5D III AF Assist and 580EXII

The 580 EX II only support up to 41 focus point. Hence, does anyone find any limitation on a 5D mark III focusing in a dimly lit place which the 5D won't be able to focus without the 580EXII AF assist?

I am really confused by your question. The 5d mkIII focuses in lower light than any other Canon camera ever made. Period. Use the assist light for even better results.

I think he's refering to the 20 new AF points available on the 5Dm3 that were not present on any canon camera when the 580EX II was designed...Good question and I don't really know the answer, but I suspect that there would be enough spill-over from the assist beam connected with the other 41 pts so that the AF will work fine.

Listening.

This last weekend I was at a retreat and one night we were having a campfire with the only light being from that fire. It wasn't a huge fire either. So, most people were pretty dark with maybe a hint of orange glow from the fire on them. I definitely couldn't see them with my eyes normally. The widest lens I have is just the 50 1.8, so I was using that on my 5D3. At times it was having trouble focusing (as there were times i was shooting into almost total darkness. I slapped on my 580exII and used the focus assist and it worked pretty well. Granted, I only used the cross-points to focus. I never really use the single ones.