Cuprite and cross polarization

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Geopressure
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Cuprite and cross polarization

Post by Geopressure »

The following Cuprite specimen had proven unusually difficult to photograph. For some background, Cuprite has a refractive index of 2.8 which is even higher than Diamond at 2.4. Diffuse lighting does reduce the intensity of specular reflections, but also results in partial reflections which obscure the color on nearly every crystal face (see the 10X stack below). Lou Jost had previously suggested using cross polarization and I finally tried it.

The second stack was shot at 5X using directional, polarized LED illumination, and a circular polarizing filter in between the objective and tube lens. This suppressed most of the reflections and allowed the transmitted color to show through. Processing was similar on both images.

The method isn't perfect since there invariably seem to be some orientations of bright reflections that cannot be eliminated. Maybe this behavior is related to Brewster's angle? Perhaps a less directional light source would help. Another potential issue is the significant loss of light intensity. Thanks to all the forum members who have posted helpful information on this subject.

Image

Image

rjlittlefield
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Post by rjlittlefield »

Cross-polarization should kill single reflections, but double reflections are problematic. See http://www.photomacrography.net/forum/v ... hp?t=33842 for explanation.

I don't see how Brewster's angle is going to play in here, but maybe it does somehow or other.

--Rik

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Post by Pau »

Interesting application.
In fact, despite being a clear fan of cross polarization, I prefer the first image. I suggest you to try partially crossing the polarizers to control but not totally suppress the main reflections of the crystal faces.
And of course you can combine cross pol and diffusion if you place the polarizer after the diffuser.
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Post by Geopressure »

Rik: Thanks for the input.

Pau: I'm very surprised you prefer the first image! As a color loving mineral guy I really dislike that one. The cuprite looks much better than that under a scope, though it does exhibit more reflections than the second image.

Lou Jost
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Post by Lou Jost »

I'm very glad you tried it, and I much prefer the second image. But as Pau says, the great thing about the method is that you can control it.

Another suggestion would be to add a bit of backlight to highlight the color of the translucent crystals.

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Post by zzffnn »

I much prefer the second cross pol image as well, even though I do not like cross pol light on most invertebrates. To me, by taking away surface reflections, cross pol makes surface of invertebrates too dull. I personally would rather have a small hot spot than a very dull image.

But cross pol really helps if you want to see UNDER the surface (in this case, it is the red mineral color from under the surface; in other case, it could be fish under the water surface, for example).

The second composition has 1-2 darker areas though. Maybe play with software (shadow, highlight, brightness, gamma, contrast, curves), do not completely cross pol, or add another speedlite to enhance it (and try to mix in some diffused light in some areas)? I know this is only a comparison experiment though.

Geopressure
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Post by Geopressure »

As this was a test, I didn't worry too much about shadows. In any case I'm quite pleased with the improvement in color, and also the reduction in stacking halos. Thanks to all for the comments.

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Post by Pau »

Of course the colors are much nicer in the second image but the aspect of the crystals is much better shown in the first one, this is why I suggested trying partial cross pol.
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Post by Geopressure »

I see what you mean Pau, maybe I can find a compromise with better contrast.

soldevilla
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Post by soldevilla »

I understand what Pau says. There must be a midpoint between both images in which you can show the reflection, very characteristic of the cuprite, and the deep red color. I have never got good results with polarizers, but maybe because I only have linear polarizers. another thing to try.

Nice sample and images... ¿from France?

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Post by Lou Jost »

I think there are enough reflections to show the crystal structure. I'd just add a a weak diffuse rim light coming off from the right side and slightly behind, that would illuminate the inside of the crystals.

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