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 (the full field) 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.

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 (more sophisticated electronic shutters are global, not rolling), 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 (flatbed or hand).

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 (along the slit) and the opposite axis represents time (along the exposure time). For example, a photo finish shows one strip (e.g., the finish line) over time, where the scanning direction (e.g., horizontal) 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 (a spatial dimension) and later in the shot (the time dimension). 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.

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

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 (approximate) sync, which can be done for racing, the image can look almost undistorted.