Something different happens when you're viewing three-dimensional motion projected onto a flat surface. When a helicopter flies off the screen in Monsters vs. Aliens, our eyeballs rotate inward to follow it, as they would in the real world. Reflexively, our eyes want to make a corresponding change in shape, to shift their plane of focus. If that happened, though, we'd be focusing our eyes somewhere in front of the screen, and the movie itself (which is, after all, projected on the screen) would go a little blurry. So we end up making one eye movement but not the other; the illusion forces our eyes to converge without accommodating. (In fact, our eye movements seem to oscillate between their natural inclination and the artificial state demanded by the film.) This inevitable decoupling, spread over 90 minutes in the theater, may well be the cause of 3-D eyestrain. There's nothing new about the idea—an article published in the Atlantic in 1953 refers to the breakdown of the accommodation-convergence ratio as a "difficulty [that] is inherent to the medium." And there's no reason to expect that newfangled RealD technology will solve this basic problem of biomechanics.
(There's also little reason to believe new technology will overcome another fundamental problem with the 3-D business model: Five percent to 8 percent of the population is stereoblind and can't convert binocular disparity into depth information. That means they can't appreciate any of the 3-D effects in a RealD or Imax movie. An additional 20 to 30 percent of the population suffers from a lesser form of the deficit, which could diminish the experience of 3-D effects or make them especially uncomfortable to watch.)
The eye-movement issue may even carry other, more serious risks. A long session of 3-D viewing tends to cause an adaptive response in the oculomotor system, temporarily changing the relationship between accommodation and convergence. That is to say, audience-members may experience very mild, short-term vision impairment after a movie ends. I won't pretend there's any hard evidence that these transient effects could develop into permanent problems. But if 3-D becomes as widespread as some in the industry claim—every movie in three dimensions, for example, and television programs, too—we'll no doubt have plenty of data: Small children, their vision systems still in development, could one day be digesting five or six hours of stereo entertainment per day. There's already been one published case study, from the late-1980s, of a 5-year-old child in Japan who became permanently cross-eyed after viewing an anaglyph 3-D movie at a theater.
There are plenty of other problems with 3-D movies that might contribute to the sore eyes, headaches, and nausea. As a general rule, the greater the disparity between the two image tracks—that is to say, the farther apart the two cameras are placed during shooting—the greater the illusion of depth in the finished product. That's a plus for the filmmakers, who tend to favor extreme special effects, pickaxes flying off the screen and all that. On the other hand, the more pronounced the disparity, the more difficult it is for the viewer to fuse the two perspectives into a coherent scene. That could lead to double-vision, uncomfortable flickering, and—yes—eyestrain.
So if the new 3-D movies are still giving us headaches, why has no one bothered to mention them? It may be that the visual fatigue, however pervasive, is small enough to hide in the novelty of the experience—we're so jazzed up that we barely notice our eyes hurt. If we did become aware of some discomfort, we might not recognize where it came from: Were my eyes tired from watching Monsters vs. Aliens last night or from having sat in front of my computer all through that morning and afternoon? Did the RealD projection give me a headache or was it the movie's lamebrained script? Indeed, several of the critics who reviewed the film seem to be suffering from a form of source amnesia: A.O. Scott calls Monsters vs. Aliens "strenuous, noisy, 3-D fun;" Anthony Lane describes growing "fuzzy with exhaustion;" even Time's Josh Quittner must confess, "After watching all that 3-D, I was a bit wiped out."
So here's one theory for why 3-D movies have failed to catch on in the past. It's not because the glasses were "cheesy" or because the projection systems were crude. It's not because the movies were poorly made. (Some truly amazing stereo films have been produced, like Hitchcock's Dial M for Murder.) No, the bubbles always pop because 3-D movies hurt our eyes. We may not notice the discomfort at first, when the gimmicks are still fresh and distracting. But eventually, inevitably, perhaps unconsciously, they creep off the screen and into our minds. It's happened before and it will happen again: At some point soon, 3-D cinema will regain its well-earned status as a sublime and ridiculous headache.