Two polarizers?

[Eastree]

I just read an article, and there's a photograph of minerals near the end of it all. The caption describes it as "The photo was taken with partly crossed polarizing filters; the width of the entire field is 1.62 mm."

Would that be two CPL or two linear polarizers? And how exactly does that work, when cross polarization so often blocks a great deal more light, or can even block light completely (at a perfect 90 degree cross)?

The image:


The article:
Global Volcanism Program | Dabbahu | Monthly Reports

[OsmosisStudios]

2 circular polarizers, i would imagine. that would allow the user to control how much light is blocked.

POlarizers arent on/off switches: they have a certain degree of modularity. im not sure what Im looking at: it may have been necessary to view a certain portion of the image to cross both polarizers. The light coming from behind the subject could also be VERY bright.

[daft_biker]

Originally Posted by Eastree

Would that be two CPL or two linear polarizers? And how exactly does that work, when cross polarization so often blocks a great deal more light, or can even block light completely (at a perfect 90 degree cross)?

Cross-polarised (UK spelling ) flash is a method that is sometimes used to control highlights on reflective surfaces. I use Lee polarising sheet on the flashes and a circular polariser on the lens.

Here's an article discussing the technique.

I suspect if you want to use AF then you need a circular polariser on the lens as normal and it you are focussing with available light the kind of polariser on the flashes is irrelevant. Not that I've ever tried to use AF but even with the polarising sheet covering the modelling lights on my macro twin-flash so I can preview the effect through the viefinder it never goes completely dark and I can manually focus.....haven't a clue about the physics of why it works...it just does

[sk66]

How do you "cross" two CPL's?
I would have to guess it's linear CPL's used to "shut down" the light. It looks to me to be an image as might be seen thru a microscope...i.e. a very thin slice, high magnification, backlit...this would make sense.

[daft_biker]

Originally Posted by sk66

It looks to me to be an image as might be seen thru a microscope...i.e. a very thin slice, high magnification, backlit...this would make sense.

The backlighting you refer to might be dark-field illumination. Clicky for Wiki

[kencaleno]

Basically two circular polarizers stacked can operate as a variable 9 stop graduated neutral density filter. Ken

[fletch]

I think to get the effect in the image above you need one circular polariser and one linear polariser. Then you just play with the angles. I saw it in a magazine but can't find the article online at the moment.

[sfisher]

If I understand the physics correctly, the front polariser of the two must be a linear polariser, otherwise the rear polariser won't do anything (other than act as a ND filter). The rear one should be a circular polariser, to avoid confusing any AF systems in the camera.