It might be feasible (and fun, perhaps) to construct your own film scanner using some components that are cannibalized from other sources.

Choosing a Film Scanner

The most expensive part of building your own system will be the film scanner. You will require one that has a straight path through the unit, and you will also have to manufacture a gate to hold the film steady as it passes through. This need not be anything more complex than a film guide, but you must be careful to protect the imaging area of the film to avoid scratching it.

Rather than use a slide scanner, consider carefully dismantling a video camera and using the imaging system in that. Some high-end Telecine manufacturers such as CTM Debrie Cinematography use Sony HDCAM components on an OEM basis to build their
systems.

Film Transport

Next you have to construct a framework to create a film path through the scanner. You will need a couple of film pulleys. Most important for driving the film through, you will require a suitable sprocket for the film gauge you are using. But there is no need to manufacture a proper film gate with a claw mechanism. Dismantling a broken film projector will yield the parts you need.

If you place a friction pulley on one side of the gate and a motorized sprocket to pull it through the gate from the other side, this will provide sufficient control. The film must remain taut. There are other tensioning devices available, and perhaps the cannibalized projector will yield up a sprung double-pulley tensioner.

A projector synchronizes the movements of the film and the shutter by a complex mechanism of gears and cams. That is unnecessary here because our exposure time happens under software control. The film must be moved through the gate, advancing by one frame’s worth of distance each time. Attach a stepper motor to the sprocket drive to provide the motive force. Stepper motors are obtained by dismantling old floppy-disk drives to remove the head-positioning motor. An interface is required to contrive some kind of electrical connection to the computer, and one of the servo control interfaces used by amateur robotics enthusiasts would do. Some of these I/O controllers are now available as USB connected devices. Controlling this machinery should not involve any particularly complex driver writing.

Software Support

Once the whole machine is assembled, two sets of utility code must be provided.
The first one will control the stepper motor. This must be moved one step forward at a time. The code is called a number of times depending on the distance from one frame to the next, measured in steps. Hopefully the step distance will be small enough that there is an integer relationship between the two.

The second part of the utility software is designed to drive the film scanner in order to capture a frame. Scanning the whole 35mm film surface area is not necessary, as the device was likely a 35mm slide scanner to start with. The API support in the
TWAIN interface will provide the support necessary to frame the scanning area so it just covers the imaged part of the film. This is known to be feasible here because it is done all the time with paper scanners during a preview and crop. They use the same TWAIN driver code.

Running the Process

Scanning the full 8mm width will include the sprocket holes. That provides a reference that will reduce the software-process tracking errors later on.

An application uses these utilities to drive the stepper motor and take a snapshot. That application could just capture the frames as a series of still images or make a movie file one frame at a time.

After a successful run, the application will have created a file containing a digitized version of the film at probably as good a resolution as it is possible to achieve (depending on the quality of the film scanner you start with). Some remedial work to stabilize any camera gate weave and crop the output will be necessary.

Some variants on this theme are possible if the scanner is driven one line of pixels at a time. In that case alternating steps of the film transport and the scanning head will yield a continuous series of scan lines that will then need to be sliced into frames. This will deliver a slightly more stable scanned result. The quality of the whole project is dependent on your mechanical skills and how well you conceive the software to process the results. This design will eliminate the gate weave you normally get with a projector because the film is being moved far more gently through the gate. None of the scanners can eliminate the gate weave from the camera because that is an integral part of the film once it has been exposed. We stabilize the footage in software to remove that problem.