James Cameron: "I think it's a myth that you want deep focus in 3-D shots. I find the opposite is true. Selective focus, created by working at low f-stops with longer lenses, evolved as a cinematic technique to direct the audience's attention to the character of greatest narrative importance at a given moment. With 3-D, the director needs to lead the audience's eye, not let it roam around the screen to areas which are not converged. So all the usual cinematic techniques of selective focus, separation lighting, composition, etc., that one would use in a 2-D film to direct the eye to the subject of interest, still apply, and are perhaps even more important. We all see the world in 3-D. The difference between really being witness to an event vs. seeing it as a stereo image is that when you're really there, your eye can adjust its convergence as it roves over subjects at different distances. Convergence is the natural toe-in that the eye does to align the left and right eye images of objects at specific planes of depth. In a filmed image, the convergence was baked in at the moment of photography, so you can't adjust it. In order to cut naturally and rapidly from one subject to another, it's necessary for the filmmaker (actually his/her camera team) to put the convergence at the place in the shot where the audience is most likely to look. This sounds complicated but in fact we do it all the time, in every shot, and have since the beginning of cinema. It's called focus. We focus where we think people are most likely to look. So I've found that just slaving the convergence function to the focus works exceedingly well, and makes good stereo a no-brainer on the set."
(David S. Cohen, 10 April 2008, Variety Magazine)