Stereo Base (the distance between the camera lenses).
mm
Film Pair Width (the total width of the stereo pair as recorded on a single roll of film).
mm
Maximum Accomodated On-Film Deviation (the difference between the film gate separation and the stereobase).
mm
Film Chip Width (the width of single frame of the stereo image).
mm
Septum Width (the unused central part between the two individual frames).
mm
Camera Focal Length.
m
In-Camera Stereo Window Distance (from the film plane).
Using the calculator
There are numerous ways this calculator may be used. I have tried to make the calculator accessible, but you may initially find it overwhelming, so I would like to suggest a couple of sample use cases:
If you have a Sputnik, Rolleidscop or TL-120, you can select your camera from the "preset" dropdown menu and see the values of the various parameters. If you unlock Wseptum and change the value of MAOFD to 3 mm (i.e. viewer_lens_separation minus mount_aperture_separation), the value of Wchip is the widest possible aperture mount that your camera will support without window violations after cropping. If instead you change the value of Wseptum, you can see the effect on MAOFD and Wchip if you were to file down the septum of your camera! You can see for example, that it might make sense to reduce the septum width of the Rolleidoscop by filing, but it would make very little sense to reduce the septum of the Sputnik.
If you are designing a stereo camera, you can enter the design parameters which you have decided on, and the calculator will determine the values for the remaining parameters. For example, say I wanted to design a camera which uses a red window and the backing paper to advance the film. The 6x6 backing paper markings are positioned approximately 64 mm apart, so maximum width of the stereo pair is 128 mm, but I would like 2mm between each stereo pair for handling, so I will design the overall width of both film gates to be 126 mm (i.e. Wpair=126 mm). Let's also say that I would like the stereobase of the camera to be 65 mm, and I would like to support an on-film deviation of 3 mm. Entering these values, I can see that maximum possible width of the stereo image will be 58 mm (or a mounted image width of 52 mm plus 6 mm of wiggle room for cropping) and that there is little point in capturing the central 10 mm portion between the frames.
There are two independent equations (see below) and five parameters (the first five parameters in the table above), and therefore three of the parameters must be defined as inputs, and the calculator will automatically calculate the other two parameters as outputs.
The three most recently modified values are designated as inputs. If you change the value of an output, it will become designated as an input, and the input which was modified the longest time ago will be bumped and designated as an output. If you find that an input field becomes designated as an output despite you wanting it to remain an input, you may click the "lock" icon for that parameter. Only 3 parameters may be locked at a given time.
Equations
The maximum accomodated on-film deviation is defined as the difference between the film gate separation and the lens separation. Since the film gate separation is the sum of the width of the chip and the width of the septum, the first equation is: Eqn. 1) MAOFD = Wchip + Wseptum - B
The second equation simply introduces another parameter, the width of the pair on film, which is the sum of the both film gate widths and the septum in between: Eqn. 2) Wpair = (2×Wchip) + Wseptum
Notes and Comments
This calculator computes the in-camera stereo window, and correspondingly, the in-camera MAOFD, which is only of moderate importance. Of course, by cropping during mounting, you are able to adjust the location of the stereo window. This is a tradeoff: if the in-camera maximum accomodated OFD is too small, more deviation may be accomodated by cropping during mounting, however you must sacrifice the width of the frame. For example, if you choose the Sputnik preset, you will see that the in-camera stereo window only accommodates 1.25 mm of on-film deviation, placing the window at 3.75 m. Does this mean the near point in your scene must be at least 3.75 m away from the camera? No, but it does mean that you will have artificially increase the septum width during mounting by cropping out the central part of the images (outside edges after transposing the film chips), and therfore the maximum width of the stereo image (Wchip) will be less than what is calculated by this calculator.
This calculator assumes that infinity is present in the scene when interpreting MAOFD. In other words, it defines MAOFD as the difference between the separation of homologs located at infinity and the separation of homologs located at the in-camera stereo window. If you plan to design a camera for macro photography, this tool will not provide useful information to offset the film gates correctly.
If you are using two "mono" cameras, it is safe to asume that the film gates will be centered behind the lenses (B = Wchip + Wseptum, ∴ MAOFD = 0). This calculator will not be of much use to you. The in-camera window will be positioned at infinity, and the inside edges of film chips will need to be cropped by half of the actual desired MAOFD, and Wchip will be correspondingly lower.
If you are developing a stereo camera, the main takeaway is that the film gates ought to be offset to the outside (i.e. not centered directly behind the lenses, but rather separated by a slightly larger distance than the lens separation). The actual area masked off by the film gates doesn't really mean anything, what determines the maximum possible window-violation-free frame width is the lens spacing, the OFD and the width of the stereo pair. This calculator may assist you in determining the balance of film economy, stereobase and final stereo image width.
If the Wseptum parameter is an output, it calculates the septum width which will place the in-camera stereo window at distance required to achieve the MAOFD specified. If both Wseptum and MAOFD are outputs, Wseptum is calculated from Wpair and Wchip according to equation 2. The only reason to select this parameter as an input is if you are designing a camera. For the best film economy, the septum width should be minimized, but you may want a bit of film between the frames for handling during mounting.
