Speed up Cycles Render: Only calculate whats visible to the camera I remember reading an article not so long ago about a node setup to significantly speed up render times in Cycles by telling the engine to only calculate materials based on the portion of the object visible to the camera and not the entire object.

I think it was posted on the official blender page/ blender nation but I cant seem to find this information anymore.

Does anyone know where that article/video is? Does anyone else remember this?
It's not really limiting the render to what's visible to the camera, but you can limit some effects to camera, such as bump maps.

Mixing your bump BW factor with a 50% grey color with light path's "is camera ray" to make your bump map only visible to the camera. Add (math node, clamp checked) "is camera ray" with "is glossy ray" to have your bump visible in glossy reflections too.