Motion Picture News (Mar-Apr 1923)

1714 Motion Picture News Description of the Motion Picture Camera (Continued from page 1708) movement. Should one side, for instance, wear a trifle more than the other three sides, every fourth picture in the negative would be slightly out of register with the other three. In addition to this, slight variations in the thickness of the negative film, or its pliability, cause it to ride the intermittent sprocket more or less snugly, causing a variation in the frame line, or an up and down movement of the picture. The Revolving Cam The harmonic cam, on the other hand, revolves once for each frame taken. Any small amount of wear, being the same for each successive picture, is not appreciable. This wear may be readily taken up in most constructions by loosening two screws which hold one of the guides between which the cam runs, and the guides may be adjusted firmly against the cam. The shutter opening with the Lumiere movement may be greater than 180 degrees, which is much more than any other mivement in common use. The shutter blade could be reduced to 120 degrees were it not for the fact that it must have an additional width sufficient to cover the aperture opening, so that the smallest shutter blade that can be used in any movement is that fraction of a revolution during which the film moves downward plus a segment wide enough to completely cover the aperture opening from corner to corner. The Pathe, Prevost, the Universal, the Gillon, and many other makes of cameras, use the harmonic cam. Almost all other movements are some variation of the rod and crank principle. That is, a rod, or other connection, fitted to a crank pin on the shutter shaft actuates the up and down movement of the claws. Since the downward movement of the crank is one half of a revolution, no rod and crank motion can have as wide a shutter opening as the harmonic cam. Some of them decrease the time in which the film is moved down by having a crank whose throw is greater than the distance from picture to picture, and use only a portion of the crank throw for drawing the film down, the engagement of the pins or claws witih the film taking place after the crank has commenced to move downward and releasing before the crank reaches the bottom of its throw. There are many variations of the rod and crank movement. In the Pittman model the fingers are upon springs actuated by a crank. The fingers move in a circular path except when drawing down the film, where they are forced to subtend a chord of the circle by a friction plate in the plane through which the film moves. This friction plate being struck 16 times per second by the revolving spring claws makes this movement a very noisy one. In the Williamson movement a small arcshaped slot guides the pins in an approximately straight line during the period of their engagement with the film. In other movements a double crank is used, giving both the in and out and up and down movement to the claws. A third variety of movement which was much used a few years ago was called the slip claw movement. In this movement the claws were ratchet-shaped and in their upward travel slipped along the perforation as a pawl slips over a ratchet. Faults with a Particular Movement The Pathe Freres formerly made an amateur model which used the slip claw movement. The slip claw movement has almost entirely gone out of use because it could not be reversed. No matter in which direction the crank of the camera was turned the slip claws would pull the film down in the same direction. An inadvertent throwing back of the crank, for even a fraction of a revolution, would cause the film to lose its upper loop. Unless there was a great nicety of adjustment between the friction at the gate and the pressure of the spring claws thev were liable to push the film backward on their upward travel, causing the frames to overlap, thereby making what is called a creep in the film. The in and out movement, or the movement of the pins in and out of the perforation, is accomplished in various cameras by many different methods. A positive in and out movement is much to be preferred over one which is accomplished by some sort of spring pressure. A positive in and out movement is one in which the pins are pressed in and withdrawn by a mechanical movement, such as a cam or drunken screw. In the Pathe or Gillon types the in and out movement is accomplished by a drunken screw. A drunken screw is a thread having an irregular pitch, the thread used for the in and out movement being a continuous one with the contours so placed as to force the pins into the perforations at the beginning of the downward throw of the cam and withdraw them at the bottom of the throw. In the Prevost movement the in and out throw of the pins is accomplished by small harmonic cams of the same design as the larger cam which produces the up and down movement. Most of the rod and crank types of movement have a cam working against a spring to push the fingers in and out, the cam pressing the fingers in and the spring pushing them out when released by the cam. It is possible to operate -a movement of this type so fast that the spring does not have an opportunity to withdraw the fingers quickly enough, thereby causing creeping and losing of the loop. The Ernemann camera has a rod and crank movement with cam and spring for the in and out finger movement. As nearly all parts of a camera movement, shift backward and forward 16 times per second they are subjected to a great deal of wear. All of these parts subject to wear must, of course, be kept constantly but lightly lubricated, and should be provided with means for adjustment so that there is the least possible amount of play between bearing surfaces. The finger shuttle, that is a frame bearing the fingers, which moves up and down, is carried in some sort of guides which should be provided with adjustable gibs for taking up wear. The shutter is the revolving blade which cuts off the light from the lens while the film is being moved downward for the next picture, or exposure. The circular revolving shutter is so universally used in motion picture cameras that it is almost unnecessary to take any other type into consideration. The shutter should consist of two blades, one of which is set immovably with a minimum area for keeping the aperture closed during the downward movement of the film. Another blade should be provided which may be adjusted so as to decrease the opening in the shutter by revolving it past the fixed blade, so that the opening may be entirely closed if necessary. While it is preferable to use the maximum opening of the shutter in most instances, there are many times when it is desirable, for various reasons, to cut down the exposure by means of the shutter opening instead of a smaller diaphragm opening. The Shutter Dissolve A means for decreasing the shutter opening while the camera is in operation is called a shutter dissolve. By its employment are obtained such effects as fade-outs, fade-ins, dissolves, etc. There are two types of shutter dissolve, the automatic and the hand oper(ConUnued on page 1716)