Motography (Jan-Jun 1913)

April 19, 1913 MOTOG^APHY 277 following table, and are expressed in the percentages of reflection that they give in regard to a surface giving total of 100 per cent reflection. Value in Material. Per Cent. Polished silver 92— .93 Mirror, silvered on back 82 — .38 Plaster walls, white 35 — .40 Aluminum, frosted 60 — .65 To prevent the distortion of the image, the screen should always be placed perpendicular to the optical center of the projector. If the screen is tilted out of this position on a horizontal axis, the vertical distances will be shortened with a constant horizontal length. If turned to the right or left, the vertical lengths will remain the same, but the horizontal will be reduced. The distortion will be the grt test at the farther edge of the screen as the distance included between the angle of two adjacent rays is greater at the greater distance. The screen should be given a backward inclination when the projector is pointed down at the screen from a balcony or other elevated position so that the screen is perpendicular to the optical center of the projector. CHAPTER II. MAKING THE PICTURE. IN a general way, the process of taking and finishing motion pictures is the same as that followed by the amateur photographer in taking and finishing snap shot pictures. In both cases a "negative" film is obtained by exposing a sensitized strip in a camera which is afterwards developed and printed. This negative is then used in obtaining a "positive"' print by allowing light to pass through the image on the negative and onto the positive sensitized film. This reproduces the image but in a reversed form, all of the light portions of the negative being dark on the positive, and vice-versa. Instead of using paper for the positive print as in Kodak photography, the motion picture manufacturer makes his on a celluloid strip that is similar to the negative film. The light of the projector passes through the transparent positive print and traces the image on the screen. The principal difference between the snap shot and motion picture camera lies in the shutter action and the film feeding mechanism, the action of these parts being practically continuous in the motion picture camera. Externally the latter type of machine resembles a large box camera that has a crank, a film measuring dial, and focusing aperture in addition to the equipment of the hand camera. An exceptionally fast lens is required, the usual lens being an anastigmat with a working aperture of f/3.5 to f/3.0, while lenses of f/2.0 are not uncommon. All of the cameras are provided with adjustable diaphragms similar to those used with view cameras. The focal length of the lens ranges from 2 to 4 inches. The usual focal length, about three inches, gives an angle of view of about twenty degrees. A two-inch focal length gives about thirty degrees. When more field is required in the foreground than is given by the threeinch lens, a lens of shorter focal length is substituted. Two independent mechanisms, the shutter and the film feeding device, are actuated by the crank in such a manner that the film is fed forward for a new exposure with the shutter closed, and is held stationary while the exposure is being made, the film progressing through the camera by a series of jerks. At each movement the film is fed forward through a distance equal to the height of one picture (%-inch). As the camera operator continues to crank the machine, the shutter and film movements are repeated over and over again so that a number of pictures are made in a row down the center of the film. These miniature photographs are placed so close that the top of one coincides with the bottom edge of the picture lying next to it. To insure accurate spacing, the film in some cases is positively driven through a sprocket wheel that engages with a series of perforations in the edges of the film. In this way a fixed relation is maintained between the shutter and the pictures so that each picture will be in the proper place in the projector on the opening of the shutter. In other cameras the toothed sprocket wheel is supplanted by a reciprocating hooked rod or claw, the points of which engage with the perforations in the edges of the film. In either case the result is the same. The claw points engage with the perforations at the upper end of their stroke, and as the crank revolves they are jerked down suddenly, pulling the film with them through a distance equal to the height of the picture. At the lower end of the stroke a special motion disengages the Fig. 17. Motion Picture Camera and Operator Ready For Action. claws from the perforations and they at once begin thenupward travel without moving the film. The claw type of intermittent mechanism as shown in Fig. 13 in Chapter 1, is better adapted to the camera than the projector, for in the camera the period of film rest is shorter and the wear due to the claw movement is practically negligible as the film passes through the camera but once. A revolving shutter of the vane or sector type is generally used which is gear connected with the operating crank. This shutter is simply a circular sheet metal disk with a "V" or sector shaped opening cut in it for the admission of light to the film. As it revolves, this opening comes opposite to the lens intermittentlv and in fixed relation to the film movement. The disc shutter is used in nearly all cameras, and is very similar to that used in the projector except for the proportions of the vanes or blades. It is generally placed between the lens and the film. The openings in the shut