Wrong, Wrong, Wrong: Myths of Stereoscopic Filmmaking

Every field has its mythology. Myths can build a collective spirit, and help people reach for a goal. But sometimes myths are destructive because they are based on fiction and can prevent people from making properly informed decisions. This is especially true in a nascent field like stereoscopic filmmaking. Although stereoscopic filmmaking has been around for a long time, people haven’t had a chance to practice their skills, so from a craft point of view, it’s still in a relatively early stage. Until lately there has been a lack of proper technology so that theatrical filmmakers could learn from experience. You learn from experience, which often means your mistakes. Although the maxim is learning from mistakes, you also learn from what you do right and what works. Who’s to know which is more important–doing it right or doing it wrong?  Obviously, nobody wants to do it wrong, so people tend to get conservative and cautious, especially in an undeveloped field like stereoscopic filmmaking.

One of the vexing issues has to do with the perception of projected 3-D images. We’re not used to looking at stereoscopic images on a big screen. We see stereoscopically in the real world every day, but looking at 3-D movies is not the same as looking at the real world, just as looking at motion in the real world is not the same as looking at movies–that is, photographed images that move. With that in mind, I am going to rail against some of the things I have read and heard recently that annoyed me. I don’t know the answers to all 3-D issues, but many people have made assumptions that they could test if they took the trouble to do so.

The first wrong, wrong, wrong myth is that you can’t do fast cuts in stereo. A recent test done by DreamWorks Animation proved the wrong-headedness of this notion. The test was supervised by our own “Captain 3-D,” Phil McNally. He took a section of Kung Fu Panda, about five minutes long, that has rapid cuts. It’s an action sequence of a tiger being released from imprisonment, and it’s spectacular. It leaves the audience gasping and cheering. Why people think you can’t do rapid cutting in 3-D is probably related to the fact that prior 3-D systems of photography and projection were so terrible that fast cuts got blamed for the pain. But what Phil has supervised proves that it’s not the rate of cutting that’s responsible for discomfort.

The next myth is that the breakdown of convergence and accommodation applies to the stereoscopic cinema. Wrong, wrong, wrong. The breakdown of convergence and accommodation is the habituated response we have between the neurological pathway for eye muscles that control vergence (fixation on a point in space) and a separate set of eye muscles and their neurological pathway controlling focusing. These are coordinated in the visual field. When you look at something in the visual world what you are focused on is also converged on. But for projected stereoscopic images that’s not the case for much of what’s going on on the screen. Only action that’s at the plane of the screen with at or near zero parallax involves no breakdown of convergence and accommodation. The very word “breakdown” is a scary term. It sounds like something is falling apart, and the whole ball of wax is going to crumble. That’s the accepted nomenclature and it refers to the fact that this habituated response doesn’t happen for plano-stereoscopic stereoscopic displays. It is a habituated response but the two neurological systems are independent.

When you’re viewing stereoscopic movies, typically you’re sitting tens of feet from a big screen. That’s good, because at such distances there is no breakdown of convergence and accommodation. It’s possible that people sitting in the first row or so of a small theater will have some problem. But the way it works is this:  Past a certain distance A/C breakdown doesn’t happen. I’m tired of reading ill-informed articles saying that the breakdown of convergence and accommodation is a big deal for projected 3-D movies. It of concern for small screens like TVs and desktop monitors, because people sit close to them.

Here’s another wrong, wrong, wrong:  Films that are composed for IMAX won’t work in Real D. But two shows that I consulted on from National Geographic–Sea Monsters: A Prehistoric Adventure and Lions 3D: Roar of the Kalahari–work perfectly fine when translated from IMAX to Real D. The major concern was aspect ratio–cropping from 1.4 to 1.85. Another concern was to make sure that the zero-parallax plane was properly adjusted. IMAX doesn’t care about making the zero-parallax condition correspond with the plane of the screen, because the screen surround is almost beyond the periphery of the visual field. Real D cinemas have the equivalent effect by using floating windows. I’ve been looking at a clip of The Polar Express for three years on a Real D screen. It looks great and it was prepared for IMAX. You can translate from one to the other. You can translate from Real D to IMAX and vice versa, and the results look good.

Another wrong, wrong, wrong is that everything in a 3-D movie needs to be sharp. For photographers that means lots of depth of field. Perversely, the nomenclature that has grown up for people in CG animation is to “turn off the depth of field.”  If photographers say they want a lot of depth of field, the people in CG say, “Let’s turn off the depth of field.”  Maddeningly enough, they mean the same thing. For a long time people have thought you need to have everything in a 3-D movie in focus.

The out-of-focus or depth-of-field cue that occurs in cinematography is a unique depth cue because it was created by photography. In the real world you have so much depth of field that nothing is out of focus so there is no resultant depth cue. But we have learned to associate out-of-focus images with images that are in the background. Depth of field is a depth cue but an invented imaging depth cue. It is a depth cue that you won’t see in paintings prior to the invention of photography, but it’s one that we’ve learned to appreciate because of photography. The history of stereoscopy and the history of photography are linked. Wheatstone enunciated stereoscopic imaging in 1838, and a year later he made the first stereoscopic photographs. His initial work was with drawings.

It turns out that, based on tests I’ve done and seen, there’s nothing wrong with out-of-focus backgrounds for 3-D. The question that comes up is: do you need this cue when you’re doing stereoscopic imaging?  You probably don’t need the background focus cue that much anymore, because you’ve got the stereoscopic cue. But the background cue doesn’t hurt, and you can use it. However, the foreground out-of-focus thing–like the tip of a finger coming off-screen–might make sense in a 2-D show, but in a 3-D show you’re better off having off-screen objects sharp.

Another wrong, wrong, wrong myth of stereoscopic filmmaking that people have been rapidly learning is wrong in the past year or so, is that stereoscopic photography should be done with the interaxial (distance between camera lens axes) and the interocular or interpupillary (distance between the eyes) with the same value. It turns out that a lot of good stereoscopic cinematography can be done–and, especially on a set, must be done–with the interaxial less than the interpupillary separation. If you don’t do this you can blow out background points. That is, you will have strongly divergent background points if you insist upon having the interaxial equal to the interpupillary. You can also wind up with exaggerated stereoscopic depth effects in which objects look elongated. The point of stereoscopic filmmaking is to make images that are enjoyable to look at, and that leads me to my next point.

Wrong, wrong, wrong:  Stereoscopic movies must be orthostereoscopic. There’s a concept in photography called orthoscopy in which certain geometric constraints have to be taken into account in order for an image to have the same appearance that it would have in the visual world. Given a certain focal length of a lens you have to be a certain distance from a projected image, given its magnification, for the image to be orthoscopic. In other words, it needs to subtend the same height on your retina as it would have in the visual field from a given distance. Human beings can’t change the focal lengths of their eyes (unless you’re looking through a telescope or binoculars). But filmmakers can. Typically, for orthoscopy to work, images shot with wide angle lenses have to be viewed close, and images shot with telephoto lenses have be viewed from far. To pull it off, everybody in a theater would have to keep moving around to different seats or you’d have to change the size of the projected image for on a shot-by-shot basis. But this is not done because we have learned how to look at projected images shot with different lenses.

The planar orthoscopy conditions apply to orthostereoscopic conditions. The orthostereoscopic condition also includes the condition that the interaxial equals the interpupillary. This is a condition that can only be fulfilled for some of the people in the audience. That’s because the mean interaxial separation for human beings is 65 millimeters. I don’t know the value of the standard deviation, but from kids to adults (with big heads) it’s anyplace from 45 to 74 millimeters. So that means that if you shot for one set of eyes the majority of people would not be seeing an orthostereoscopic image. For an orthostereoscopic image to be seen, in addition, you need to be sitting in the dead nuts middle of a row, at exactly the right distance given the focal length and magnification. And there’s no good reason to do any of that, because the point of stereoscopic filmmaking is to create an enjoyable image, not a scientifically correct one.

Another wrong, wrong, wrong, myth, has to do with divergence–that you’re going to tear people’s eyeballs out of their heads if you have more than 2-1/2 inches (the nominal interpupillary separation) of background parallax. It’s a good thing to avoid divergence, but think about it this way:  For people who are sitting an average viewing distance from the screen, the difference between 2-1/2 inches and 3 or 4 inches of background parallax is mice nuts. That’s because the best measure of parallax is angular measure, not that which you can measure by laying a ruler on a screen. Large values of parallax, either off-screen or divergent parallax, have the biggest effect on people who are sitting up close. But for the vast majority of people in a theater, a little divergence is probably not going to hurt them very much. Discomfort also depends on how long the shot is on the screen, how sharp it is, and a whole bunch of other things. Stereoscopic filmmaking is an art. But for CG animation there’s no excuse, if you’re shooting for a certain size screen, to have divergence. Cinematography (with cameras) is a more difficult art.

But there’s a lot of flexibility, and you’ve got to use common sense. Human beings have a flexible visual system.

“Silver screens are crap.”  Wrong, wrong, wrong. There’s a prejudice about silver screens, despite the fact the movie industry itself is described by the term “the silver screen.”  There are some powerful producers and technical people in this town who hate silver screens. It’s not altogether an unreasoned prejudice, because there have been problems with silver screens in the past. There has been blotching, visible seams, and hot spotting. Modern silver screens made in the past couple of years can be good. Well-made screens no longer show seams, no longer have blotching, and no longer have hot spotting. Silver screens are different from matte screens. They have higher gain, and they conserve polarization. But they can be used for showing both 2-D and 3-D. The major disadvantage of silver screens is the same disadvantage you have when you sit in the worst seats in the house and you’re looking at a matte screen. Not only is the image “keystone” distorted because you’re way off on the side, but you get shading. By “shading,” I mean that one side of the picture is going to be brighter than the other. The worst seats in the house for matte screens are even worse for silver screens; there’s no denying it. But the vast majority of seats in a normal theater are good. Silver screens, when you project 2-D images on them, have more contrast. 2-D can look better on a silver screen.

Silver screens sometimes have different colorimetric characteristics. Some silver screens may slightly tint the image so that it’s colder. If they tinted the image so that the image was warmer, or more towards the red, I don’t think people would object. But it’s the coldness that people don’t like, and that can be corrected easily when projecting digitally, which is the major use of silver screens.

2 Responses to “Wrong, Wrong, Wrong: Myths of Stereoscopic Filmmaking”

  1. danielesiragusano Says:

    Hi Lenny,

    nice and true Words,

    For the »out of Focus point« I have a different point of view.

    Out of Focus Objects can be seen in many old paintings of the flämisch Painter like Vermeer, Rubens and Rembrandt (Ok some of them used a camera obscura – anyway). Out of Focus is not an Invention of the photography. If you focus on your thumb (I bet you did that thousands of times), you see a unfocused doubled background image.

    In fact unsharpness helps us in the visual field, when the relative disparity of Objects is bigger than 25‘ (25′ Rule). The binocular rivality is won by the objects with more detail. This is a efficient way, because in nature the normal field of view is concordant to the depth.

    You don`t notice this unsharpness, because the seen image is a viewsynthese of many images the eyes captures.

    So the use of depth of field in cinematography has to be reflected and reinvented for stereoscopic cinema, in context to the different visual perception. And yes, »everything sharp« is the wrong starting-point.

    • lennylipton Says:

      You know I never noticed out of focus in Renaissance painting but it doesn’t metter, it seems to me, whether the origin is painting or photography because this effect is simply not apparent in the visual world.

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