Tip Blender :Light portals.

Cycles is great but at times very noisy and cranking it up to 1k samples takes a lot of time. One reason why cycles is so noisy is that when using HDR as world light bounces in every direction. With the new (2.75) light portal function it's possible to give cycles hints where light should be rendered. This saves on render time because with the use of light portals renders become less noisy even at a few samples...

more here in this Tutorial ...

lesterbanks.com/2015/06/reduci…