Real ISOs
Hi,
I apologize for this intrusion. I have nothing to do here; I stumbled upon this page through a link. I am not the owner of any MFDB; my interest is only to all kinds of raw images, analyzing the characteristics of different cameras for the selfish purpose of better understanding their nature.
Anyway, I noticed the post of Greg Seitz guessing that the Sinar eM22 back does not really have different ISO gains. I suspected this before, and now took the liberty and downloaded the DNG files for an analysis.
Well, Greg was right. There is no ISO gain; the raw data is getting "lower and lower" with higher ISO.
The raw converter records following adjustments in the XMP metadata; this causes for example ACR displaying like-brightness. Without that the higher ISO images would appear very dark:
ISO 50: +0.50 EV
ISO 100: +0.85 EV
ISO 200: +1.85 EV
ISO 400: +2.80 EV
Here are the raw histograms (and I mean RAW). Note, that ISO 200 and 400 look almost identical, but look for the yellow note on the pixel level. Brumbaer's DNG converter insert strange white levels (16696, 16689, 25047), only ISO 400 gets 16383; this cases Rawnalyze to scale ISO 400 differently.
It is noteworthy, that the pixel values do not get halved by increasing "ISO" setting; the reduction is less.
Finally, the histograms:
ISO 50:
ISO 100:
ISO 200:
ISO 400: