 3 inches in front of your eye, and look at your reflection in a mirror. Looking from the camera's viewpoint (i.e. with the filter threads pointing towards you) you should see the image of your eye in the mirror (as if looking through a neutral density filter). Now turn the filter around so that the filter threads point at the mirror. If the polarizer is linear you should see the same thing that you saw before, but if it is circular it will appear black and you will not see the reflected image of your eye. This is because light reflected from your eye passing through the circular polarizer exits in the direction of the mirror as circularly polarized light (let's say it's left handed circular polarization). When it reflects from the mirror it reverses its polarization to the opposite sense - in this case it is reflected as right handed circular polarization. The filter only passes left handed circular polarization when the light enters from the "camera side" of the filter, so the reflected light is blocked. The situation is analogous to the effect of a vertically polarized linear filter on horizontally polarized light - little or no transmission of the light can occur..."

From this randomly Googled article - http://www.bobatkins.com/photography/technical/polarizers.html

 

[Blake Gibson]

"...You can also easily tell if a polarizer is linear or circular even if it is not marked. Hold the polarizer about 2 or 3 inches in front of your eye, and look at your reflection in a mirror. Looking from the camera's viewpoint (i.e. with the filter threads pointing towards you) you should see the image of your eye in the mirror (as if looking through a neutral density filter). Now turn the filter around so that the filter threads point at the mirror. If the polarizer is linear you should see the same thing that you saw before, but if it is circular it will appear black and you will not see the reflected image of your eye. This is because light reflected from your eye passing through the circular polarizer exits in the direction of the mirror as circularly polarized light (let's say it's left handed circular polarization). When it reflects from the mirror it reverses its polarization to the opposite sense - in this case it is reflected as right handed circular polarization. The filter only passes left handed circular polarization when the light enters from the "camera side" of the filter, so the reflected light is blocked. The situation is analogous to the effect of a vertically polarized linear filter on horizontally polarized light - little or no transmission of the light can occur..."

From this randomly Googled article - http://www.bobatkins.com/photography/technical/polarizers.html

Cool, thanks for this. I'll test it tonight. It's a 4x4 filter so I'll check to make sure I can see both ways the same as I suspect it's linear because all the pics I have found online for the circular have "this side out" on them, which makes sense as they only work one way as opposed to linear which is both ways. That's not a 100% way of going about it though

Thanks

 

[Ulf Krentz]

Circular polarizers are used to prevent misreading of the metering system of mirrored SLRs, for video cameras it should not really matter. I experienced linear filters to be more pronounced in actual work. My Tiffen 4x4 Polarizer is linear but only kind of 50% density of what I have in "screw in" filters though. Most important character will probably be a possible color cast.
A very easy way to check for circular / linear is to look at a LCD Computer Screen, if the filter is circular it will only turn the screen black in the intended direction, a linear filter will block the screen in both directions.

Cheers
Ulf

 

[Blake Gibson]

Thanks Les, I did the mirror test last night with my circ pol and sure enough, the one I was wondering about is a Linear!

Circular polarizers are used to prevent misreading of the metering system of mirrored SLRs, for video cameras it should not really matter. I experienced linear filters to be more pronounced in actual work. My Tiffen 4x4 Polarizer is linear but only kind of 50% density of what I have in "screw in" filters though. Most important character will probably be a possible color cast.

Thanks Ulf. I've been reading that, I don't really understand why though. Is it because the circular polarizers allow light in but no much out, and linear allow light in both ways (in and out) and it messes with the mirror because too much light? (that doesn't make sense though because with no filters there's even more light)

A very easy way to check for circular / linear is to look at a LCD Computer Screen, if the filter is circular it will only turn the screen black in the intended direction, a linear filter will block the screen in both directions.

Cheers
Ulf

I did the mirror test last night, I'll do the computer screen test tonight. Thanks again

 

[Les Hillis]

No worries Blake, glad you got it sorted out. It actually reminded me to look into using a screw-in and matte-box polarizer together for a variable ND effect, so thanks for that.

About the problem with linear polarizers and modern SLR camera's, that was summed up in the linked article too -

"...the intensity of the reflection from partially reflecting surfaces can be dependent on the linear polarization angle of the incoming light. Windows and pond surfaces are two examples of partially reflecting surfaces, but another is the main reflex mirror in many modern SLR cameras.

Older SLRs used fully reflecting mirrors, which reflect all polarizations with equal intensity.

Newer SLRs, and particularly autofocus SLRs, often use partially reflecting mirrors. The reflected light goes to the viewfinder and metering systems, while the transmitted light goes on to the auto focus sensors.

If a linear polarizer was used on such an SLR, the intensity of light sent to the metering system would depend not only on the intensity of the light, but also it's polarization angle. However, when a picture is taken the mirror is out of the light path and the film is only sensitive to the intensity of the light, not its polarization. Thus metering errors can (and do) occur when a linear polarizer is used with an SLR which has a partially reflecting main mirror.

If a circular polarizer is used, the circularly polarized light emerges from the polarizer is reflected by the partially reflecting mirror with an intensity which is independent of the angle at which the polarizer is set, eliminating metering errors..."