I am quite late to the digital back's party, and so this might not be a news to you guys. I am quite shocked that the native base ISO is not at 50 for nomal mode, or 140 for long exposure mode. However I am sure I am not the last to know about this, so I think it is necessary to make a post. I have proofs below:
As can be seen above for the case of normal mode, for the same lens at the same aperture, 50 ISO and 80 seconds has less highlight details recoverability than 100 ISO and 40 seconds.
As can be seen above for the case of long exposure mode, for the same lens at the same aperture, 140 ISO and 20 seconds has less highlight details recoverability than 200 ISO and 14 seconds.
For some reason, Phase One has chosen to not explicitly tell the truth that this Dalsa CCD is indeed working at 100 ISO voltage. If you work at 50 ISO, then the histogram and lightlight warning are NOT reliable! You end up risk blowing up your highlights without being aware of! (The same holds for 140 ISO in long exposure mode.)
The ISO sensitivity test from dxomark also confirms that this is true for the IQ180 digital back.
Further validating this, I have asked Bill Claff to run his algorithms on the IQ260 RAW files. (These test shots were taken at room temperature.) He got the following conclusions:
It can also be seen that the Dalsa CCD can not keep up competitive against the latest SONY CMOS chips in terms of dynamic range performance (especially the ability to pull up shadows for long exposure shots).Here are the normal mode results:
It's clear the base ISO is 100 since the ISO 50 result is the same.
Things go normally from ISO 100 to ISO 800.
At ISO 1600 "Sensor+" must kick in.
I noticed the ISO 1600 and ISO 3200 files are also lower resolution; 4490x3364 versus 8984x6732
The long exposure mode results are similar:
I am now quite torn between the IQ260 and the IQ250. One has ultra wide angle, while the other is much better at long exposure shots.