interesting, i was refering to the L iso setting or iso 50, basically as i heard an iso 100 processed darker (as was the case with the 1ds mark 1)
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If I recall correctly, with ISO 50 on the 1D2 the sensor saturated well before the maximum raw level (around 1/2 stop give or take) and so one had less highlight headroom above metered middle grey. The camera does however collect twice as man photons per raw level. At ISO 100 on the 1D3 the sensor already saturates a little early (well under 1/3 stop), and so ISO 50 is just ISO 100 overexposed by a stop with correspondingly less highlight headroom, with no more photons being collected per raw level at the ISO 50 setting. So the 1D2 has ISO 50, with a bit less headroom, and the 1D3 has no true ISO 50, just ISO 100 overexposed by a stop.
One can argue however that this is a good thing -- it means that the 1D3 sensor is about twice as efficient at collecting photons. Both have about the same full well capacity in terms of electrons, but the 1D3 reaches it at ISO 100 while the 1D2 needs ISO 50 and therefore a longer exposure time to fill up its photosites. This means that, for a given exposure and ISO, the 1D3 will be collecting more photons and therefore have a higher S/N ratio and correspondingly better image quality. If you need ISO 50, use a ND filter -- it's much easier to throw away photons that you don't want, than to recapture the ones you do want but the camera couldn't capture.
