Motion picture handbook; a guide for managers and operators of motion picture theatres ([c1916])

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118 MOTION PICTURE HANDBOOK rear condensing lens it not exactly square or in line with the front one does not make any serious difference, provided, of course, that the fault be not too great. I do not wish to be understood as saying that this condition ought to be allowed to obtain. The better practice is to have the entire lens system in exact line, but with present projector mounts this is ,a somewhat diffcult thing to accomplish, and failure to accomplish the lining of the two condenser factors perfectly with each other will not be a very serious matter. Another extremely important relation between the condensing lens and the objective is illustrated in Plate 4, in which A represents the extreme limit of light from the lower edge of the condensing lens when it is placed 16 inches from the aperture of the machine. You will observe that with the condenser at a distance from the aperture which will place the arc in focus (the point where the condenser ray begins to diverge), which is the point where the picture will receive evenly distributed illumination, the light will pass through the aperture and become a diverging beam. This is clearly shown in Plate 5, which shows the light beam as in actual projection, and is proven in Plate 6, in which the condenser is covered by a metal plate in which are two holes located diametrically opposite each other and about a half-inch from the edge of the lens. It will be seen from Plate 6 that the rays from the outer edge of the condenser lens actually do act precisely as indicated in diagram, Plate 4. In Plate 7 the same two rays are passed on through the objective lens. From this the inevitable conclusion is reached that, with the crater in focus at the aperture, the closer the condenser is to the aperture the more rapid will be the divergence of the beam beyond the aperture, though the increase from this will be comparativetly slight. It will also be seen that the greater the distance from the aperture plate to the objective lens aperture the wider the light beam will be at the point it encounters the lens, See Plate 8. It therefore is an undoubted fact that the diameter of the objective lens is an exceedingly important factor, particularly with long focal length lenses, and it is a factor which must be taken into very serious account in the matching up of projector lens systems. Plate 9 shows the loss of light through using a lens of too small diameter. This loss may be slight; or it may be very great. In many cases it is the latter. In this case the loss is far greater than appears, because the camera only caught the loss which fell outside the lens barrel, whereas