To understand the dilemma that RBT (or any person/company attempting to design a 35mm stereo camera) faces, consider the size of the full-frame 35mm film format: 24x36mm. The simplest solution for film economy, while maintaining full frame, is to have a full frame pair, separated by a full frame. This design will lead to the sequence: ABAB/CDCD/, etc. (the film can be cut every two pairs without splitting a pair) The only problem is that the minimum separation of a pair is 2 x 36mm = 72mm. That would have been the case if the pictures touched each other. With a bit of a margin between each pair, we arrive to the 75mm image (and lens) separation
This is configuration B, possibly the most popular configuration. The configuration leads to alternating advance sequence (1-3-1-3, etc). One advantage is that the roll looks similar to a standard roll exposed from a 2d camera, so the slides can be handled and mounted by automated machines. The disadvantage is that the lens spacing (75mm) is wider than the spacing of the eyes (which is around 65mm average).
If one does not like this 75mm wide separation of the lenses (dictated by the separation of the images on the film) then they have two options:
1) Reduce the image size. Keep the same basic format (ABAB/CDCD/, etc, alternating advance sequence) but make the size of each picture slightly smaller, 33mm instead of 36mm. By cutting 3mm from each picture, we can reduce the spacing of the images and lenses by about 9mm. The result is a 65mm lens spacing (desirable) but the image size is smaller and the roll cannot be processed by automated machines. This is configuration A.
2) Leave a gap between each pair (i.e. have no images recorded between two pairs). In this case we can keep any spacing of images (and lenses) at the expense of film economy. This is configuration C. This configuration only gives 13 pairs per 36 exp roll (vs. 20 for configuration A).
Which format is best?
It depends. Three questions to ask: 1) Is orthostereo (65mm) important to you? 2) Is automated processing important to you? 3) Is having full frame images (36mm wide) important to you?
Format B gives up orthostereo for automatic processing. If you want automatic processing, then B is the only choice, at the expense of orthostereo. If you want orthostereo but don’t care for automatic processing (because you are doing your own stereo slide mounting, for example) then go for A (film economy) or C (full frame size).
How important is the difference between 65mm and 75mm?
Some people will say that this difference is small. Some people will actually prefer the extra “depth boost” of the 75mm spacing. Those coming from the Stereo Realist format (70mm) should have no problem adjusting to 75mm. But, for me, coming from the gentle depth of the RBT S1 (59mm), the 75mm is too much. I do not like indoor shots taken with 75mm lens spacing. So I want 65mm. I also don’t care for full frame and I prefer film economy. The format of choice for me is A.
The RBT S1 camera follows a different format. For some reason, RBT designed the RBT S1 with a 59mm spacing. This leads to exactly half frame gap between each stereo pair. The film advance is fixed (20 perforations). Maybe this choice was dictated by the details of the film advance modification when joining the cameras. The RBT S1b uses a 45mm lens spacing but apparently the same advance sequence (and film waste) as the S1. This reinforces the idea that the details of the camera advance modification have led to this unsual format. I’ve heard that the RBT S1b was made especially for underwater stereo photography. Some people think that the 59mm spacing of lenses in the S1 is already too small, but personally I have grown accustomed to it.