The depth information in a stereo image must not exceed certain limits.

Why should we observe the shooting rule?

Since our brain can process only a limited amount of depth information, the depth contained in a stereo image must be limited so that the entire stereo picture can be observed as a uniform spatial image. A violation of the shooting rule leads to a decay of the spatial image into individual image sections. If we want a stereo photo that is pleasant to view, adherence to the shooting rule is a condition which should be observed under all circumstances.

Which values and tolerances are dependant upon the shooting rule?

While taking a stereo picture, the spatial depth produces a lateral displacement of the two corresponding points in the left and right stereo images. For close objects this displacement is larger - and it is smaller for distant objects. The difference between the largest (produced by the "closest object") and the smallest (produced by the "most distant object") displacement is called deviation or maximum parallax. The deviation should not exceed 1/30th of the entire image width. Thus, with the European slide format 24mm x 36mm, the deviation obtained is 36mm/30 = 1.2mm. The American format of 24mmx24mm results in 24mm/30 = 0.8mm. Both are also empirically found and generally accepted values.

How do I adhere to the shooting rule?

A very simple condition for adhering to the shooting rule can be derived from some geometrical considerations:
Distance to closest point <= Stereo base times focal length (all values in mm).

When taking pictures, you only need to ensure that no object is closer to the camera than the allowable distance to the closest point, calculated using the above rule, for example 2.3m with 35mm focal length, 3.3m with 50mm focal length or 4.5m with 70mm focal length (assuming a stereo base of 65-70mm). This condition can be somewhat modified if the distant point is not located at infinity. For these cases, refer to the special literature (e.g. Stereoscopy, Dec. '95, pp 24-25 for close-ups) or do some trial and error tests and calculations yourself.

A simple estimate of the permissible stereo depth is also possible by comparing the depth of sharpness: With stereo cameras having a 65-70mm stereo base the admissible stereo depth corresponds rather accurately to the sharpness depth zone of a 35mm lens at f-stop 8, of a 50mm lens at f-stop 11 or of a 70mm-lens at f-stop 16. Violations of the shooting rule cannot be corrected anymore during mounting or during projection.