Hi,
A longer answer…
Just to say, I have not really seen colour shifts in the darks on my P45+, but decided to look into that a bit. My background is in engineering, so I have a tendency to look fora feasible explanations for any observation. With MFD there are many myths, little observations and very few explanations.
I have a couple of tools that I originally acquired to do "sanity checks" on my printer profiles. One of those tools is PatchTool from BabelColor. That tool can read a pair of CGATS files and do a graphical comparison between the colours. Very useful.
A while ago I got interested in looking into the accuracy of colour reproduction on my P45+ compared to my Sony Alpha 99, and I was looking into getting a test chart with more colours than the classic Colour Checker. The Colour Checker SG would be a natural alternative, but it is a bit expensive and reference values are not easy to obtain.
Than I realised I could use an IT8 colour chart. It has a lot of fields and covers most parts of Adobe RGB and they always come with reference data:
http://www.targets.coloraid.deI used the C1 target, the reference data is here:
http://echophoto.dnsalias.net/ekr/Articles/darkrepro/Ref/R050301_remap.txt (note the fields are remapped for easier use)
Now, I made 5 different shots with 0, -2EV, -4EV, -7EV and -6EV exposures (the last two using a 3X ND filter)
Those images are here:
http://echophoto.dnsalias.net/ekr/Articles/darkrepro/OriginalsI made raw conversions from the 0EV and -4EV images using both LR5 (using process version 2010) and Capture One compensating for underexposure, saving the results as 16 bit TIFFS.
Next step was to use the "scanin" program from Argyll CMS to read the IT8 field values from TIFF files into CGATS files.
The last step was to read the reference file and the CGATS files from scanin into PatchToo and evaluate the differences.
I didn't go beyond -4EV as I wanted to compensate for the underexposure in LR, and LR with PV 2010 only goes to +4EV compensation
The include screen dumps from RawDigger show the three darkest grey patches in RawDigger from the -6EV exposure.
I would think that once exposure is so small that readout noise "contaminates" shot noise there would be a colour shift as the different channels would be differently affected, and that colour shift would vary by white balance and CFA design.
Best regards
Erik
This is nice. What kind of color passport did you shoot? I'd like to test it as well!