Color balance with cross-polarized flash

[luong]

I've been experimenting with photographing grains of sand using a Canon MP-E 65mm and cross-polarized flashes, which provide all the light. Cross-polarization does remove the surface reflections, but it introduces an unwanted warm color cast compared with images where the polarizing filter on the lens is set for minimal cross-polarization. However, a GretagMacbeth grey balance card exhibits very little variation between pictures with the filter set for maximal cross-polarization and  pictures with the filter set for minimal cross-polarization. Using the grey balance card to set up the white balance does not take care of the color cast.

Is that normal ? Any ideas on how to get a neutral color balance with the cross-polarization set-up ?

[theguywitha645d]

So you are saying the card looks normal, but the sand is changing color? Cross-polarization can result in phase shifts which will impact color. The problem is with the interaction of the polarized light with the sand--the gray card is not affected by polarized light. A couple of pic might help.

[PeterAit]

What I would try is to take take pics of the same sand with and without the cross-polarizers on the setup (I assume you mean 1 pol on the flash and another on the camera). The "without" shot will give you the correct color. Then see how much you have to adjust color (in LR or whatever software you use) to make the "with" image match the "without." By applying this same adjustment to all photos you may solve the problem.

[Bart_van_der_Wolf]

Any ideas on how to get a neutral color balance with the cross-polarization set-up ?

Hi,

First you'll have to establish if the color cast varies with amount of Polarization.

Shoot the GM gray-balance card at fixed degree cross-polarization filter rotation intervals, from 0 to 90 degrees. You can check for the approx. 0 degree crossing by looking through the viewfinder at a reflection of the flash filter via a mirror. Then determine the Color Temperature and Tint settings after a Click White Balance, and see whether there is a slope.

If there is a slope, then you have a really big problem.

Cheers,
Bart

[Jason DiMichele]

Two things to pay attention to:

1) the more polarization that's used, the more the reflections will be suppressed. The more the reflections are suppressed the more potentially saturated certain colours (especially red) will become.  So you may always have to desaturated some of these colours.

2) some colours will change hue slightly with different exposures. You can make a custom colour DNG camera profile but ensure that your exposure stays pretty consistent if possible. This should be mostly achievable since you are using flash.

For my art reproductions I'm using a custom colour profile which helps but there are always some tweaks required. To get even more accurate colour you can use the adobe DNG profile editor and tweak your custom profile to handle more of the most common saturation/hue modifications to get you the best starting point you can.

Cheers!

[luong]

Thanks everybody for suggestions.

Here is a (temporary) page with pictures. First with no filters, subsequent filters at 0 (min), 30, 60, 90 (max):
http://www.terragalleria.com/tmp/crosspol.html
By the way, the sand is from the Mesquite Dunes in Death Valley. More colors than expected !
I know it would have been nice to stick a bit of WB card, but the one I have at hand is too big and would not fit within my set-up.

You can see that the color gets warmer with the amount of polarization. I am still wondering why this is happening.
With polarizing filters parallel it is a bit too cold (and has more surface reflections than without filters !) and then becomes too warm at 90 degrees.

[Jason DiMichele]

You can see that the color gets warmer with the amount of polarization. I am still wondering why this is happening.

Just a quick test. I'm not at my computer right now but on my phone it doesn't appear that more polarization is causing warmth. It looks as though it's saturating the image more because of the stronger polarization. The white sand pieces look pretty white throughout the shots. It looks like the coloured sand bits are becoming more saturated.  If you hover over white sand  on all the shots do the RGB channel values change?

Cheers!

[bwana]

I noticed something similar playing with a linear and circular polarizer. If you flip the circular polarizer so that its female threads are facing the female threads of the linear polarizer then something interesting happens. As you rotate one relative to the other from 0 to 90 degrees, you dont get extinction but rather the colors go from warm to cold.

I wish I could explain that.