Figure 13-1.–Basic components of a motion-picture camera. Figure  13-2.–Rotary  shutter. shock  and  prevent  the  film  from  breaking  (fig. 13-1). The shutter in most motion-picture cameras is a focal plane type and is called a rotary disk shutter. A rotary disk shutter is a disk that has a segment cut out, causing the shutter to have a light and dark cycle as it rotates. Exposure is made when the cutout segment of the shutter passes in front of the film. The film is advanced  during  the  dark  cycle  (fig.  13-2). A  motion-picture  camera  is  used  to  photograph action in a rapid succession of still pictures on a long strip of film. Each picture area on a motion-picture film is called a frame, and the speed that the camera is operated is called frames per second (fps). The standard operating speed for 16mm cameras is 24 fps. When the camera operating speed and the rate of projection are the same, the action looks normal; therefore, the standard projection speed is also 24 fps. However, it is possible, and sometimes desirable, to make motion pictures at a slower or faster rate than 24 fps. You may do this to either slow down or speed up the action on the screen. To portray a subject in slow motion, you operate the camera at a speed faster than the standard 24 fps, but keep the projector at the standard speed. To portray a subject in fast motion, you operate the camera at a speed slower than 24 fps, and the film is projected at 24 fps. All changes to the portrayal of normal subject motion should be done by adjusting the camera speed, not the movie  projector. Camera speeds in the thousands of frames per second  are  used  in  scientific  and  experimental  research to measure and observe such things as the fall of liquids, the  speed  of  objects  in  flight,  and  the  bursting characteristics of objects. When films shot at very fast fps rates are projected at 24 fps, the illusion of subject motion on the screen is slowed down considerably. At these speeds the viewer can study details of the subject matter  and  obtain  research  data. Motion-picture cameras are classified according to the size (width) of the film they use. The most common motion-picture  film  sizes  are  as  follows:  8mm,  super  8, 16mm, and 35mm. In the Navy today, motion-picture film has almost been completely replaced with video film;  however,  Hollywood  productions  still  use motion-picture  film  as  large  as  70mm. Lenses used in motion pictures are basically the same as lenses for still photography; therefore, the information on optics presented in chapter 1 also applies to motion-picture camera lenses. The standard or normal focal length lens for a 16mm camera is 1 inch (25mm). Longer  or  shorter  focal  length  lenses  should  be considered   as   long   focal   length   (telephoto)   or wide-angle lenses, respectively, depending on what size film is used. A long focal length lens for 16mm film is 38mm or longer. A wide-angle focal length lens for this camera is 13mm-17mm. Table 13-1 illustrates some typical  camera  and  lens  combinations. FILTERS With one exception, the use of filters for motion pictures is the same as for still photography. The effects that filters produce on motion-picture film emulsions are the   same   as   the   effects   they   produce   on   still photographic film emulsions. The one exception is the use of a polarizing filter. Camera panning should be avoided because variable darkening of the image results. The information on filters presented in chapter 3 applies to   motion-picture   photography   as   well   as   still photography. 13-2