AFAIK, Carrara doesn't use AO, which, in poser at least, is a way (when using GI) to put shadowed areas in spots from which ambient light should be excluded (or occluded), but aren't. hence it's a kind of "fake" GI lighting/shadowing patch, at least in poser. carrara uses photon mapping, which is rather slow at high quality, as mentioned by jimbo. artifacts might be eliminated by rendering as a series of uncompressed tiff images, as any codec is gonna screw things up, compared to uncompressed.