International projectionist (Jan 1959-Dec 1960)

else. Unless the added light is properly directed, we might just as well increase screen illumination hy turning on the theater lights. (3) Uniformity of Screen Illumination. Uniformity of illumination over the projected picture area is dependent upon the total vignetting which occurs both in the projection lens and in the illumination system. Increasing the speed of the projection lens alone will not produce a uniform increase in illumination over the whole picture area. To accomplish a uniform increase may require a change in the illumination syslem which the manufacturer is unable to make. (4) Depth of Focus. We shall use the term depth of focus in its usually accepted sense of being the distance along the lens axis through which an object can be moved forward or backward without causing a detectable change in focus of the image of that object. Its practical importance is that it is indicative of how much film flutter or The GENARCO ELECTRIC SLIDE-CHANGER for 70 slides, VA" x 4", can now be mounted on ALL TV slide projectors ( Bodde, Transitu, etc. ) . Demonstration or literature from: GENARCO, INC 97-00B SUTPHIN BLVD. JAMAICA 35, NEW YORK buckling can be tolerated in a given projection system. Thus: Depth of Focus = ±0.000043 X (F/number)2 inch For comparison purpose, we list the depth of focus for several lens F/numbers. F /Number ±Depth of Focus, inch 2.5 0.00027 2.0 0.00017 1.8 0.00014 1.6 0.00011 1.4 0.000084 We note that going from F/2.5 to F/1.8 effectively cuts the depth of focus in half, and that at F/1.6 the depth of focus is reduced to just about five wavelengths of light. (5) Alignment of the Projection Lens With the Film Plane. It follows that as the depth of focus decreases with increased projection-lens speed, the alignment of the optical axis of the projection lens perpendicular to the plane of the film gate becomes increasingly critical. (6) Speed of the Illumination System. This, too, becomes more complex with increasing projection-lens speeds, as will he illustrated further along. Lens Only One Component These are but a few of the many factors which it must be recognized influence overall performance and which are affected by the choice of the projection-lens speed. Considering the fact that the projection lens is but one component of the projection system, it is quite apparent that a list of all the known factors of all the components contributing to the overall performance will be quite an imposing one. Moreover, it is quite apparent that the interdependence of the performance of one component upon the performance of one or more of the other components necessitates the evaluation of each component with respect to its performance in the system, rather than its performance as an entity. With these problems in mind, how shall the projectionist make his choice? Consistent Maximum Light of Greater Economy DIAMOND PROJECTION We have said, for example, that we can increase screen brightness at the expense of image contrast, but what should the compromise be? The same question applies to brightness and resolution. We have shown that depth of focus decreases with increased lens speeds. From experience we know that we cannot, nor do we have to, focus the projection lens to quite the precision indicated by the theoretical values, but we do not know at what point depth of focus does become the limiting factor in image quality. Diverse Factors Involved We know that we can change the brightness distribution on the screen by varying the speed of the illumination system; but do we know what the relationship should be between the projection-lens speed and the illumination-system speed from an overall consideration of brightness distribution, film buckling, cost and convenience of size of the lamphouse? How can we get the answers to these problems? Certainly they will not come from the manufacturers involved because the manufacturers have little or no incentive to solve the problems. Piecemeal advances in projection equipment will, of course, continue to be made, as for example, the recent introduction of the "cold" mirror. But even in this case, maximum utilization of the cold mirrors requires certain changes in the lamphouse over which the cold-mirror manufacturer has no control. It is the fact that the optimum designs 70 PINE STREET • NEW YORK 5, N. Y Wanted by CINEF0NES, BOMBAY (INDIA): Theatre Sound Engineer with good technical qualifications and thoroughly experienced in the installation, repairing and servicing of all types of theatre projection and sound equipment. Administrative experience essential. Send complete resume airmail to CINEFONES 3, NEW QUEEN'S RD., BOMBAY 4, INDIA 24 INTERNATIONAL PROJECTIONIST MARCH 1959