The connection might seem a little tenuous at first, but I think an assignment that deals with using a polarizer is a great assignment to have in conjunction with the white balance assignment. People will periodically ask my opinion on using polarizers, and my answer is usually that I don't use them much anymore since I switched to digital. Why? because I found time after time that the polarizer would throw off the color balance in my images, and no matter what I did, I just couldn't get the colors and tonalities to look right. I've done a bit of reading on the subject, and there are several reasons why this happens. First, and probably most importantly, the glass used in constructing the polarizer isn't always color neutral. You can tell how the polarizer might shift the colors by holding it in front of a white piece of paper and seeing if it the paper takes on a color tint when viewing it through the polarizer. I have an expensive B + W multicoated polarizer that is supposed to be one of the more neutral filters, but I can see a distinct green color cast when looking through the filter, and this color cast carries over into my images. You would think that a camera's auto white balance would correct for this greenish cast, but I find that the auto white balance system in Nikon cameras has a difficult time correcting any green/magenta tint in images. From my limited experience with other camera brands, I suspect they have a difficult time with green/magenta tint as well.
There is another more complicated reason why polarizers shift the colors in your images, and it is because the polarizing filter can only be optimized for a specific wavelength of polarized light, and will affect that wavelength more than others. In other words, as you rotate the polarizer to change the amount of polarization, the color balance of the scene will shift too. So the color shift can be caused both by the color of the glass in the polarizer, and the fact that the polarizer will affect some wavelengths of light more than others. Not only will the polarizer shift the colors in your scene, but it will do so inconsistently, depending on the amount of polarization and the wavelengths of the polarized light in your scene.
It is very, very difficult to correct for these color shifts just by "eyeballing" it, but there is a tool we can use, and it's the same digital gray card we're using in our white balance assignment.
I finally took the time to do a comparison between the same scene shot with no polarizer, shot with my B + W polarizer, and shot with my Sing Ray polarizer. I included a gray card in each shot so that I could compare the "as shot" white balance selected by the camera's auto white balance system with the white balance determined by clicking on the gray card. What I found was that the B+W Polarizer resulted in a significant shift towards green (requiring +15 towards magenta on the Adobe Camera Raw tint slider to correct), and a smaller shift towards blue (requiring about 300 degrees Kelvin shift towards "warmer" on the white balance slider). My Sing Ray filter shifted the colors much less, with a correction of +7 required towards magenta, and an insignificant correction of 50 degrees Kelvin towards blue required to correct the colors.
The bottom line is that if I shoot with a polarizer in the future, I will include a gray card in one shot of the scene so that I can correct the color shifts caused by the polarizer. I will also be much more likely to use my Sing Ray filter instead of the B + W filter, since the Sing Ray seems to produce more neutral colors (less color shift) under daylight conditions.
The assignment for the week of 5 - 11 September 2011 is "Polarized." You should compose a scene using a polarizer to remove reflections in the scene, darken a hazy sky, or increase the contrast between clouds and sky. However, you should also recognize that the polarizer will probably cause color shifts in the image, and be prepared to correct those color shifts in post processing. The recommended method for correcting color shifts introduced by a polarizer filter is to use a color-neutral gray card in one of the shots of your scene and then click balance on the gray card to correct the white balance.
I'll look forward to seeing your images.
Keith
