3D Brains
Watch a 3D film and take off your glasses off. You will see two images fused into an in a blurry amalgam, nothing like the real world, and yet it is what your eyes, or rather your brain, actually sees.
A stereoscopic (3D) movie relies on your brain to take those two over lapping pictures and make sense of them – a vastly more complex process than observing a regular 2D screen where the hard work processing depth cues has been done for you.
As a result it is pretty easy to overstress your audience’s hard working brains, creating distractions from the story, discomfort, or eye strain and headaches, and/or just plain weird images.
Over at Images and Sound, a post house in Auckland, they have been experimenting with their 3D equipment to work out how to stay out of that kind of trouble.
After a bit of prodding their 3D guys – after all they want their 3D gear to look good - dig out some examples of poor stereoscopic footage which is very instructive, because instead of the usual you-might-get-this, you-might-get-that warnings you can actually see what happens when it goes wrong.
Vertical alignment is critical. When you watch a stereo movie your brain expects your imaginary left and right eye to be level, which might seem a bit strange because in the real world you can tilt your head. But your brain knows the world through the screen window should be level, anything else does not compute. The 3D effect should come from separation of left and right images in the horizontal plane only.
Even in 2D your brain can't always resolve depth cues
If the images are vertically misaligned, your brain takes a guess and places objects wherever it thinks they should go, and instead of a layered 3D effect stretching into the distance you end up with optical illusions – for me it was an object obviously from the background transparently overlaid over a foreground object. It is quite disturbing and while your brain tries to figure it out you can be sure the story line will be done the brain’s priority list.
It takes time for your brain to resolve a picture and make sense of it. According to the Images and Sound 3D guys, that’s why the short fast paced cuts don’t have the same effect as they would in a conventional 2D sequence.
“When you have quick cuts depth is lost because you are not able to register the 3D. You need to tweak a lot of the music or action paced cuts that you’d use in a standard 2D film,” says Brenton Cumberpatch, 3D Flame artist.
He says it is better to extend the shots longer and get more action within the frame. This suggests that using one eye of a stereoscopic production for a 2D version may not be the best tactic. If the shots and cuts are optimised for one of the versions it will be less than optimal in the other. The camera angles alone will see to that.
“You have to plan for it up front, you can’t just go and shoot stereo and expect it to work. You have to think of both of the 2D and 3D options,” says Cumberpatch.
Paul Lear, 3D colourist, agrees. “The switch to stereoscopic means a whole different pace and it is hard to get your head around. You really need to be watching a 3D cut, if not all the time you are editing, al least refer to it often because it is so different.
“The other thing you have to keep in mind is depth of field, because in 3D you want to see almost everything in focus and in 2D you don’t.”
He explains that with a shallow depth of field and an out of focus background, it is asking too much of the audience’s brains to try and resolve two left and right overlaid backgrounds if they are also out of focus.
Again a struggling brain does not assist the story telling.
“That’s not to say you can’t have some depth of field,” says Lear, “but not nearly as dramatic as you would in 2D.”
The pair say this is an important consideration if for example you wanted a 3D and 2D version of a commercial out of a single shoot, as commercial directors often employ a shallow depth of field for dramatic effect.
Wide lenses and small apertures confer a greater depth of field, but an overriding factor is the size of the camera sensor. The idea of a large camera sensor in a small cheap package is the force behind the current DSLR craze.
But on the other hand it’s not wise to go too small. Lear warns against sacrificing resolution by using small cameras with small sensors.
“When we change convergence in post we are blowing up the shot maybe 10-15%,” he says.
