If I’m not wrong,
Native 4perf S35 (which is simple the original frame size that was used in silent 35mm)
is something like 24.86mm x18.66mm, with a x1.39 crop factor.
The APS-C sensor which is 23.6mm x 15.5mm has a x1.5 crop factor.
I think the F5 sensor is somehow between these two values, with a sensor size of 24mm x 12,7 mm (native aspect ratio of 1.89), so it falls into the S35mm sensors range.
I’d not say that the 18mm becomes a 12mm, since the focal lenght of the lens doesn’t change, what changes is the angle of view.
So a 18mm lens on the F5 (without the crop sensor mode activated though, which would allow you to use S16mm lenses) has the same field of view of a 27mm set on the reference format (35mm).
The correct process is 18 x 1,5 = 27mm
That’s the reason why with small sensors like in the black magic pocket camera you will find hard to get a proper wide angle shot.
I hope I’m all correct with what I wrote