what I meant is that the way ISO below native is implemented in KM and Sony (don't know about the other brands) is that for ISO 100 vs. native ISO 200 they basically do a +1EV exposure correction at the native ISO and then devide the signal by 2 (i.e. reduce 1 stop again)
I don't know how KM cameras are doing this. However, I do know that neither the Sony A700 nor the A900 is doing this. Actually, the DR @ ISO 100 is somewhat greater than @ ISO 200 with both cameras, but one needs to know how to expose: the exposure @ ISO 100 should be only 0.5 EV higher than @ ISO 200 with the A700, and only 1/3 EV higher with the A900.
This is why in this set up you lower DR. An oversaturated pixel will still be oversaturated with the signal devided by 2
Even if it worked this way, that would not reduce the dynamic range. The dynamic range does not depend on how you are arriving at a certain exposure. The camera's metering is incorrect in these cases in principle
, but it may be the best by chance (i.e. by circumstances not recognized by the metering); beside, you can use exposure bias or even manual metering.
So, it boils down to the correct exposure.
However, there are cases, when digital adjustments cause loss of DR:
1. the fake ISOs in the high range, as adopted by DSLRs: doubling the pixel values causes clipping those pixels, which are in the top EV but not clipped,
2. some cameras create 1/3 stop ISO values from the next lower respectively next higher full stop ISO. The x-1/3 ISO reduces the number of levels (not an issue with 14bit anyway), but the x+1/3 ISO "outmultiplies" the pixels in the top 1/3 stop.
Don't understand, never seen this before, can you explain more as I would like to understand this better.
I am working on a complete set of demonstration; I will post it soon.