As reported some time ago in this post, models, when rendered at a z of 0 can seem to be floating above the background. This means that shadows appear to never meet the objects. I can guess how far below the model is that the background or floor is and try to move it down but hey, the floor moves too.

Now if I start a new document and simply add a cube to it at z=0 and render it, it appears to be sat on the floor. If I move it up to z=100 the the background seems to move with it. It becomes impossible to make the item float without adding something new under it. However, I can move the model to z=100 and cut from it another block at z=0 with a height of, say, 150, thereby giving the impression that the item now has a z=150 and the shadow moves up with it.

That then gives me a clue - perhaps there is something under my model? Nope. Checked that. Well at least I think I have. I can't find something below it. Mind you, there were things below my model which aren't ever seen - like objects that I have cut from visible objects.

Is there a way to fix this? How can I move my objects onto the background or, more obviously, move the background to be an infinite plane at z=0? Is there a way of managing the background's position? What is it that actually sets the position of the background?