Both Blackmagic Cinema 6K and Sony A9 II have Full frame sized 24.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the Cinema Camera 6K and A9 Mark II sensor size comparison.
Sensor Size and Resolution Comparison image of Blackmagic Cinema 6K and Sony A9 II Cameras
Blackmagic Cinema 6K and Sony A9 II have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Blackmagic Cinema 6K
or the
Sony A9 II ,
including hyperfocal distance.
