I can’t see

I rarely go out to movies, but that doesn’t mean I don’t consider the options. “Coraline” looked good. “Monsters vs. Aliens” got good reviews. The problem, however, is that both were offered in 3-D. And the prospect of staring through 3-D glasses for that period of time doesn’t enthrall me. The last 3-D film I saw was in November, and it was made with very new technology at the Newseum. It was cool, but I was glad it didn’t last beyond 20 minutes.

Even with technological advances, why do 3-D movies still hurt our eyes? Thankfully, the good people at Slate (specifically, Daniel Engber) have explained it.

Outside of the 3-D movie theater, our eyes move in two distinct ways when we see something move toward us: First, our eyeballs rotate inward towards the nose (the closer the target comes, the more cross-eyed we get); second, we squeeze the lenses in our eyes to change their shape and keep the target in focus (as you would with a camera). Those two eye movements—called “vergence” and “accommodation”—are automatic in everyday life, and they go hand-in-hand.

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.