Moon Photography: Auto White Balance Mystery

Started Aug 20, 2013 | Discussions thread
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Bob in Baltimore
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Moon Photography: Auto White Balance Mystery
Aug 20, 2013

I have a number of photos of the rising Moon with interesting landscape features. I am preparing a talk on this topic for the NASA Goddard Photography Club next month. One of the things I did in preparation for this was to photograph the rising Moon to measure atmospheric extinction ... changes of up to 5 f-stops (!) occur within 15 minutes when you start with the Moon right on the horizon.

I was able to clearly document a phenomena that I had previously observed ... the auto-white balance does some very strange things. Within 2 minutes the color of the Moon (as photographed) changed dramatically. The color of the Moon that I was looking at (when not clicking the shutter) did not change.

I was shooting raw, so the white balance should not affect the raw data, but I am unable to reconcile these two images, even if I force a color temperature in Nikon's raw conversion software.

The images were shot on a rather murky evening, with high extinction. I had traveled to the country to get a clear, low horizon. Unfortunately, I left my tripod adapter at home, and so had to shoot at ISO 1600 to get an adequate exposure when the Moon was as low as 1 degree.

I converted the image in Nikon Capture, but made no adjustments other than those automatically applied. In Nikoneeze that means a "standard" picture control, auto WB, ADL set to "Normal") The ISO, aperture and exposure settings were identical. The Moon's altitude increased from 4.1 to 4.4 degrees in the two minute interval, which should have brightened the Moon by 0.6 f-stops. The measured increase in brightness in the green is 0.4 f-stops, a satisfactory agreement in my opinion.

The color as I recall it was closer to that of the second image, but it is insufficiently red. The first image color is not correct.

I measured the brightness of a spot near the disk center, which is in the lunar highlands. As you can see from the readings in the picture, the the green reading increased a small amount while the red dropped by 23% and the blue more than doubled.

So, what is going on here? I have only one thought, and it is insufficient to explain what is going on. Can anybody offer some thoughts on this?

My one thought: There were a few spots on the first image where the red is saturated, and that seems to have driven the white balance out of balance, so to speak. But I estimate them at only a few percent of the area.

I am going to cross post this to the Nikon FX forum. I know that some of you are in both, but there may be some Nikon gurus who dont read this forum. And many of you here don't shoot Nikon, but I have a hunch that this is not a Nikon specific issue.

(And as long as I have your attention, let me suggest you check out some of my Moon in the Landscape shots at www.ScientiaPhoto.com in the Hawaiian Landscapes collection.)

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Bob in Baltimore

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