This is an important issue that can cause problems for a beginner. Every time you take a stereo picture by shifting a camera/lens parallel, the “stereo window” is placed at infinity. This creates two wide bands on the left side of the left image and the right side of the right image, which not only do not contribute anything but they also do not help to properly locate the image in space. To restore the proper location of the stereo window, some cropping is required.
Consider the picture shown here. This is a close up of a 10 inch doll taken by shifting a digital camera. Can you see these wide bands? A rule for proper placement of the stereo window (3L rule) says that “The Left eye should see Less on the Left side”. In this case clearly there is more to see on the left side of the left image, not less. We need to crop out these bands. By doing this, the entire picture will now properly be behind the stereo window, as you see in the corrected pair.
How do you get rid of these bands? It is easy to do it in digital photography or when making prints (cut off part the left side of the left print and the right side of the right print). But what do you do if shoot slide film? In this case you use the stereo mount to crop the edges, by shifting the film chips away from each other until the bands are hidden behind the mount.
The amount of cropping needed depends on the stereo base (B) and the magnification (M). It is given by basic stereoscopic formula: Cropping = P = M B. See this blog for derivation.
Consider some special cases: As infinity (M=0) no cropping is needed. At low magnifications (regular stereo camera distances) M = F/I, and the formula becomes Cropping = FB/I, where I is the distance of the subject. This is about 1.2mm for most stereo cameras.
At M = 1 we have the interesting situation where a shift of B in the object space creates the same shift B in the image plane, so cropping = B. In a previous slide bar example we showed that for M=1, a good value for B is 5mm. The final image will require cropping by 5mm. So our 36mm length of 35mm film image area is now reduced to a useable 31mm length. This is important for slide film stereo photographers because it means that you cannot use 33mm or 31.5mm (all available by RBT) to mount this stereo pair and the next available size is 30mm. If you want to preserve more of the stereo image, you might want to consider less stereo base, just to reduce image loss. You might need to work this the other way around. Let's say that you want to mount your image in a 31.5mm mount. You cannot crop more than 4.5mm. To have some room in mounting, you put a limit of 4mm cropping. This corresponds to 4mm shift. So, instead of 5mm, you shift by 4mm only, the choice being dicated by cropping considerations only.
Most stereo cameras from the '50s are constructed with a built-in stereo window. This is achieved by shifting the film gates with respect to the lenses. For example, the Realist lenses are separated by 70mm while the film gates are separated by 71.2mm. This shift creates a window at 7ft from the camera, which saves film and makes automatic mounting easier (by just centering the chips in the mount we get a window at 7ft.)
Consider the schematic here (click at it to enlarge it). In the camera on the left, the film gates are centered under the lenses. The stereo window is at infinity and you will notice that at any distance from the camera the left eye sees more on the left side instead of less, which means that this point is in front of the stereo window. The correct stereo window can be set by trimming parts of the final image. The camera on the right creates a stereo window by separating the film gates wider than the lenses. The left eye now sees less in the left side for objects past the stereo window. In this case, if the film chips are centered, the window is set automatically and less film trimming is required for close-ups.