Two synchronized 35mm cameras mounted in tandem — enables stereoscopic 3D capture or dual redundant takes without resetting. Rare but essential for large-scale 3D productions.
Two 35mm cameras, mechanically and electronically synchronized, mounted one behind the other on an axis – that's the core idea. The double projector allows you to capture two identical images at the exact same moment, either for true stereoscopy or as a safety measure when a second, simultaneous recording of the same take is indispensable. This configuration was and is extremely rare in everyday production because the technical requirements are considerable, and modern digital cameras have long offered more elegant solutions.
Historically, the double projector was used during the stereoscopy wave of the 1950s – films like Bwana Devil or early 3D experiments operated on this principle: two cameras with slightly offset lens distances (interocular distance) captured the scene simultaneously to create spatial depth through corresponding projection later. The two negatives were developed separately and registered and combined in post-production. The effort was immense – synchronization via gears, shared exposure control systems, identical optics.
In modern production contexts, the double projector works on a different principle: you mount two digital or digitized 35mm cameras so close together that they run in absolute time synchronization. This can be necessary when you need two separate negatives – for example, in action scenes or expensive stunts – without having to repeat the scene. Some sequences were so expensive that a reshoot was impossible; here, the double camera guaranteed you a backup version in a single take. The editor received two full, time-synchronous rushes from one shot.
The practical hurdles remain considerable: mechanical tolerances, synchronization drift over longer takes, different filtration, and light loss due to beam-splitter optics (if used) – all of this requires meticulous preparation. Modern solutions like redundant digital capture or simply two spatially offset cameras with subsequent synchronization in the NLE have relegated the classic double projector to niche areas. Nevertheless, for certain stereoscopy projects or extreme budget scenarios where repetition is impossible, this setup remains relevant – it is one of the last mechanical solutions for absolutely synchronous dual capture.