Amateur Cine World (November 1961)

Making a Start / BY H. A. POSTLETHWAITE I HAVE MENTIONED already how the functions of a camera and a projector are in many ways similar. Film goes through the projector with the emulsion (dull) side towards the lens (except copies of reversal originals) and with the image upside down. This is, of course, the same way it went through the camera when the film was made. With &mm. film the perforations face you as you thread up because, of course, the sprockets which engage them are also so placed. How to thread up the film will probably be indicated on the projector or clearly shown in an accompanying diagram. In either event it is a simple operation. With some projectors threading is automatic: the end of the film is inserted in a slot and the motor switched on. It only then remains to take the film on to the take-up spool. This is undoubtedly a time-saver if you are going to project a number of short lengths of film, but with most projectors threading by hand presents no difficulty. The sprocket wheels which engage the film perforations above and below the gate regulate the speed at which the film goes forward. They are similar to the sprocket wheels in 16mm. cameras and in a few 8mm. models. Little guide rollers may be provided to keep the film in position as it passes over the sprockets. Between the upper sprocket wheel and the gate, the film should go up in a small loop, and a second loop is made between the gate and the lower sprocket wheel. This enables the film to lie flat in the gate channel to offer minimum resistance to the claw which pulls it forward. You will recognise this as an important factor as soon as you have a projector and start threading up. It is essential that the loops “stay put” during projection. If a loop is “lost”? and the film is consequently pulled tight between a THE PROJECTOR: Film Transport, Lens sprocket wheel and the gate, one part of the film transport system will attempt to pull the film onwards faster than the other parts are mechanically able to let it go, with consequent damage to the film. To make sure the perforations are properly engaged, the film should be given a gentle tug as it is placed over each sprocket wheel. You may also ascertain this quite easily with the naked eye, but do not consider this quite good enough when you sstart.) Then, with the gate closed, the film should be advanced a few frames by means of the knob provided for this purpose— the inching knob. Or, when you have gained experience and confidence, this check may be made by switching the motor on and off quickly. If either loop increases or decreases in size, the threading must be done again. If a loop should be lost while a film is being shown, the picture on the screen will jump about or flicker and the projector make strange noises. It must be stopped at once and the cause ascertained or you may find your film is getting damaged. If both film and projector are in good condition, however, loops are seldom lost. The most common cause is a torn perforation, or slipping, because the teeth of a sprocket wheel are not properly engaged in the perforations. But the fault could occur if the claw were out of adjustment or if the tension of the take-up too strong, so that it pulled the film along faster than the sprocket wheel could pass it. The latter faults require expert attention. In a type of projector which provides for rewinding the film by attaching a belt to the feed spindle, the upper loop might be lost if this belt were left on the spindle when projecting. All the foregoing contingencies, however, are. rare and the threading up of a film on a modern projector is simplicity itself, After passing over the lower sprocket wheel, the film is led to the take-up reel, and in most 8mm. projectors it may pass under two or three rollers on the way. The spindle which holds the take-up reel may be driven by a belt or by a series of geared wheels. In either case it is fitted with a slipping clutch, because the speed at. which the reel winds up the film decreases aS successive quantities are wound on. At first the reel will take up only a few inches of film with each revolution, and must therefore turn fairly rapidly. When the reel is nearly full, obviously the amount to be wound on may be fifteen or twenty inches per revolution, and the reel must consequently turn more slowly.