One of the most interesting things about stereoscopy, to me, is not so much the replication of reality, but the plasticity of stereo space; the possiblity of manipulating depth. Just as we choreograph the lighting, color and actions of a movie, we can control the stereo illusion to our dramatic end. So let's try some experiments to see how the camera variables affect the stereo image.

You can see in the two windows below the simple scene I set up for this tutorial. You might like to replicate this in your own computer to further explore these ideas. (These actors are all on the A:M CD) But for now, you can grab your anaglyph glasses, and follow along. Sometimes you'll want to take off the specs and look at the double image. Since the left and right images are complentary colors, you can learn a lot by observing the color fringes. I've placed these fellows in a gray limbo, specifically to not obscure the concepts with background noise. And note that the shortest actor is closest to the camera, while the tallest is furthest away.

Put the RED filter over your LEFT eye for these anaglyphs! This web page is optimized for 1024x768 display--which you pretty much need to run Animation Master anyway!

Note how moving the camera changes both the depth of the image and the perspective. The perspective has to do with the size in the frame of the actors, relative to one another. Obviously, moving closer to a subject will make the subject appear larger in the frame, but note how this effect is most pronounced in the nearest actor.

Also, moving the camera closer increases the deviaton--the "shift" of the images, or the width of the fringes. Deviation determines the apparent depth in the stereo image. Note how the "far away" image has very little stereo effect. Humans only see stereo out a few hundred feet; beyond that we must use other depth cues to estimate distance. Stereo is most effective at close ranges.

Note how different changing the focal length is from moving the camera. The images size in the frame changes, but the actor's sizes relative to one another does not change. Also note that increasing the focal length increases the stereo deviaton--increasing the apparent depth in the stereo image, but proportionately to the magnification.

Compare the leftmost image of the focal length set with the leftmost image of the camera distance set, above. Observe how, even though increasing the focal length increases the deviation (and depth), the foreground actor looks rather flat, almost like a cardboard cutout. The background actors are even more "cardboardy." But since moving closer changes the perspective, the actors there look more round.

Increasing the stereo base ("Eye spacing" in the stereo properties panel) increases the stereo deviation.

Note how altering the camera spacing changes the apparent depth in the scene, without changing the relative on-screen sizes of the actors. BUT, that changes how we interpret the sizes of the characters in space: in the deepest image on the left, they seem further apart and the actors seems smaller. In the shallowest image, on the right, the actors all seem larger.

You can use this effect to control the preceived scale of the image. If you are making a movie from the point of view of mouse, you want to use a smaller base. By using a very large stereo base, you can make a city look like a toy. This is called "hypostereo" and "hyperstereo," respectively.

But look how different this is! These are the same pictures, but with the stereo window (the frame) set at the foreground actor instead of behind the background actor. The plane where the stereo pair is superimposed affects your interpretation of the depth. (See also, below, the section on varying the frame distance.)

But note also how the anaglyphs on the left and right ghost more than the anaglyph in the center. The extreme frame positions maximize the size of the fringes, so you're more likely to see ghosting. The center image has a frame position that minimizes fringing (by splitting the difference), so the ghosting is less apparent. With anaglyphs, often the most important factor in setting your frame distance will be minimizing ghosting.

This is the one aspect of the stereo image that can be changed after photography: by horizontally shifting the Left and Right images in relation to one another, you can accomplish the same thing as changing the frame distance. Of course, since you're working with two 2D images, not in the 3D space of A:M, you can't call this process "changing the Frame Distance" anymore. The most common term is "convergence," but I'm not fond of that useage (I think it leads to confusion with other aspects of stereoscopy), so I prefer "alignment." Lenny Lipton suggests "HIT" for Horizontal Image Translation, which is rather more descriptive.

I've varied the camera-to-subject distance and the focal length, keeping the center actor the same height in the frame. The camera spacing is the same in the first three images, and doubled for the last. Go back and review what I said earlier about the "cardboard cutout" effect (in the focal length section), and look at the images here. You should be able to see that increasing both the focal length AND the camera spacing together will NOT give the same effect as moving the camera closer to the subject--although in the physical world, that's the only way some things can be photographed. (How close do you want to get to a wild lion?) But in the virtual world, you can put your camera anywhere you please.

We can begin to see now that our interpretation of depth in a stereograph is not just a function of the deviation, the stereo base, but is combined with perspective (and other depth cues not discussed here, such as overlapping objects and atmospheric effects) to create an impression of space.

You should play around with these variables to help develop a sense of how you can manipulate space in your animations and stills. If you like, try animating some of the above examples, and see how the effect looks over time.

This page was inspired in part by Jim Sherwood's "Fun With Focal Length" tutorial and George Themelis' articles in "Stereogram", newsletter of the Ohio Stereo Photographic Society.