International projectionist (Jan-Dec 1947)

rear surface of the lens to the focal point in parallel light [B.F.] the equivalent focal length [E.F.] of a lens is defined as the distance from the second nodal point (cardinal point) to the second principal focal point [see Fig. 5]. We are all familiar with the fact that even with infinitely distant objects the image size is proportional to the focal length ot the lens used. The 50-min lens on miniature cameras will yield an image one-third the size of that formed by a 6-inch lens. Now, it is a most unfortunate fact that the cardinal points have positions dependent on the refractive index of the glass comprising the lens. Expressed in other words, each color has its own cardinal points which can be distinct from those of any other color. Now, if the lens is color-corrected axially, i.e., if the red and blue colors unite in one focal point on the axis, and if the nodal points in the two colors are different, automatically the lens must have different focal lengths in the two colors. This is shown in Fig. 6. However, these different focal lengths, as we saw previously, mean differently sized images: that is to say, the magnification will be different, depending on the color [see Fig. 7]. It is for this reason that this second type of chromatism is spoken of as chromatic difference of magnification, or lateral color. Lateral Color Aberration In a lens afflicted with this aberration the points in the margin of field are drawn into spectra, or rainbows. This particular aberration is most annoying even with black-and-white film, for it quickly sets the limit of usable field by producing a graded soft focus near the margins. In the correction of this aberration, advantage is taken of the fact that some lens systems will give a spectrum with the red end closer to the axis; while others will give a rainbow oriented the other way around. In the correction of lateral color, then, systems of these opposing characteristics are balanced against each other until a satisfactory compromise is found. We have now seen the two types of pure chromatic aberration, longitudinal chromatic aberration, and chromatic difference of magnification. The former, the simpler type, is manifested by the absence of one single focal point for all colors, with the existence of a distant point for each separate color. This is a very troublesome aberration and is corrected in all but the most inexpensive lenses. These lenses can be spoken of as being "color-corrected" only to the extent that there is little or no shift of focal point with color. This aberration is in theory independent of the aperture of the lens, though in practice its effects will be mitigated by stopping-down, for then the effective depth of focus is increased and the image is less sensitive to improper focus. Longitudinal chromatic aberration is corrected by combining a positive lens with a negative lens of higher refractive index and dispersion and of such power that the dispersion of the positive lens is annulled, but its convergence not cancelled, so that a union of the red and blue colors occurs on the axis. The second chromatic aberration is more complex and manifests itself in differently sized images in different colors. This chromatic difference of magnification is troublesome even in blackand-white photography, and the best lenses are corrected for this aberration. Only the photographic objectives designed with this aberration pared to tolerable limits can be spoken of as "fully color-corrected." This aberration is proportional to the image height, thus becoming worse toward the corners of the picture. Stopping-down the lens is without effect on this aberration. It can be corrected by employing combinations of elements of opposing tendencies, so that one set cancels the effects of the preceding. The corrected lenses of the reputable makers have all been designed with this aberration in mind and can justly be spoken of as being fully color-corrected. However, it is well to remember that nothing perfect is manufactured, and that with the most refined tests it might FIGURE 7 RED BORDER be possible to demonstrate some lateral color with the most nearly perfect lens made. This would be without significance, however, since the test necessarily would be artificial and would not correspond to the conditions of use of the lens. If the longitudinal chromatic aberration has been corrected, and if the lateral aberration cannot be detected on the film, then for all practical purposes the lens is "fully corrected." Full color-correction is as essential in the best photography and projection with black-and-white emulsions as it is with color film. The effect of lateral color in the case of the former is to create a soft focus effect toward the margins, while with color film registration difficulties are the consequence of this aberration. Color photography and projection has introduced no new element into the design of good objectives. With color film the color aberrations, particularly lateral color, become visible as color defects. The best lenses have always been fully color-corrected. Greatly Expanded SMPE Program Results in Dues Rise To provide finances for many new and essential projects covering technical problems of production, distribution and exhibition, the Society of Motion Picture Engineers is launching a campaign for additional revenue through slight increases in membership dues and subscription to the monthly Journal. Special effort also will be made to obtain a more equitable portion of financial and personnel support from the motion picture industry proper — the producers, distributors and exhibitors — which at present is contributing only about 25% of the dues collected by the S.M.P.E. although it derives the greatest benefits from the research, technical developments, standardization activities and other work done by the engineers. Though it costs the Society an average of $14.27 per member-subscriber to operate each year, dues for Associate Members, who make up the largest body, have been only 17.50 a year. The Society also has a Student Membership, for registered students in engineering, which has been available for $3 a year. All members receive a copy of the monthly Journal, which in itself costs $4.61 per member annually to publish and mail. Starting January 1, 1947, dues for Associate Members will be $10 annually, while student memberships will cost $5. Subscription to the Journal, for non-members, is being increased from $8 to $10 annually, with single copies costing $1.25 instead of $1. With a present membership close to 2,500, of which nearly 500 joined in the past year alone, it will be necessary to obtain additional support from the industry in order to carry out the Society's plans for greater service to the industry. RED a BLUE AXIAL POINTS COINCIDING 7 N2 FOR RED LIGHT Natco South American Distributor Natco 16-mm sound projection equipment will be handled in Central and South American territories exclusively by Bellows, Schausser & Co., Inc., of N. Y. City. Personal supervision by Robert Schausser. 12 INTERNATIONAL PROJECTIONIST • January 1947