Strip Photography and Scanning Photographic Systems
Rochester Institute of Technology
Introduction: Focal Plane Shutters
When we take a photograph and later on we state that it is a record of a “slide of time” or an “instantaneous” photograph, we are confusing our¬selves and the general public as far as a realistic perception of the manner in which most photographs are made today.
The reason for the above statement is that most of our cameras are equipped with focal plane shutters. Inherent to the operation of these shutters is the fact that the focal plane of the camera is uncovered (and covered) in a sequential, rather than instantaneous, manner.
It is true that for most photographic situations the small time difference between which the first edge of the film is uncovered and the last strip is exposed is insignificant. However, with any moving subject, distortion in the recorded image is a real occurrence and the greater the rate of subject movement the greater the final distortion on the film for a given focal plane shutter curtain velocity.
An object which is square and moving perpendicular to the direction of curtain motion will become rhomboidal in shape on the film. If its image moves parallel and in the same direction as the curtain, it will become a rectangle and it may eventually extend the whole length of the film gate when image speed and curtain speed are just equal. Conversely, when it moves in the opposite direction, a square subject’s image will become compressed and if the speed is high enough it will approach the width of the exposing shutter slit.
Basic Strip Photography
A single lens reflex camera as we know it holds the film stationary during the time it takes the shutter slit to travel from one edge of the film to the other. A streak, or strip recording system keeps the shutter slit stationary and continuously changes the recording surface behind the slit.
At first thought one might dismiss this idea as useless and assume that it could not record anything but blurs. This assumption would be correct as long as the camera was used to photograph subjects whose images cannot be made to move with respect to the film plane in a direction perpendicular to the orientation of the slit shutter.
However, as a square object crosses the field of view of a strip camera and its image traverses the slit shutter in such a direction and speed that it matches that of the moving film behind the slit, the film will perceive the subject as stationary and record an (almost) normal-looking image of the object; i.e., a square will look square.
When the film and image motions do not match in speed or direction, the image recorded on film will appear distorted relative to the original subject in ways which can be predicted from the previous discussion on the operation of focal plane shutters.
Streak, smear, slit, or strip photographic systems are much more common than most of us realize. One of the most common applications of these techniques (and one which is most misunderstood) is the photography of the order of finish of participants in races, be they horses, dogs, or athletes. In this system the slit shutter is aligned in such a way that the leading edge of the slit lies just next to the area in space desig¬nated as the end of the race. The film in the camera is set in motion as the participants are approaching the finish area. As they individually cross the finish “line” they leave an impression of their bodies on the film.
The displacement between racers is directly translatable to time. The fact that a ‘natural” looking subject is recorded is due to the fact that the film is made to match the expected speed of the image of the runners at the finish line. However, regardless of the speed of the runners or the speed of the film, the photo-finish photograph is an almost indis¬putable record to be used for determining the winner of a race.
In the above case we set up the system so that the camera was stationary while the subject moved. A similar system is that which photographs a stationary subject but which induces image motion by moving the camera past the subject. The most well-known application here is in certain aerial mapping cameras.
This particular method, when brought closer to home and upon more detailed examination, yields photographs which our eyes could never see but which generally our brain accepts as a possible interpretation of reality. To accomplish this “feat” and gain some more insight into strip recording, the subject we will use for this example is a row of cars parked in clearly designated spaces in a large parking lot. The strip camera is mounted in a car and driven at a constant speed past the cars so that the rate at which the image of the cars moves past the slit matches the speed of the film in the camera.
Upon development we notice that between every car a clearly delineated marker is evident. The system, therefore, can be said to be devoid of perspective in the direction perpendicular to the slit orientation. That direction is a representation of time, which is generally an “invisible” factor in our minds.
A third application of strip photography is the making of continuous panoramic photographs such as those sent back from the surface of Mars by the Viking lander.
At this time we must remember that as long as the image of the subject can be made to move with respect to the slit in the camera, a picture can be recorded with a strip camera. An image of a stationary subject, such as the world around us, can be made to move over the slit by moving or turning the camera.
If the rate at which the camera turns is constant, then the speed of the film within the camera has to be adjusted for the particular focal length of the lens attached to the camera. This is primarily dependent on the angle of view of the lens along the slit, its relationship to 360 (the full circle of a panorama) and the distance over which the angle of view of the lens is measured; i.e., the dimension of the film along the slit in the camera. A good example is the attempt to photo¬graph “everything”; that is, to produce a photograph with 180 vertical and 360 horizontal coverage. In this record the panoramic camera must advance 48 mm. of film for every revolution of the camera since the 180-degree vertical coverage is measured along the 24 mm. film dimension. By tipping the camera so that its angle of view extends behind the axis of rotation one may photograph beyond infinity!
Finally, the opposite of recording panoramas, or 360 “looking out,” would be to record 360 “looking in” or peripheral photography. With this technique the primary applications lie in surface photography of pistons and art objects such as pottery.
In this case the strip camera remains stationary and the subject is made to rotate so that it presents a different surface to the open slit and moving film at all times. The speed of the image of the surface of the subject at the slit is made to match the speed of the film and as the subject turns its surface is optically “stripped” and recorded onto the moving film. The method is not limited to perfectly cylindrical subjects, and interesting results can be obtained with such imperfect cylinders as human heads. The system will easily produce 360 portraits. Distortions are introduced when the subject’s center of rotation does not fall precisely on the slit or if there is movement along the orientation of the slit during the recording of one revolution.
Strip photography can be accomplished easily as long as the photographer is more interested in visual impact than absolute technical perfection. To begin with, a narrow slit (about 1 mm.) made of black photographic paper is installed between the focal plane guide rails of a 35 mm. camera. With the lens cap on, the film is advanced to the take-up side and then the shutter is set to B or T and locked open. After activating the rewind release button, the film is slowly rewound into the supply cassette by turning the rewind crank by hand. Subjects which cross in front of the camera will be recorded on the film and after development one looks over the film in search of just the right image. The same camera can be used to take peripheral pictures, or photo-finish pictures, or to take photo¬graphs out of a moving vehicle.
By making the camera independent through motorizing the rewind procedure, it can then be mounted on a motorized turntable or even a tripod and panoramic exposures can be recorded. Of course, there are a number of commercially available units but they are so specialized that in general their price is prohibitive for strictly experimental use. A new panoramic camera, appropriately called Rotocamera, was introduced at the 1976 Photokina. Strip cameras are made by Robot and distributed by Karl Heitz of New York. High-speed scientific cameras are available through Red Lake, Gordon Enterprises, Photosonics, and others.
If you would like to develop your own system, I would be happy to help you do so and to try to answer any questions you may have on this topic. You can write to me at the Rochester Institute of Technology, One Lomb Memorial Drive, Rochester, NY 14623.