> What makes it possible for cameras like Sony's a7s series to shoot in nearly dark conditions?
Mostly because the A7S(2) has a 35mm sensor with "only" 12 MP - it's simple physics, the individual pixels are so huge compared to 50 MP+ cameras and thus much less susceptible to noise.
In addition the A7S line, during 4k recording, does not do binning or other quality degrading post processing (because its resolution is so "low" that 1:1 4k can be done). This reduces processing load as well.
Mostly because the A7S(2) has a 35mm sensor with "only" 12 MP - it's simple physics, the individual pixels are so huge compared to 50 MP+ cameras and thus much less susceptible to noise.
In addition the A7S line, during 4k recording, does not do binning or other quality degrading post processing (because its resolution is so "low" that 1:1 4k can be done). This reduces processing load as well.