Strip photography


Strip photography, or slit photography, is a photographic technique of capturing a two-dimensional image as a sequence of one-dimensional images over time, in contrast to a normal photo which is a single two-dimensional image at one point in time. A moving scene is recorded, over a period of time, using a camera that observes a narrow strip rather than the full field. If the subject is moving through this observed strip at constant speed, they will appear in the finished photo as a visible object. Stationary objects, like the background, will be the same the whole way across the photo and appear as stripes along the time axis; see examples on this page.
The image can be understood as a collection of thin vertical or horizontal strips patched together, hence the name. Digital sensors do produce discrete strips of pixels that are captured and arranged one line at a time. In film photography, the image is instead produced continuously, so there are no discrete strips, just a smooth gradation.

Implementation

Many photographic devices use a form of strip photography due to the use of a rolling shutter for engineering reasons, and exhibit similar effects. This is common both on cheaper cameras with an electronic shutter, as well as cameras with mechanical focal-plane shutters.
This technique can be implemented in multiple ways. In film photography, a camera with a vertical slit aperture can either have fixed film and a moving slit, or a fixed slit and moving film. In digital photography, one can use a line sensor, generally one that is moving, as in a rotating line camera, but also an image scanner.

Aesthetics

The fundamental property of strip photography is that one axis of the photo shows the scene changing over time, while the other axis does not. The simplest method of this is recording a stationary slice, perpendicular to the frame, so one axis of the photo is a spatial dimension and the opposite axis represents time. For example, a photo finish shows one strip over time, where the scanning direction represents time, not space.
If the camera moves during the shot, like when taking a panoramic photo, there is no longer just one space axis and one time axis. Instead, for the example of a camera moving horizontally from left to right to take a panoramic shot of a landscape, the vertical axis is still just a spatial axis, but as you look from left to right along the photo, you see an image that is both further to the right of the subject and later in the shot. The vertical axis of the photo is therefore a mixed spatial and time axis, and the panorama represents a period of time not a single instant in time.

Characteristics

Strip photography has a number of distinctive characteristics, particularly fixed slit.

Distortion

Moving objects are distorted based on the relative speed of their motion and the image capture. For objects moving in a fixed direction at a constant or almost constant rate, as is the case of racing photographs, notably photo finishes, this yields an approximately constant rate of distortion, so the image is stretched or compressed. If the speeds are in sync, which can be done for racing, the image can look almost undistorted.
In strip photography, distance is interchanged with time, so width in the scanning direction is proportional to time, and thus inversely proportional to speed. Slower-moving objects occupy more time, and thus appear wider, while faster-moving objects are narrower, as they occupy the slit for a shorter period of time. In extreme cases a very rapidly moving object can be captured for only a single strip or even none at all, while a stationary object will appear as a horizontal line. These differences are particularly notable in cases of movement at differing constant speeds, such as parallax from a train window or differing speeds of traffic or people walking. Further, in the case of motion towards or away from the camera, size changes, creating additional distortion.
In the case of diagonal motion in the direction of capture and towards or away from the camera, objects flare as they approach and taper as they recede; this is because objects appear larger the closer they are, with increase in horizontal size yielding faster movement and thus decreased size in the strip photograph. These effects are inverse in magnitude, so objects effectively undergo a squeeze mapping, properly an inhomogeneous squeeze, hence the flared shapes.
In other cases, however, particularly movement not in the direction of capture, very unusual distortions result, resembling smears. These may be compared with surrealism, such as the work of Pablo Picasso or Salvador Dalí. In cases when the exposure time is slow relative to movement, this distortion combines with motion blur, yielding soft blurs.
In the case of runners, the torso will be moving at an approximately constant rate, but the extremities will be moving rapidly in other directions, yielding distorted extremities. Particularly notable are relay runners, due to the combination of regular racing and the irregular transfer.

Striped background

For fixed slit photography, non-moving objects, particularly in the background, are rendered as a constant stripe, yielding a striped background. For moving objects, the width of the object in the image is inversely proportional to the object's speed relative to the camera as discussed above ; a very slow or stationary object therefore has a very large or unlimited width, which is a stripe.

Perspective

More subtly, for fixed slit photography, as all capture is in a constant direction, there is no perspective in the image. This is conspicuous in long strips of races, where all the racers are viewed directly from the side, rather than from an angle depending on their position. This effect looks the same as the maximum possible perspective distortion in a photo taken from very far away, in which case perspective flattens.

Changing scene

Because the photograph is not at a single instant, the scene can change during the exposure, including such features as duplication of a single object. Notably, extreme ends may be quite separated in time. For example: in a rollout photograph of a head that makes more than one revolution, the subject's expression may change each time; in a race photo, a racer may go behind the camera and pass the finish line repeatedly, or reverse direction and cross the finish line in the opposite direction; in a panoramic photo, a subject may be captured at one side, go behind the camera, and then re-enter the frame and be captured at the other side. Before the digital age, this was a common ploy on school photographs that were taken with a slit camera that rolled across the scene, the film moving by the same mechanism in the reverse direction. It can also be used to tell a temporally authentic story, as in a comic strip - events at one end, and consequences or reactions at the other, later in time and space.

Layout

Vertical strips – so time is horizontal – is most common, and accords with horizontal scanning, as in reading, though horizontal strips – so time is vertical – are also found. In addition to horizontal and vertical strips, other forms are possible, such as radial strips. Aspect ratio varies, with some photos being similar to ordinary photos, emphasizing a single image, while others are long, emphasizing the passage of time, as in a comic strip or traditional scroll paintings.

Applications

This is most basically used in panoramic photography, to capture a large, static scene that would be difficult to capture via other techniques; scanning cameras are designed for this.
Other applications include:
Sports are a common use of strip photography, both for photo finishes and artistic purposes. It is particularly common for racing, where movement is largely regular and predictable, but by no means limited to it. Due to the movement in sports, which is a combination of movement at a regular rate and at a changing rate, various forms of distortion are possible. An early accidental example of distortion is "Grand Prix de Circuit de la Seine" by Jacques Henri Lartigue, where the skew caused by the vertically traveling slit makes the race car appear to lean forward, creating a sense of speed.
Strip photography was notably used by George Silk at the US tryouts for the 1960 Summer Olympics, Further photography at Life and Sports Illustrated that used strip photography included John G. Zimmerman, who borrowed Silk's camera to photograph Pete Rose and later photographed basketball players Nate Archibald and Julius Erving using a slit-scan camera for Sports Illustrated, and Neil Leifer, who used it frequently in the 1970s for athletes including Gaylord Perry and Billy Kidd, and for sports such as IndyCar racing. More recently, Bill Frakes captured Marion Jones winning the 100m event at the 2000 Summer Olympics using a strip camera.

Artistic uses

Strip photography can be used for artistic effect, which has been done regularly since the 1960s. In addition to sports, early examples include work by Silk and other Life photographers for various subjects, such as the cover of the Halloween issue of Life 1960. William Larson pioneered modern artistic uses of strip photography from the late 1960s. Michael Golembewski has been a practitioner of scanography.
More recently, Jay Mark Johnson has used slit cameras for artistic effect. Adam Magyar used a custom "slit scan" camera to record city traffic over time in his panoramic photo series Urban Flow. In his next project, Stainless, Magyar made use of an industrial line scan camera and custom software to capture panoramic photos of moving subway traffic in major metropolitan cities, including New York, Paris and Tokyo. He later included high speed video from the perspective of the moving subway car, which captured exceptional three-dimensional detail of people waiting on the platform over a very small amount of time.
The Processing graphics application has an example script for video manipulation that outputs an endlessly rewriting strip photograph.