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156 MOTION PICTURE HANDBOOK
which is sharp all over, and if the machine be set much to one side it will be found practically impossible to get even a reasonably good picture. It is no unusual thing, however, to have a machine giving a fairly sharp definition all over the picture with a drop in projection of fully 40 feet in 100. Of course a portion of this difference in effect is accounted for by the fact that the picture is wider than it is high, but this does not seem to explain the whole thing, as a fairly sharp picture may be had with very steep downward pitch.
The keystone effect, so far as the outline of the picture is concerned, may be corrected by filling in the projector aperture with hard solder, and then carefully filing it out until the picture assumes its normal shape on the screen.
The best and in fact the only practical way to do this is to fill in with solder and file the aperture to shape when the light is on, first, however, having removed one of the condensing lenses so that the spot will be very large, since otherwise it will be too hot to work in. By this method you can watch the exact effect of every stroke of the file upon the outline at the screen. Be very careful that you do not get a little too much off, because if you do you will have to do the whole job over again. If the machine sets above the screen the filing will have to be done on the sides of the aperture, the lower part of the aperture being made widest. If the machine sets to one side of the screen then the top and bottom will have to be filled in. Before beginning, hang a narrow strip of black tape, weighted at its lower end, with its upper end just where the lower end of the upper corner bend comes. This will supply guides so that you will get the side lines perfectly straight and perpendicular. Bevel the sides of the aperture opening slightly on the screen side.
As before stated the outline of the picture can be corrected in this way, but the distortion of the picture will remain. That cannot possibly be corrected, except by setting the machine lens central up, down and sidewise, with the center of the screen.
The out-of-focus effect which accompanies keystone effect where the machine is set to one side of the center of the screen may, if it be not too great, be corrected by loosening the aperture plate and placing a thin strip of metal under one side, the idea being to slightly raise one side of the aperture plate, provided it be a type of machine which will allow of its gate being squared with the aperture in its new position. Up and down keystone effect can also be corrected by blocking the upper end of the aperture plate out somewhat ; but