Another way to put it is that I want to create a material that will do the cropping for me.

If I apply this shader to a cube in the middle of the shot, then my render will have a cube-shaped block of transparency in the middle of the shot, even if there are objects behind the cube.

Can anyone provide me a pointer on this? I'm new to nodes.

Thank you thank you!

EDIT: I'm using Cycles.

EDIT EDIT: Thank you for your answer, Denis. I realized today that there's one thing missing from the solution you provided. I also need the object to cast shadows and show up in reflections. I can't quite get my head around to make that happen. Any help out there?

EDIT EDIT EDIT: Figured it out! enter image description here

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    $\begingroup$ which render engine are you using? $\endgroup$ – Denis Aug 27 '15 at 19:11

Cycles Render

All you have to do is to add a Holdout node to the material output on the masking object.

enter image description here

Blender Render

Enable Transparency on the material of the masking object, set transparency method to Mask and change Alpha to 0

enter image description here

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