Hi again, I've attached a mockup scene of my issue.
I have a cube that encapsulates the shot and use a fog material to create a low lying fog which has two cylinders in it (Representing some general objects, say people)
I render the shot off (generally with these people hidden to camera but casting shadows etc) then rerender a viewport selection (i.e. the cylinders) so that i can comp them in photoshop to add motionblur.
Issue is, when you viewport select the cylinders, they don't render, it'll just render black as if nothing is selected.
i think it's the interference of the box. it's visibility to the camera is maybe in the way.
Anyway if you need any guidance let me know