are you suggesting that ISO 200 is the true "native" ISO of the P25+ back?
I am suggesting, that there is no "native" ISO, because there is only one. The ISO selection changes the metering, and the question is, which ISO yields the best exposure (i.e. utilizing the dynamic range and minimizing the noise).
Guy posted a set with ISO 100, 200, 400 and 800, with successively reduced exposure. From those, the ISO 100 shot is perfectly exposed (1/3 EV down from the right edge); however, the question is, how good that metering reflected the entire scenery, i.e. how reliable the camera's metering is in that particular setting.
On the other hand, when I see ACR's automatic adjustments, ISO 200 seems to be the basis. Is ACR reliable in this relation? Did they make respective tests? I doubt it, because ACR applies the same adjustment to P45+ images as well, which do not need to be adjusted, for it has different ISOs.
And further suggesting that one gains nothing in terms of image quality by shooting at ISO 50 or ISO 100?
Let's see the meaning of ISO selection on different cameras, on the histograms, which show the effect of the exposure.
First, Canon 40D;
40D ISO 20040D ISO 40040D ISO 80040D ISO 1600The histograms show, that *roughly* identical image data has been delivered with increasing ISO setting, although the shutter was 1/20s, 1/40s, 1/80s and 1/160s. The increased ISO gain "made up" for the reduced exposure. Of course this "substitute exposure" is not a 100% substitute, i.e. the noise will be higher with the lower exposure.
Now, let's see what is happening if there is no ISO implemented on that level; following are the histograms from Guy's shots:
P25+ ISO 100P25+ ISO 200P25+ ISO 400P25+ ISO 800Here too the shutter has been halved with each stop increase of the ISO; however the pixel data got halved as well (the thicker white bars under the histograms mark one stop, the thinner bars 1/3 stop).
The higher ISO setting did not make up for the lower exposure. In other words, the higher ISO means metering as if there was a higher ISO implemented, but the camera does not implement it, it has to be compensated for in the raw processing.
This is equaivalent to the "ISO by pushing the exposure in raw conversion".
What is the result? If you pick a certain spot on the image, its brightness (in terms of pixel values) goes down with each ISO increase. Something, which has been in the sixth stop (from the very right edge) at ISO 100 will be in the range of the ninths stop with ISO 800. Accordingly, the noise will be higher and the details less.
There is nothing wrong with it, but it is important to understand: the higher ISO is in no way a substitute for higher exposure. Therefor my suggestion: always shoot with that ISO, which yields a close to ETTR exposure. If you can't afford the required exposure (moving subject, low light), then you will see, that you are underexposing, but you get the same as if you had increased the ISO.
Downsides:
- the display on the LCD will be darker,
- you have to increase the intensity in raw processing.
Of course you may shoot with higher ISO *when you realise, that you can't go with the required exposure*; the resulting image is the same without the downsides. But, again: be aware, that this is an "emergency measure", just like I am shooting with ISO 800 and 1600 only in "emergency cases".