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258 HANDBOOK OF PROJECTION FOR
center of which is a hole about tyi of an inch in diameter Next hang cords at the screen so that they will pass down exactly over the marks you have made at the bottom of the upper corner curves. Now strike an arc, and, with the light through the hole in the plate over the cone to guide you and show you the exact effect of every move, file out the aperture sides until the light comes exactly to the lines on the screen.
The light enables the worker to watch the exact effect of every stroke of the file. He is thus enabled to do a very accurate job. It is necessary to be extremely careful in filing because if you get a bit too much metal off at any stroke of the file it means the job must all be done over again.
Keystone effect is invariably accompanied by a greater or less tendency to out of focus. This is especially true if it be side keystone, since the picture is wider than it is high. It is caused by the fact that a projection lens is presumed to focus at a given distance, (Fig. 36 D), and since with distortion of the kind we have described the distance to the screen varies, it will readily be seen that a strain is placed on the powers of the projection lens in the matter of focusing. The lens may be given increased depth of focus, or in other words may be caused to focus over a greater distance by reducing its diameter, and this is why large diameter objectives are a very hard proposition to handle where there is any decided tendency to distortion in the way of keystone effect. If the projectionist is working under conditions of heavy distortion or keystone and is unable to get a sharp focus all over his picture, let him try stopping down the diameter of his projection lens by inserting a ring of black cardboard in the front end of the lens barrel, right up against the front factor of the projection lens. In the center of this cardboard cut out a circle say one inch in diameter. If this sharpens the picture, then he can know where the trouble lies and can increase the size of the hole in the stop until the trouble again appears, after which a new metal disc with an opening just a little bit smaller will serve the purpose. We know of no other means of remedying such a condition, so long as the distortion remains, and the remedy we have suggested may be quite expensive in light, therefore it is a waste of electric current.
CHARACTERISTICS OF SCREEN SURFACES.— The following data is extracted from a paper presented to the Society of Motion Picture Engineers by Lloyd A. Jones and