Seeing double: How do 3D movies really work?

Movies are great, movies with explosions are better and movie explosions three inches from your face are the best.  Three-dimensional technology brings us there, and it seems like lately everything is being released in 3D. What does that mean, beyond higher ticket prices and lots of dorky glasses?

To understand 3D movies, first you have to understand how vision works. When you look at an object, your brain receives two different images of that object – one from each eye. The brain fuses these two flat images to create one image in three dimensions.

When we watch 3D movies we see two images at once, each from a slightly different perspective. Filmmakers can achieve this in a few ways: James Cameron uses a fusion camera system, designed to capture two sets of images at once, while other live-action directors use a rig with two separate cameras for all shots. Computer-generated movies can be set up to render two images from different perspectives of the generated scene, which is why almost every big-budget animated movie is released in 3D these days. If the original data is still available, this can even be done years later (as with the Toy Story films) without any loss of quality.

The classic red and blue glasses, first used for “The Power of Love” in 1922, filter movie images so each eye sees only its intended half of the movie. By tinting one set of images red and the other blue, filmmakers can trick our eyes into seeing one image through the red lens and the other through the blue. The brain processes the images as two perspectives of the same object, creating the illusion of a 3D image.*

Since 2003, black plastic glasses manufactured by RealD Cinema have flooded theaters. Instead of using different colors of light, these glasses use different polarizations. Polarized light is forced to move in only one direction. The lenses act like horizontal blinds on a window: If you half close the blinds and look down, you won’t see any light–but if you bend down and look up through the slats, you’ll be able to see light shining through.

When you watch a RealD film, the left- and right-eye versions of each frame alternate at a rate of 144 images per second. A filter in front of the projector switches the polarization of each alternating image to match the filter for either the right or left lens of the glasses. The result is the same as with the red and blue glasses, but with more clarity.

But all that eye activity might cause headaches for some. Many viewers complain of nausea and discomfort, and there are enough 3D-dissenters to warrant the invention of glasses to cancel out the 3D effect. Plus, 3D movies cost an average of 18 percent more to make, and it’s movie ticket buyers who absorb the difference. But movie goers still buy expensive tickets in droves because 3D movies, through their visual trickery, help the public escape to fantasy worlds more effectively.

The increased sense of immersion caused by 3D movies is “primal and direct,” according to Shun-nan Yang, research director in optometry at Pacific University in Forest Grove, Oregon. That primal feeling results from the parts of the brain activated by viewing 3D movies. When watching flat movies, Yang says, we rely on visual cues to interpret the depth of the image. When watching 3D, our sensory system responds directly to the visual stimulation. Instead of “knowing” the depth, we “feel” it. This, along with an exaggerated perception of motion caused by the illusion of depth, allows us to become more immersed in a film. Rick Heineman, spokesman for RealD, agrees. “If watching a 2D movie is like watching through a window,” Heineman says, “Then watching in 3D is like being in the room.”

*Correction, December 15, 2012:

Originally, this article read that only red and blue 3D glasses were used during the 3D “golden era” of the mid-50’s. This is inaccurate. Polarized glasses were also used in cinemas at that time.