Oh lord.  RR has the most complicated way of determining this issue!  I got caught in it a while back, so I can tell you what I found out at the time.  (Here's a link if you want all the information ... I warn you, it got a bit dull, but at least you can see in writing what they told me.)

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2763377

I hope that helps!

Tara

"If your pictures aren't good enough, you're not close enough."  ...  Robert Capa