[vilhelm_s]

If the pixels were literally black that would be the case, but even on very underexposed pictures I don't think that ever happens. There's still photons on the sensor, it's just that they are swamped by the noise. Like, most DSLRs nowadays capture 12-bit values, which suggests you would have to underexpose by something on the order of 12EV to truncate the output to zero.

[CHY872]

Yes, and that is very, very possible. In any case, doing that would give you some relatively painful quantisation error, which is why the HDR approach works better.