Slit-Scan

Time is recorded across space rather than frozen at an instant, so a moving subject stretches into long streaks. The frame stops being a moment and becomes a record of a duration.

Because nothing in ordinary vision behaves this way, the result reads as travel through somewhere that is not a place. It is why the technique attached itself to the idea of passing through time.

A narrow slit sits between the subject and the film, and one of them moves while the shutter stays open. Only a thin strip is exposed at any instant, and the strip written a moment later lands beside the one before it.

Douglas Trumbull built a rig for exactly this, running the camera along a track toward artwork behind a moving slit, exposing a frame at a time over many hours. The streaks are the accumulated record of that travel.

Digital versions do the same arithmetic on a video file, taking one column of pixels from each frame and assembling them side by side. The mechanism differs and the rule does not: continuous movement is what produces the streaks.

Use it for passages through time or space, for title sequences, and for any moment meant to leave ordinary physics behind.

Avoid it when the subject has to stay recognisable. Slit scan destroys shape, so a face put through it stops being a face.

A kaleidoscope repeats and mirrors the frame into a symmetrical pattern. The source image stays intact inside each wedge, and the effect is spatial.

Slit scan does not repeat anything. It smears one continuous movement across the frame, and the effect is temporal. The test is symmetry: a kaleidoscope has it, and a slit scan does not.

Douglas Trumbull created the Star Gate sequence in 2001: A Space Odyssey (1968) with a purpose built slit scan rig, and the corridors of light that Bowman travels through are single frames exposed over long periods rather than anything photographed at speed.

The model returns a still frame with motion blur on it. Say the movement is continuous across the whole shot, because a still subject produces flat bands and nothing else.

The second failure is short streaks that read as ordinary blur. Ask for the streaks to run the length of the frame, and name the direction they travel in.