ISO vs RAW brightness compensation

[pwnage101]

Is ISO in DSLRs done by physically changing the voltage/current in the sensor, or is it post processing done by the camera's CPU to modify the histogram?

Is it advantageous to make the ISO faster in low light situations with X exposure rather than capture the image at X minus 3 exposure and bump up the exposure by 3 in ACR?

Thanks,
troy

[Colin Southern]

Is ISO in DSLRs done by physically changing the voltage/current in the sensor, or is it post processing done by the camera's CPU to modify the histogram?

As a rule manufactures don't reveal details, but usually whole ISO steps (100 -> 200 -> 400 etc) are achieved by analog amplification of the signal read off the sensor whereas intermediate steps are done via software (and are thus slightly more noisy). The exception (with Canon anyway) is that the "H" and "L" ISO modes are done digitally.

Is it advantageous to make the ISO faster in low light situations with X exposure rather than capture the image at X minus 3 exposure and bump up the exposure by 3 in ACR?

Yes. A correct exposure at a higher ISO will yield less noise than an under-exposed shot at a lower ISO that's adjusted in PP.