Don't sit too close to the (3D) TV

A look at how seating distance can affect stereoscopic 3D-viewing enjoyment.

There's no hard and fast answer for how close you should sit to a 3D TV. It all depends on the content. David Katzmaier/Joseph Kaminsky

When I read Roger Ebert's latest blog post bashing 3D last week, modestly titled "Why 3D doesn't work and never will. Case closed," I couldn't help but wonder whether he was right. As a TV reviewer, my job sure would be a lot easier if I could just ignore 3D and focus on what my readers have overwhelmingly expressed as more important to them: good picture quality in 2D mode.

Alas, I don't think Ebert is right. There's the little fact that 3D works well enough to adequately entertain millions of viewers of blockbusters like "Avatar" and "Toy Story 3." And "never" is a long time.

Regardless of the headline's hyperbole, Ebert's most damning piece of anti-3D evidence has merit. The crux is a statement by Walter Murch, a highly respected film and sound editor, who says:

The biggest problem with 3D, though, is the "convergence/focus" issue. [T]he audience must focus their eyes at the plane of the screen--say it is 80 feet away. But their eyes must converge at perhaps 10 feet away, then 60 feet, then 120 feet, and so on, depending on what the illusion is. So 3D films require us to focus at one distance and converge at another. And 600 million years of evolution has never presented this problem before. All living things with eyes have always focused and converged at the same point.

That made logical sense to me, but it turns out to be more complex than "it doesn't work." Humans, as Murch admits, can tolerate a certain amount of disparity, or decoupling, in the distance between the plane of focus (the screen) and the target of convergence (the illusory 3D object, whether it seems to float in front of or hang behind the actual screen). The question then becomes how much of such a disparity is tolerable before 3D viewing, especially over longer periods of time, induces headaches or worse.

Sitting closer to extreme 3D more "worrisome"
I asked Martin Banks, a professor at the Visual Space Perception Laboratory at UC Berkeley. My main concern wasn't theatrical 3D but instead 3D TV in the home, where the plane of focus--the TV--is generally a lot closer than the screen at the multiplex.

Banks said that 3D content producers are very aware of the convergence/focus issue, and the best 3D films and TV shows are produced in ways where large disparities (like the ones Murch cites) are minimized. He said they will generally produce 3D effects that are relatively muted, or "close to the screen," as opposed to gotcha effects that pop out or recede inward excessively. Banks cited "Avatar" as a good case for muted 3D effects and "My Bloody Valentine 3D" as a not-so-muted one.

In his words, "It depends entirely on the content."

He also said that the convergence/focus issue becomes more "worrisome" at a shorter seating distance, such as a TV or a video game system like the Nintendo 3DS. Having to focus on a closer screen means that your eyes have to do more "work" when objects appear in front of or behind the screen.

Unfortunately when I asked Banks to pin down a "far enough" seating distance, such as Panasonic's recommendation of 3x the screen height (that's 6 feet, 2 inches for a 50-inch 16:9 TV showing full-frame video, for example), he declined to do so. He said "Avatar" shouldn't be any trouble at Panasonic's distance, but some other, more extreme 3D content could cause problems.

For 2D theaters, THX recommends a viewing angle of 36 degrees, which works out to 5.6 feet for a 50-inch TV. That's closer than Panasonic's recommendation, and definitely closer than most viewers sit to their TVs anyway. But it's safe to say that if you sat that close, seeking the most immersive 2D image, you'd run a higher risk of headaches or other ill effects from viewing some 3D content.

Beyond seating distance, screen size itself has an impact on potential decoupling-caused discomfort according to Banks. A larger screen has the advantage of allowing you to sit farther away, but can introduce another problem. If the content produced for a certain screen size is simply expanded to fit a larger one, it moves the 3D illusion closer to or farther away from the viewer, increasing the amount of decoupling. Banks told me producers from Sony, for example, had mentioned this very issue to him earlier, but when I asked for the "ideal" or "reference" screen size at which producers were creating 3D Blu-ray transfers for the home, he said he didn't know.

Advice for the young at heart: Sit back
Banks also mentioned a few other factors contributing to focus/convergence decoupling fatigue. A new study result, presented last week at the Stereoscopic Displays and Applications conference in San Francisco, showed that older viewers were actually more comfortable with decoupling than younger ones. I was surprised by this finding, since I figured "young eyes" could better cope with larger disparities, but Banks was not. "As you get older, your ability to focus your eye is reduced, so you slowly learn to 'uncouple' that response," he said.

I asked whether wearing prescription lenses in addition to 3D glasses played a role beyond the obvious discomfort of having to wear two pairs of glasses, and he said that while the jury was still out, early indications are that near-sighted viewers have an easier time with 3D than far-sighted ones. He also had seen no indication that ambient light played a role in discomfort caused by decoupling, although he mentioned that for active eyewear, certain lights can cause strobing or other undesired effects.

Throughout our conversation, Banks stressed that 3D affects different viewers differently, and that real data on potential risks of 3D viewing is still largely nonexistent. My takeaway from my conversation is that I'll sit no closer than Panasonic's recommended distance when watching 3D, and if I feel headaches, I'll blame the content first, not 3D itself.

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