Exposing the same frame multiple times — creates layered, ghosted images through additive light. Now mostly achieved in post via compositing.
You expose the same frame multiple times — either analog on camera negative or digitally in post. The images layer on top of each other, add together, creating transparencies and ghost images. Classic examples: multiple motion phases of a person in one shot, or double-exposed portraits with landscapes superimposed on the face. This was long the only way to achieve such effects if you didn't want to work with mattes.
Analog on Set — the old craft: You shoot scene one, rewind the negative (partly manually, partly via camera markings), and expose scene two onto the same film. Each exposure had to be underexposed by one to one and a half stops, otherwise the final image would be overexposed. The cinematographer needed a steady hand, precise notes, and confidence in their camera knowledge. Rollback mechanisms weren't equally reliable everywhere — some 16mm cameras would drift, and your registration would be off. Each repeat was a risky take.
Digital Today — much more controllable. You shoot your elements individually, cleanly exposed separately, and composite them later. In DaVinci, Nuke, or After Effects, you layer the clips, adjust opacity, blend modes (Screen, Add, Multiply — depending on the effect), and fine-tune the color. No physical rewinding, no exposure calculations in your head. This saves stress and material wear.
Practical advice on set: If you have to work analog or want a retro look, remember — a tripod is mandatory. Small camera movements between takes will destroy registration precision. Use frame counters or digital markers, photograph the markers. In a digital workflow, the biggest hurdle is consistently lighting and positioning the separate takes. Lighting changes between take one and two are immediately noticeable. Therefore: same camera position, identical lighting, only the performers or props move. The compositing phase then becomes artisanal — rotoscoping, matte refinement, grain matching, to create the impression of a single, multiple-exposed shot. With modern tools, this is cleaner than ever before.