having two eyes is vastly overrated

A few years ago I had to go to the RMV to get my driver’s license renewed. This involved, aside from the paperwork and general atmosphere of human degradation, a quick eye exam. In Boston this is done by peering into what looks like a table-mounted View Master, only instead of slides of the Superfriends, this one has an eye chart.

“Read the letters above the green line, please.”

“X-O-Q-T,” I say. This is followed by a slightly too-long pause.

“And the rest of the line, sir?”

“Oh,” I say, quickly recognizing the problem. I need to turn on my right eye. I squint my left eye shut to give my right eye a jumpstart, and the rest of the line pops into existence. “R-P-V-M.”

Without another word she moves on to the rest of the paperwork. I didn’t get my license renewed that day, but that’s a different story. The point of my telling you this is to let you know that like many people born with cerebral palsy, I have a lazy eye. Or rather, I had a lazy eye, surgically corrected when I was about two years old. As with language, where there’s a sensitive period during which the brain can soak up a native language with ease, there’s a similar period for the wiring of depth-sensitive neurons. This period occurs at a very early time in development, so my depth-sensitive neurons didn’t develop in an ideal manner.

I’ve spent most of my life with this crippling visual disability. Unable to play catch or aim a frisbee. Terrible at estimating how far away I am from anything. And most damningly, utterly unable to shake hands or open doors without accidentally punching someone in the mouth or mashing my fingers against the wall. My hands, they ache from being bitten and bludgeoned so.

Or, you know. Definitely not.

This is why articles like “How 3D Works (And Why It’s Back!)”, by Erez Ben-Ari, never fail to tick me off. They inevitably equate depth perception with stereopsis (literally “solid sight”), a phenomenon experienced when the two slightly different images hitting your eyes merge to produce a sensation of depth (this process is also called binocular fusion1). In reality, stereopsis is a minor contributor to depth perception, most useful within a range of five feet and essentially useless beyond twenty. The lion’s share of depth perception arises from monocular (one-eyed) cues. We tend to take these for granted since they seem so basic: the way solid objects overlap each other, the way things get smaller as they get farther away, changes in texture and other visible details, to say nothing of a little thing called motion, which always seems to get neglected in discussions of depth perception, academic or otherwise.

Mr. Ben-Ari’s article makes a mistake pretty early on when he claims that “3D imagery has been around for ages, mostly as a gimmick, but things have changed in the past few years.”

Well, sort of. Stereoscopy, or the process of creating a sensation of depth from a pair of 2D images, has been around since 1840. Sir Charles Wheatstone invented the first stereoscope (among a few dozen other things). In fact, Wheatstone’s stereoscope is still used in vision research today, as the apparatus is both cheap and easily adjustable for each observer. You can make one yourself, either with mirrors and cardboard or, if you’re feeling particularly American, with iPods. Even setting the research applications of stereoscopy aside, Ben-Ari’s claim that it’s mostly a “gimmick” is debatable. Any doctor who’s ever gazed into a professional-grade microscope will tell you how useful that extra depth cue can be. Of course, research into stereopsis eventually led Béla Julesz to the random dot stereogram, which in turn gave us the Magic Eye. That’s not just a gimmick, that’s torture. There is no Easter Bunny.

Ben-Ari can also be faulted for failing to do some basic research into the history of 3D movies. Discussing the various methods of projecting 3D movies he says, “…the most popular way, initially, was to use the notorious red-blue glasses…A few years ago, a new delivery method came about, based on polarizer glasses.”

Ben-Ari gives the impression that this polarization technique is all newfangled, arriving on the scene “a few years ago.” I suppose that’s true, if by “a few years,” you mean seventy-four. Polarized 3D movies were patented and marketed by the brilliant Edwin H. Land in 1936. In fact, he started a little Mom and Pop business called Polaroid, perhaps you’ve heard of it? Most of the 3D films shown during the “golden era” of 3D in the 1950s were projected using the polarized method, with the more well-remembered red-blue lens system being used for comic books and later TV adaptations.

All this is window dressing that hides Ben-Ari’s real whopper. After a discussion of the basics of binocular depth perception and before his inaccurate recounting of the history of 3D film, he casually says, “For this reason, people with a damaged eye cannot judge distances correctly.”

Oh, I beg to differ. So would cinematographers, surveyors, and snipers (and anyone who spends a lot of timing estimating distances with one eye shut, really), as well as Bryan Berad, the one-eyed professional hockey player. While we’re on the subject of sports, I found at least three Major League pitchers who are blind in one eye: Thomas Sunkel, who pitched for the Cardinals, the wonderfully named Whammy Douglas, who pitched for the Pirates, and Abe Alvarez, who pitched a few games for the Boston Red Sox during—get ready for it—the 2004 season.People with damaged eyes should be concerned more with their diminished field of view than anything else. Judging distances is not a problem.

Let it also be known that not all “damaged eyes” are equal. If you’re like me and you have some form of amblyopia then not all is lost. Evidence suggests that special visual exercises can restore an amblyope’s visual function to normal levels. At a large vision conference last year, I had the pleasure of meeting Dr. Sue Barry, who showed me that I can even restore my stereopsis, provided I’m looking at the right things. My recent experiences at 3D movies like Up and Coraline have shown me that I am indeed capable of perceiving stereoscopic effects, which suggests that with training I might be able to fully restore my 3D vision.2

So let’s review. Erez Ben-Ari wrote an article on the resurgence of 3D movies in which he botches the science, rewrites history, and fundamentally misunderstands why this resurgence is taking place. It’s not really about improved stereoscopic technology (although it’s certainly easier today than it was in 1936, when two reels of film had to be meticulously synchronized to prevent the audience from dying of eye strain). It’s more about 3D as an attraction. You still can’t get good 3D in your home, which means that the theater is the place to be. For this reason, I doubt we’ll ever see a usable home solution for full-color 3D. If the movie industry hasn’t already gummed up the works on that project, they really should. It’s where the money is. Just look at Avatar.


  1. By the way, binocular fusion’s evil twin is binocular rivalry, which occurs when the two images hitting your eyes are too dissimilar to be merged together. Rather than fusing them into a depth percept, your brain has a big argument over which picture you should be seeing, with your dominant eye (yes, just like you have a dominant hand, you have a dominant eye) usually winning out by default. This is why red-blue 3D glasses almost never work for me, and it’s also why I had that problem at the RMV.

  2. It really seems to depend on how much form information my brain has to work with. I’m hopeless on a random dot stereogram, where a perception of depth arises purely from binocular fusion, but as the object is more clearly defined (more realistic) and/or takes up more of my visual field, it gets easier to induce stereopsis.