Why bad 3D, not 3D glasses, is what gives you a headache

2 Aug 2010

This is the second in a series of blogs based on a seminar by Buzz Hays, chief instructor for the Sony 3D Technology Center in Culver City, California.

3D is an ever-evolving process, which is why the effect can be such a hit-and-miss affair. But those who insist 3D glasses give them headaches are a little wide of the mark, according to the man who trains the filmmaking pros.

“It’s not the technology's fault, it's really the content that can cause these problems,” explains Buzz Hays. “The more care taken when making the content, the better off everyone’s going to be. My mantra is that it’s easy to make 3D but it’s hard to make it good – and by ‘good’ I mean taking care to make sure that this isn’t going to cause eyestrain.”

There are several common mistakes that can cause discomfort, and easy ways for that to be reduced, yet they’re only just being learned and put into regular use.

Interaxial distance

The interaxial, or the distance between the two cameras, controls the overall depth of the 3D effect. Objects will appear closer or further away but they won't change in size, so it's important not to increase the interaxial distance too much. Filmmakers are gradually gaining experience with what types of scene work with different depths of 3D, and Buzz was keen to point out that framing a scene for 3D has similarities to composition for still photographers.

“When it comes to composing in 3D… by using the heads of the audience [in a U2 concert clip] or the ground plane, or some continuous sense of depth in the shot, it holds the shot together. One of the complaints people sometimes have about 3D is that it feels like a cardboard cutout: that there’s a cardboard cutout, then some space and then another cardboard cutout. By using a careful choice of interaxial spacing, and also by having something in the frame like the ground plane, or smoke or atmosphere or something, then you can start to hold the shot together.”

Convergence

Our eyes converge inward as we look at an object moving towards us. In 3D it’s essentially the same thing: we converge (or "toe-in") the angle of the left and right cameras, and this alters the particular 3D plane to which our focus is drawn. Objects in front of the convergence point appear to be coming out at us, while objects behind do the opposite. Care needs to be taken, however, particularly when fast cutting is used.

“There’s a situation where every time we cut to a new shot, the subject of interest is at a slightly different distance from us,” explained Buzz, demonstrating a rapidly cut clip of two people at different convergence points. “What’s happening is on every single cut, your eyes are making an adjustment to depth – you’re trying to find that object. It’s a very subtle distance, it’s not a great distance, but that’s what you’re feeling in your eye muscles as you’re trying to work to catch up with the shot. That’s called the vergence-accommodation conflict.”

“The way we make it much easier to look at is by using convergence in post-production. In that same sequence I adjust the convergence in post [production] to massage the depth. Now your eyes are making the adjustment once in the very first shot, and from that point on they don’t have to adjust again. It’s very subtle but if you don’t do it, it’s the difference between a comfortable experience and a splitting headache after 90 minutes.”

What filmmakers are now learning is that trying to control the convergence during filmmaking is, as Buzz bluntly puts it, “a waste of time”. As cuts are made and scenes are shifted around, it’s difficult to know exactly what shot will follow another, so trying to predict it all is futile.

“It’s far better to find the comfortable place to put the convergence level during shooting, then adjust it in post-production once the edit is finished – that ultimately makes the difference between good and bad 3D.”

Divergence

The opposite of convergence is divergence, and just as our eyes can only converge to a certain point before we go cross-eyed, so they can only diverge to parallel. Overuse of divergence can cause big problems.

“Typically, when we look at an object in the world our eyes are either parallel if it’s at distance, or they’re converged inwards for objects that are closer,” continued Buzz. “There’s a condition that can be created unintentionally where your eyes are forced to rotate outward in order to fuse this image – which frankly only works if you’re a horse or a goldfish, and they don’t buy movie tickets.”

At this point Buzz put a scene on the screen in front of us and had us don our specs. A figure at the back of the image was simply impossible to bring into focus, and even trying was as uncomfortable as you’d expect. Removing the glasses showed why: the left and right views of the figure were several feet apart on the big screen.

“Divergence occurs based on the size of screen you’re using. You might make a neat adjustment [during filming] so it looks great on a monitor, but when you scale it up to 40ft it hurts like heck. Experienced stereographers will be able to avoid it, but some low-budget 3D films have been filled with divergence, as they’ve made the cardinal mistake of falling in love with the image on a video monitor when it was really intended for a cinema display. They’re dialling the depth to within an inch of its life and getting everything they wanted on the small monitor, so their camera settings are out of whack. It can’t be fixed in post – unless you just abandon [the image for] one eye and convert 3D from the other.”

These were just a few of the common faults covered in our brief time with Buzz, and it was clear from his honesty about current 3D's shortcomings that there really isn’t a true 3D expert in existence. The people teaching it are still learning while they go, and doing their best to pass that knowledge on. The hope is that viewers will benefit from gradually better 3D – and, hopefully, fewer headaches.

Read more:
Why we can't ditch 3D glasses just yet.
From the Pole to Pandora: the shaky progress of modern 3D.
Why 3D and modern filmmaking techniques don't mix.
3D TV: in the home, on a budget and… on the news?

Goldfish image courtesy of bensonkua. Convergence diagram courtesy of Panasonic.

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