Slit-Scan Photography: Time, Motion, and the Art of Progressive Exposure

Slit-scan photography is a technique that collapses time and space into a single image by exposing the film (or sensor) through a narrow slit that moves across the image plane during a continuous exposure. Rather than capturing a single frozen instant — the fundamental action of conventional photography — slit-scan records different parts of the scene at different moments, creating images where time is literally spread across the frame. Objects that move during the scan are stretched, compressed, warped, or fragmented in ways that reveal the temporal dimension of experience that conventional photographs suppress. The result is imagery of extraordinary visual strangeness: runners whose bodies flow like liquid, cityscapes that undulate like rivers, portraits where features shift and distort as the subject moves during the scan.

Slit-scan has a rich history spanning fine art, commercial photography, scientific instrumentation, and cinema. The technique was used by photo-finish cameras at racetracks from the 1940s onward — a slit camera aimed at the finish line records whatever passes the slit over time, creating an image where the horizontal axis represents time rather than space. Stanley Kubrick's 2001: A Space Odyssey (1968) used elaborate slit-scan machinery designed by Douglas Trumbull to create the psychedelic "Star Gate" sequence — one of the most famous visual effects sequences in film history. Contemporary artists like Andrew Davidhazy, Adam Magyar, and Jay Mark Johnson continue to push slit-scan photography into new creative territory.

How Slit-Scan Works: The Fundamental Principle

In conventional photography, the entire image is exposed simultaneously (or very nearly so — focal-plane shutters expose progressively, but at speeds fast enough that the difference is usually imperceptible). In slit-scan, only a narrow strip of the image is exposed at any given moment. The slit moves across the film or sensor over time, so the left edge of the image might be exposed at time T=0, the centre at T=5 seconds, and the right edge at T=10 seconds. Anything stationary in the scene appears normal — it looks the same at T=0 as at T=10. But anything that moves during the scan is recorded at different positions by different parts of the image, creating spatial distortion that maps motion over time.

The direction and speed of the slit's movement relative to the subject's movement determines the nature of the distortion. If the slit moves in the same direction as the subject and at the same speed, the subject appears wider (stretched) because it occupies more of the scan than it would in a single instantaneous exposure. If the slit moves opposite to the subject's motion, the subject appears compressed or even reversed. If the slit moves perpendicular to the subject's motion, the subject appears sheared — tilted at an angle that corresponds to the ratio of subject speed to slit speed. These spatial-temporal distortions are not arbitrary — they are mathematically precise representations of how the subject moved through space and time during the scan.

Analogue Slit-Scan Methods

The simplest analogue slit-scan technique requires only a camera with a removable lens, a piece of cardboard or metal with a narrow vertical slit cut in it, and a means of moving the film past the slit during exposure. Remove the lens carrying a pinhole or narrow lens behind the slit. Place the slit where the film gate normally sits (directly in front of the film plane). Wind the film smoothly past the slit during exposure while the camera is pointed at the scene. The film records whatever passes the slit, one narrow strip at a time, building up a continuous image as the film advances.

A more practical approach uses a large-format camera with a focal-plane shutter-like mechanism: mount a piece of opaque material with a narrow slit (1–5mm wide) on a track that can be pulled smoothly across the film plane inside the camera back. Open the lens shutter on "B" (bulb) to admit light continuously, and pull the slit across the film at a steady speed. Each position of the slit exposes a narrow strip of film to whatever the lens projects onto it at that moment. The scan time — the time it takes the slit to traverse the entire film frame — determines how much temporal information the image contains.

The photo-finish camera variant — strip photography — uses a different approach: the slit remains stationary at the centre of the frame, and the film moves continuously past the slit while the camera is aimed at a fixed point (like a finish line). Whatever passes the camera's field of view is recorded as a continuous strip on the advancing film. The horizontal axis of the resulting image represents time rather than space — the image is a temporal panorama rather than a spatial one.

Digital Slit-Scan Techniques

Digital slit-scan is typically achieved by recording video and then assembling the frames computationally. The simplest method: record a video of a scene with the camera stationary. Then, using software (Processing, TouchDesigner, custom Python/OpenCV scripts, or dedicated slit-scan applications), extract a single vertical column of pixels (e.g., the center column) from each frame and arrange these columns side by side to build a new image. Frame 1's center column becomes the leftmost column of the new image, frame 2's center column becomes the next column, and so on. The resulting image is a temporal cross-section — each column represents a different moment in time, and any motion in the scene creates spatial distortion in the assembled image.

More sophisticated digital slit-scan techniques allow the scan line to be positioned anywhere in the frame, angled rather than vertical, curved, or even varied over time. The slit width (number of columns extracted per frame) can be adjusted to control the balance between temporal resolution and spatial coverage. Multiple slits can be extracted simultaneously from different positions in the frame. Some software allows real-time slit-scan processing of live video — the results appear on screen as the action unfolds, enabling interactive exploration of temporal imagery.

Kubrick, Trumbull, and the Star Gate: Slit-Scan in Cinema

The most famous application of slit-scan in cinema is Douglas Trumbull's "Star Gate" sequence in Stanley Kubrick's 2001: A Space Odyssey (1968). Trumbull built a custom slit-scan machine: a horizontal track with a camera at one end and a large illuminated transparency (painted artwork backlit by powerful lights) at the other end. The transparency had a narrow slit cut in front of it. The camera's shutter was opened for the duration of a single film frame, and during that exposure, the camera physically moved toward the transparency along the track. Because the slit was narrow, only a thin strip of the artwork was visible at any given distance — and as the camera moved closer, the perspective changed continuously, creating the streaking, tunnel-like effect that defined the Star Gate sequence.

The exposure for each frame took approximately 60 seconds — the camera advancing slowly along the track while the transparencies were illuminated through the slit — and the entire Star Gate sequence required months of continuous, automated photography. The result was a visual experience unprecedented in cinema: corridors of light stretching to infinity, colour fields flowing and transforming, geometric patterns warping through impossible spaces. Trumbull's slit-scan work influenced every subsequent attempt at representing transcendent or psychedelic visual experience in film, from the warp-speed effects in Star Trek to the digital tunnel sequences of countless subsequent science fiction films.

Strip Photography: Time as the Horizontal Axis

Strip photography (or streak photography) is a specialised form of slit-scan in which the camera views a fixed point in space while the film (or digital equivalent) moves continuously. The result is an image where the vertical axis represents space (the scene visible through the slit at the fixed point) and the horizontal axis represents time. Nothing in the scene appears "frozen" in the conventional photographic sense — every element is a temporal trace, its width representing how long it occupied the slit's field of view and its shape representing how it moved during that time.

Photo-finish cameras at horse races and athletics events are the most familiar application. The camera is aimed at the finish line with a narrow slit aligned to the line. The film advances at a speed calibrated to match the typical speed of the competitors. When a runner or horse crosses the finish line, their image is traced onto the moving film — a fast runner moving in the same direction as the film appears stretched (wide), while a slower runner appears compressed (narrow). Two runners crossing the line simultaneously occupy the same horizontal position on the film, while sequential crossings are separated horizontally. The resulting image is a perfect temporal record of every competitor's exact arrival time, readable to thousandths of a second — far more accurate than any conventional photograph or human observation.

Contemporary Slit-Scan Artists

Andrew Davidhazy, professor emeritus at RIT, has spent decades exploring slit-scan and peripheral photography (a related technique where the camera or subject rotates during a slit exposure, producing 360-degree peripheral "unwrapped" images of three-dimensional objects). His work demonstrates the extraordinary range of visual possibilities in slit-scan: from abstract colour studies to peripheral portraits that unfold a face around its own circumference to technical imaging applications in industry and science.

Adam Magyar's Stainless series uses digital slit-scan techniques to photograph subway stations in cities worldwide. A high-speed camera captures video of passengers waiting on the platform as a train passes. The slit-scan assembly transforms the video into vast panoramic images where each person is rendered with photographic precision but in a single, continuous temporal field — all figures occupy the same strip of time, and their spatial relationships to each other are preserved, but the image reveals temporal details (micro-expressions, transitional gestures, momentary interactions) that casual observation would miss. The results are monumental in scale and profound in their meditation on urban anonymity and shared public space.

Jay Mark Johnson's work uses a digital slit-scan approach where the camera scans horizontally while recording video, producing images that explicitly map time along the horizontal axis. Waves become rhythmic patterns, walking pedestrians become flowing organic shapes, and trees swaying in wind create undulating abstract compositions. Johnson's work makes the invisible temporal structure of everyday scenes visible — the rhythm of ocean waves, the pace of city traffic, the gentle oscillation of tree branches — transforming mundane temporal experience into visually stunning abstract imagery.

Building a DIY Slit-Scan Setup

For digital slit-scan experimentation, you need only a camera capable of recording video and a computer. Record a 30–60 second video of a scene with movement — traffic, pedestrians, waves, swaying trees, flowing water. Transfer the video to your computer. Using free software like Processing (with the slit-scan library), Photoshop (via scripting or the animation/video timeline), or command-line tools like FFmpeg combined with ImageMagick, extract a single column of pixels from each frame and assemble them into a new image. The pixel column position determines which part of the scene is sampled; the video duration determines the temporal span of the resulting image.

For analogue slit-scan, build a simple focal-plane slit mechanism for a large-format or medium-format camera. Cut a slit 1–3mm wide in a piece of matte black card. Mount it on thin rails inside the camera back, behind the lens plane and directly in front of the film. Open the lens shutter on "B" and pull the card steadily across the film, slit side facing the lens. The speed at which you pull the card determines the scan duration and thus the degree of temporal distortion. Faster pull = shorter scan time = less temporal effect (closer to a normal photograph); slower pull = longer scan time = more temporal effect (more visible distortion of moving subjects). Experiment with pull speeds from 1 second to 30 seconds for a full frame traverse.

Slit-Scan as a Way of Seeing

Slit-scan photography challenges the most fundamental assumption of conventional photography — that an image represents a single moment. In reality, no photograph is truly instantaneous (even the fastest shutter speeds have finite duration), and our experience of the world is inherently temporal — we never perceive a frozen instant, only a continuous flow of changing sensory input that our brains synthesise into the illusion of a stable present moment. Slit-scan photography makes this temporal dimension visible and explicit, creating images that are arguably closer to genuine perceptual experience than conventional still photographs.

For photographers interested in experimental and conceptual work, slit-scan offers a technique that is endlessly generative. Every variable — slit width, scan speed, scan direction, subject motion, camera motion — produces radically different results. The same scene scanned at different speeds reveals entirely different temporal structures. The technique can be applied to virtually any subject — portraiture, landscape, sport, architecture, abstract — and consistently produces imagery that is visually compelling and conceptually rich.