What reason is there to not actually film a movie in 3D?

[John Marston]

if films like the Journey to the Center of the Earth films, the recent Resident Evil films, The Three Musketeers and even freaking Yogi Bear can actually be filmed with the actual 3D camera, what reason is there for films like John Carter, Wrath of the Titans, Avengers etc. to not be actually filmed in 3D and have to be post converted?

[24Lost]

One reason is that to shoot in 3D it has to be digital and some directors prefer film

[Plain Old Tele]

You also can't shoot 3D anamorphically (yet). And with projection still being a significant issue (light levels, etc) I think a fair number of creators are still leery.

[SchumacherFTW]

I think it's partly down to how the director uses the camera in a way. As an example, Ghost Rider: Spirit of Vengence wouldn't have been filmable in 3D due to Neveldine and Taylor's overly shaky and kinetic camera work. The guys need smallish cameras, and at the moment 3D ones are likely too big for their style.