E-M5 metering, WB, middle-gray exposure

Started Aug 2, 2012 | Discussions thread
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gollywop
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E-M5 metering, WB, middle-gray exposure
Aug 2, 2012

What do you make of this? While cycling around the Vineyard Airport I began wondering if there was any effect of WB on the OM-D "middle gray" metering. Somehow, in principle, I figured it ought be minimal, and that the resulting raw files would look similar. But when I got back to the cottage, I tried the following experiment:

I put the OM-D on a tripod in front of a grayish target (it's actually skewed slightly to the blue -- but that doesn't make any difference for this test). I filled the frame (using the 12-50mm) at f7.1, Aperture Priority, all camera settings "off," focus set manually at a blur, and took shots sequentially with UniWB, AutoWB, Daylight WB, and Tungsten WB. I then ran the raw files through RawDigger with a fixed selection from the center of the image.

Needless to say, each color channel was rather narrowly defined near the middle and strongly peaked. Here are the averages for each channel over the selected area

UniWB AutoWB Daylight Tungsten
R 144 114 116 115
G 328 255 260 258
B 208 162 165 164

It is quite clear that the AutoWB, Daylight, and Tungsten "middle gray" exposure levels are metered just about the same, as I would have expected. But when using UniWB, the meter exposed "middle gray" 1/3EV higher.

I repeated the UniWB and Daylight to see if something had happened twixt the cup and the lip, but the repeat results were essentially the same -- the lighting had changed color slightly and my RawDigger sample wasn't exactly the same, but the general principle was exactly the same. I included Shade WB in this set as well, and it too looked like the other non UniWB figures.

The resulting raw histograms do indeed look essentially the same, as would be expected. The UniWB raw histogram also looks the same, except all the spikes are shifted about 1/3EV upwards.

Thoughts?
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gollywop

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