Rendering with this technique
This is a trick that I learned from a friend several years ago, and I'm finally coming around to posting about it. This is also a trick that adds the Ambient/Reflective Occlusion map to your rendering (I also have alot of people ask me how to add AO to a render). Basically the idea is that you plug the AO map into an omni light, and the occlusion gets added to all of the geometry in the scene.
This trick is done using 1 omni light, and it is the only time that you should ever use an omni light if you're rendering with mental ray. If you forgot where it is, you'll have to switch from Photometric back to Standard.
The main catch to making this work is that the omni light must be placed at 0,0,0. If the light is anywhere else in the scene, the occlusion won't line up with the geometry in the scene, and you'll get really psychedelic results!
I usually leave most of the settings at default. But I almost always increase the Max distance to something like 4' all the way to sometimes 10'.
To see the results of this in your renderings be sure to turn off any exposure control settings. You can also turn off any Indirect Illumination in the render settings.
That's it! What I really like about this setup, especially for complex animations, is that there is no light calculating process, and the renders from frame to frame are very stable and consistent. The soft shadow effect (ao) also adds virtually no extra render time to your finished rendering.
If you have a tight production turnaround, and are concerned about the lighting in a scene not being designed yet, this is a great solution.
Two more renderings with this technique: