Correct me if i'm wrong, but i was under the impression that maxwell calculates lightpaths independent from the camera position so that the light situation is 'solved' no matter what angle you look at it (unlike GI methods). This would also explain why asmall adjustments to the camera settings (film iso, shutter, burn) can have an istantaneous effect on the render even if it used other settings in the hours before. Why depth of field isn't post-adjustable might be cause the effect relies on many iterations to clean up.
Could be all wrong though, i have only googled a bit about the topic
