As I know, ISO (in digital photography) is simply a gain power of electronic amplification of an electric signal read from the sensor. Is it guaranteed that minimal ISO setting is just a "minimal amplification", but not a signal reduction? My question is can my camera at low ISO setting (50) actually reduce the actual CCD/CMOS signal? My camera is Sigma dp1x.
Sensors only have the one sensitivity, called native sensitivity, of how many photons were collected.
Higher ISO simply multiplies, to shift the histogram data up, brighter, but bringing low noise up with it.
Lower ISO 50 simply shifts the histogram down, dimmer, which lowers noise too, but also limiting the dynamic range, the top half of range is unused. The visual histogram is adjusted accordingly (blank top half is not shown). Nikon does not ever call it ISO 50 since range does not meet ISO specifications. Nikon calls it LO 1 (1 EV down).
Since the specs of the DP1x I can find online list 100 as the lowest (and presumably base iso) setting, and 3200 as a boosted ISO level, my guess would be that the iso 50 setting isn't done by controlling gain across the sensor. It is probably done in in-camera processing, as digital pull processing. That is, the image is actually shot at iso 100, but exposed for twice as long, and then the exposure is brought down in post. This actually decreases the dynamic range, but can be useful if you need a 1-stop ND filter in the field and forgot to bring one with you.
I have a Canon 5DMkII, and the iso 50 setting on it is done in this way.