When I posted my thoughts on Avatar I talked briefly about those moments when their was a short depth of field and it clashed with the 3D effect. Basically when an object in the foreground is out of focus and your eye passes over it you want it to be in focus, it’s the way your eyes function and in a 3D movie you want to create a natural viewing plane that is essentially like looking through a window. Failing to do so and having objects in the foreground out of focus can become jarring and confusing for viewers because as their eyes migrate over them they expect them to come into focus.
Avatar kept this problem to a minimal because it was conceived and designed to be shown in 3D. James Cameron is a very technically minded director and a lot of thought went into the 3D effect and the films depth of field. Even with so much thought going into the process there were still times when Avatar suffered from this problem with depth of field. Perhaps this is because they made the film with the idea that it would be shown in both 3D and a traditional 2D prints.

