I have the measurements from the images in the post immediately preceding this one.
Same regime as with the Leica M9 images posted earlier, but with the Leica M240. I made an ETTR image at ISO 640. Then I made five more images, each one stop more underexposed than the previous one. I left the f-stop the same, and made the exposure changes by successively increasing the shutter speed. I started at 1/10 sec, so there wouldn’t be any double-exposure noise reduction taking place.
I brought all the images into Lightroom 5.2, white-balanced for Daylight, and applied an Exposure (PV2012) boost equal to the amount of underexposure. The ETTR image got no boost; the 1 stop under image got a 1-stop boost, the 2 stop under image got a 2-stop boost, the 3 stop under image got a 3-stop boost, the 4 stop under image got a 4-stop boost, and the 5 stop under image got a 5-stop boost .
I exported the images as layers into Photoshop. I set the eyedropper for a 101×101 pixel average, and I measured the CIELab values of the odd-numbered small patches in the Sekonic target. That gave me 12 patches, and 6 values per patch. In a perfect world, all the values for each patch would be the same.
Here are luminance curves for all the patch values plotted against the average L* for that patch:
You can see that the images receiving a four or five stop push are noticeably too dark, even in the lightest tones, and they get way too dark for the dark tones.
For consistency with the previous results, here are the differences between the L* values for a given patch and the average value for that patch:
It’s pretty clear that for the M240 with its systematic dark tone errors, this is the wrong way to look at things. The unpushed values should be considered right, not the average values. When you do that, here’s what you get:
Here are the chromaticity errors, which I define as the square root of the sum of the difference between the a* and b* values of the unpushed and the pushed image.
The errors are acceptable for the one and two stop pushes, marginal for the three stop push, and unacceptable for the four and five stop pushes. The only reason the chromaticity errors for the five stop push turn around at the darkest tones is that the luminance values at those tones is so low that the concical nature of the CIEL*a*b* space constrains the chromaticity.
This is yet another manifestation of the Leica M240 green shadows problem, which I’ve discussed previously. Note that this is a case where pushing hard in post really does increase contrast. The M240 is highly unusual — actually unique, in my experience — in this regard.