Two *very* interesting articles:
Excerpt: "Based on that observation, I thought that instead of taking a picture at 800, I can increase the ISO to 1600 and dial +1 stop compensation to overexpose the sensor by 1 stop, that is by a factor of 2. (Don't dial the compensation when you are in Manual, of course, just keep the settings the same). Then (I presume that shooting is in RAW) one can dial down the exposure by one stop during the RAW development. The expected signal to noise gain would be 2/1.25=1.6 compared to just shooting at 800."
Excerpt: "A 12 bit image is capable of recording 4,096 (2^12) discrete tonal values. One would think that therefore each F/Stop of the 5 stop range would be able to record some 850 (4096 / 5) of these steps. But, alas, this is not the case. The way that it really works is that the first (brightest) stop's worth of data contains 2048 of these steps — fully half of those available.
"Why? Because CCD and CMOS chips are linear devices. And, of course, each F/Stop records half of the light of the previous one, and therefore half the remaining data space available. This little table tells the tale."