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LENS AND THE PHOTOGRAPHED FRAME 159
coming to rest in the aperture and is therefore neither stationary nor flat during a considerable portion of the exposure interval. The theoretical optimum in a film movement is one which moves the film very quickly and then permits almost immediate exposure of the film with the same order of flatness and absence of motion that is found in the finest still cameras. Because of the "bouncing around" referred to, it is quite conceivable that a camera with a somewhat longer movement time may be better than one with a shorter movement time. There is only one criterion — what kind of pictures does the camera take?
Many low-priced cameras use a claw to perform the functions of moving and registering ; cameras such as the Eastman Model E do rather well considering the simplicity of construction. Others with more costly parts but with poorer design are not as satisfactory from the performance standpoint. Registration accuracy with respect to the location of the perforations cannot be expected to be as good in poor cameras as it is in cameras such as the Bell and Howell Filmo, the Mitchell, or Maurer cameras.
It is desirable that frame-line shift from the nominal due to all causes shall not exceed 0.003 in., and that frame-line "jiggle" (unsteadiness of the picture due to the differences in frame-line location from one frame to the next) be less than 0.001 in. With really good movements, these errors may be reduced to one-half and even less. Fortunately, Kodak film particularly is quite stable dimensionally, and, in addition, is accurately perforated. If unsteadiness of a photographed picture occurs, the reason is usually to be found elsewhere than in the film.
Camera Lens and the Photographed Frame
The problem of exacting control of the location of all points within the frame being photographed with respect to the "ideal" flat plane is a problem little appreciated by camera users — unless their pictures seem out of focus in one portion of the frame and are in focus at another portion. Ordinarily, it is not possible to place a mechanical supporting member at the back of the film at the center of the aperture — and, unfortunately, the designer is "up against it" because it is an axiom of mechanical design that a flat plate is the weakest possible mechanical section. To add to the complexities of the design problem, the film has a tendency to curl. The base and the emulsion of a film act as the two metals of a bimetallic strip, the emulsion having a much larger expansion coefficient than the base. The amount of curl changes with