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3D movies have gone in and out of fashion for over a hundred years. Ever since the Lumière brothers first shocked an 1895 audience with The Arrival of a Train at La Ciotat, filmmakers have tried to trick audiences into believing that the images on screen are real. This past year has seen a resurgence of interest in 3D movies from blockbuster hits like Avatar; to newly debuted 3DTVs for personal home use; and even — plans for 3D cell phones. It appears that 3D may be here to stay — but, what is 3D and how do 3D movies work?

3D movies are first produced by setting up two cameras side-by-side like a set of eyeballs recording the action. Each camera records the action in almost exactly the same manner, except that the image will be slightly shifted between the two. When you watch a 3D film your left eye is watching the left camera and your right eye is watching the right camera. When your brain puts these two images together it makes one image that looks three dimensional. But in order to do this, your eyes have to watch these images separately. If you look at a 3D film without a way to separate the images you would see double – and that’s not very fun to watch.

Your grandparents can probably tell you about going to watch 3D movies in the 1950s with those funny blue and red glasses. That’s because filmmakers used to use color filters to divide the film into two separate images. The left film is passed through a blue filter and the right film is passed through the red. Two projectors would project these images together creating a very weird, blurry image on screen. But, when you watch the filtered movie with special 3D red/blue glasses, only the red lens will see the blue filtered images and the blue lens will see the red filtered images. This would separate the images for both eyes but this method also distorted the color and the picture still came out fuzzy. Not to mention that those cardboard frames would dig into your skin and make red marks all over your face. 3D films were soon considered an empty gimmick and by the end of the 1960s they were abandoned as a form of entertainment. But today, 3D filmmakers have improved how we can watch 3D movies by using a process called polarization.

When 3D movies are polarized, special polarized lenses divide the picture in two so that they can be read by each eye separately. You may have a pair of polarized sunglasses somewhere in your house. These lenses divide the rays of light into vertical and horizontal waveforms. However, you have to use the polarized glasses at the movie theater to watch a 3D movie, because they’re made to match how the movie itself was polarized.

Not all 3D movies were made for 3D. Since the popularity of Avatar, many filmmakers have begun converting their 2D movies to 3D. They do this by separating the original 2D image on different planes, from objects that seem closer to ones that seem far away. Once these images have been separated, they can increase the illusion of distance between them to help create the illusion of 3D. But, these films often fall flat when compared to a movie that was produced specifically for 3D projection.

To achieve the best effect, images must be composed for 3D. Because 3D relies on convincing your brain that there is more distance between objects than there really is, filmmakers must pay special attention to how their images increase the perception of depth. Artists have used 3D tricks for hundreds of years on their 2D canvases by distorting shapes as they approach a horizon line, painting shadows, layering colors from dark to light, and centering the composition towards one focal point. When these methods are used in combination with movement—like in a movie–the filmmaker can use the edges of the screen like the outer edges of your field of vision. From there they can create the illusion of objects flying in and out of the screen and around your head. When these images have been divided in two, and are processed by the right and left eye—you’ve just asked your brain to do a whole lot of work.

Are you getting a headache? If you ever walked away from a 3D movie with a headache, you might have experienced what optometry researchers are calling “vergence-accommodation conflict.”

Let’s look again at how we see three-dimensional objects in real life. First, the eyes must “verge”—they rotate towards the object so that it’s always in the center of the retinas. Then the eyes must “accommodate”—they focus by bending the shape of the lens to make the image crisp and clear. “Without appropriate vergence, you would see double, and without appropriate accommodation, you’d see blurry,” says Martin Banks, a professor of optometry at the University of California at Berkeley who is researching the potential side effects of watching 3D movies.

Artificial 3D can confuse the eyes because they will try to focus on two different distances at once. The first distance is usually the illusory 3D image, the second distance is on the screen where the image is actually taking shape. This shifting in focus may be the source of headaches and other discomforts, says Banks. — (Technology Review, April 5, 2010)

If you’re experiencing eye-strain or headaches, it may help to take off the 3D glasses and try to focus on something other than the screen to give your eyes a rest. It’s also helpful to sit in the center of the movie theater where the difference between the right and left images are even and the focal points are parallel, requiring less on the eyes to focus. After the show you might want to focus on distant objects and even play a game of catch to readjust to real three-dimensional world.