Stereoscopic filmmaking is gaining some traction and filmmakers are faced with new concerns. The unknown is daunting, but when faced, may not be as perilous as it seems. All the concerns of the creative filmmakers – whether they’re shooting live action or doing CGI – remain. As a first approximation you light and compose shots just the way you always have. There are additional concerns, because there are some new parameters. Objects are going to exist with a new “flavor” in the z-axis: They’re going to play at the plane of the screen, or behind the plane of the screen, or in front of the plane of the screen. In addition, there are more subtle concerns that need to be addressed.

The control of space is different. What is accomplished in planar, or 2D, filmmaking with focus and lighting and perspective can now be addressed in additional ways. It’s also important to realize that the stereoscopic material that filmmakers are creating will almost certainly be exhibited two-dimensionally, either in theaters or on DVDs – so the film has to work for both 2D and 3D. There are two major things we’re concerned with, and there’s one ancillary concern. The two major new creative controls are:

Placing objects in relationship to the plane of the screen. This is entirely in control of the filmmakers. In other words, pay attention to what plays at the plane of the screen, what plays in audience space, and what plays in screen space or behind the plane of the screen. This is controlled by horizontal or lateral shifts of the left and right images, and it’s sometimes called convergence. If the left and right images are superimposed when projected on the screen, then the image plays at the plane of the screen. So if you take off the glasses when you’re looking at a 3D movie and the images appear to be single, they’re going to look like they’re playing at the plane of the screen. If the images are double, they’re either going to play in front of the screen in audience space, or behind the screen.

It’s a perceptual conundrum that the working space in the screen is limited to infinity but the extent in front of the screen, in audience space, seems to be even greater.

The other concern I call the strength of the stereoscopic image. That is determined by two interrelated factors. One of them is completely new to the stereoscopic cinema, and has no direct counterpart in the planar cinema – and that is the distance between the spacing of the cameras or the camera lenses. Whether we’re shooting live action, or we’re in a CGI virtual space, the distance between the camera’s lenses is a critical factor. We’re going to call whatever we’re shooting with a stereo camera. A stereo camera, unlike a conventional camera, captures two perspective viewpoints. So we’re not going to refer to stereoscopic cameras. We’re going to call it a stereoscopic camera, and we’re going to say it has two lenses – a left lens and a right lens. The distance or the spacing between the two lenses is called the interaxial spacing – that is, the distance between the lens axes. If you think about it, if the lenses are superimposed – in other words, if the axes had zero spacing – you’re shooting a planar movie. The farther apart the lenses go, the deeper the image looks. The use of this control is closely related to the focal length you use. Wide angle lenses tend to stress perspective, because objects that are closer to the lens appear to be proportionately larger, and the background appears to be smaller. The stereoscopic depth sense, which is technically known as stereopsis, is weighted or scaled by extra-stereoscopic cues – that is, by non-stereoscopic or monocular cues. One of the strongest of these is perspective – and perspective is often determined by the choice of focal length. So it turns out that with wide angle lenses you can use a reduced interaxial, and for telephoto lenses you can use a larger interaxial.

The other extra-stereoscopic cue of note is motion parallax, and that effect is created by a moving camera, especially when the camera lens is pointed in any direction other than the direction of camera motion. The relative rejuxtaposition of foreground and background are a kind of temporal perspective cue and this kind of camera motion tends to exaggerate the stereopsis cue.

This is an absorbingly rich subject because aesthetic decisions are involved. Although the principles I’ve enunciated here are relatively simple, they maybe difficult to visualize. In other words, if you say, “Well, I’m going to use this focal length, and the lenses are going to be this far apart,” it’s really tough to know what you’re doing unless you can see what you’re doing on a screen. The best thing to do is to project the image, or to look at it using some kind of a stereoscopic monitor system. The beautiful thing about electronic stereoscopic images is that past filmmakers needed tables and calculators, and you may not need to rely on such tools now. The marriage of stereoscopic technology with electronic display technology is one that was literally made in heaven, because it is so easy to visualize the final result. You don’t have to wait for the dailies.