Originally Posted by fire407
Everything in the frame is in focus, but you can only focus on one thing at a time..
I think what you're referring to is eye convergence, this is different from focus.
Seeing double objects means your eyes are converged on a specific object. (you are basically doing cross eyes to converge the two different images)
On the other hand, focus can be done with one or both eyes if you want to.
Try it: Hold your hand close to your eye (either one eye open or both), and focus on something far away and it will become blurry, focus on your hand and far away objects will blur.
Eye convergence works in 3d movies because you are seeing two different images of slightly different perspective, just like you do in real objects.
But focus doesn't work, because the two images are superimposed in the exact same physical space. (so you're only really focusing the screen, so focus simply doesn't really change)
So, eye convergence = crossed eyes.
This is how 3d stereo is done.
The part of the image that is converged will seem to be at where the screen is, and crosseyed stuff will appear to be in front of the screen.
This means that stuff that "pops out" is inverted:
The left image will be closer to your right eye, and the right image will be closer to your left eye, that's how "pop out" effects are done.
The part where the two images overlap eachother will appear to be right where the screen is, and anything further back will be "normal", in the sense that the left image is closer to the left eye, and the right image is closer to the right eye, but they're still different points of views so they'll appear to have depth.
What James Cameron did on Avatar, is that unlike other movies, he always has the convergence where he wants the audience to be staring at, this is why "pop out" effects in Avatar are very rare, and will always be out of focus.
Other directors may want you to stare at the "pop out effect" instead, and simply not put the convergence at where you're supposed to be staring.
I personally think that both ways have their justified use.
The Avatar approach can even work on a serious drama with out making it seem "gimmicky" by focusing mainly on the story. And some action movies and just about every videogame can use the more traditional "gimmicky" approach for extra impact.
The best example i can think off of using every 3d trick in the book for added impact may be a Christmas Carol, and they used it very successfully IMO, you really feel taken on a ride on that one.