Double 8 spools actually contain a 16 mm film with twice as many perforations along each edge as normal 16 mm film; on its first pass through the camera, the film is exposed only along half of its width. When the first pass is complete, the operator opens the camera and flips and swaps the spools (the design of the spool hole ensures that the operator does this properly) and the same film is subsequently exposed along its other edge, the edge left unexposed on the first pass. After the film is developed, the processor splits it down the middle, resulting in two lengths of 8 mm film, each with a single row of perforations along one edge. Each frame is half the width and half the height of a 16 mm frame, so there are four times the number of frames in a given film area, which is what makes it cost less. Because of the two passes of the film, the format was sometimes called Double 8. The frame size of regular 8 mm is 4.8 mm × 3.5 mm, and 1 meter of film contains 264 pictures. Normally, Double 8 is filmed at 16 or 18 frames per second.
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