Here’s a video with a great explanation of the Hexagon’s film transport, including some cutaway views of it actually operating. From the narration:
The length of the film path from each supply assembly through each camera system and into the first recovery vehicle is almost 100 feet. Each supply assembly provides 150,000 feet of film to each camera at controlled constant velocities of up to 70 inches per second under specified tension.
The looper assembly in each film path serves as the interface between the coarse and fine film transport systems and allows the total length of the film stored in it to be constant. This enables the fine film transport system to supply film at the speeds up to 200 inches per second required at the camera’s focal plane.
Each camera system contains active steerers and passive articulators to center the film on supporting rollers at critical points in the film path. Dual take-up assemblies in each of the four reentry vehicles have a film capacity of one fourth of the supply film load. Prior to Hexagon, no other satellite camera system had been called upon to transport very large quantities of ultra-thin base film at speeds of over 200 inches per second and be able to reverse direction both at the take-up and supply spools.