International projectionist (Jan-Dec 1947)

New Lens Calibration System Developed AN IMPROVED system for calibrating diaphragm openings of photographic lenses has been developed by Dr. Irvine C. Gardner of the National Bureau of Standards. This new method of marking apertures takes into account the losses of light from absorption, reflection and scattering within the lens, thus permitting more precise and accurate control of exposure light. The method now in use is based entirely upon the ratio of the equivalent focal length of the lens to the diameter of the aperture. This ratio, known as the /-number, does not take into consideration the great differences in the useful light transmitted by various lenses. The introduction of reflection-reducing coatings has accentuated this performance variation to such an extent that for a given /-number the light transmitted by two lenses may differ by a factor of two. In the new calibration method, the lens marking does not correspond to an aperture ratio such as 1:8, but rather to an opening large enough to permit the transmission of as much light as would be transmitted by a lens set at 1:8 aperture, if there were no losses. Such a system may be referred to as equivalent or compensated apertures. A standard designation for the new system has not yet been decided, but suggested terms are: effective /-number or r-number (f-transmission). The usefulness of this method is evident from the following example. A lens of large relative aperture and no coating may have eight surfaces in which case the transmission may be as low as 66 per cent. Yet, a lens with low-reflection coatings and few surfaces, may transmit FIGURE 1. AB represents uniformly bright surface; CD, a metal plate with small hole at O' EF a second plate with larger opening. Shaded area indicates cone of light proceeding from screen AB through both apertures, falling upon photocell receiver, R. If opening at EF is so chosen that sine of half the angle of the shaded cone is Vs, photocell receives same light through apertures in both plates as if plate EF were removed and an ideal lens with an f:4 aperture setting were substituted FIGURE 2. Plate EF (in Fig. 1) is substituted by f:4 lens, decreasing intensity of light because of absorption, reflection and scattering. If iris diaphragm of lens is adjusted to increase cone of light as indicated by dashes, the same photocell readings can be obtained as for the opening in the diagram shown in Fig. 1. Lens setting then corresponds to a r-number of 4. Other values may be determined by varying the size of opening in plate EF. 98 per cent of the incident light. If exposures are made with the two lenses, for the same length of time, at the same /-number, the effective exposure will actually differ 50 per cent. However, if the diaphragms are marked according to the proposed system, the exposures will be identical. In the accompanying diagrams, the basic photometric principle of the system and its application in actual photography are shown. Involving only a matching of two photometer readings, the new system is simple, direct and accurate. As each calibration is essentially a substitution procedure in which the two values of brightness are approximately the same, errors arising from non-linearity of response of the photometric apparatus are largely eliminated. The need for carefully calibrated filters is also avoided and a means is provided by which different laboratories may arrive at the same calibration values without the interchange of physical standards, since apertures in metal plates, the only standard required, can be readily constructed to the required sizes. PERSONNEL H. Kulhow has been named vice-president in charge of manufacturing for Ampro Corp. of Chicago, 111. Kulhow formerly was associated with General Precision Equipment Corp., which now controls Ampro. Samuel E. Ewing, Jr., prominent Philadelphia lawyer has joined the legal department of RCA Victor. Ewing will supervise legal matters for the Engineering Products Department. Formerly a member of the law firm of Saul, Ewing, Remick and Saul, he is a graduate of Princeton University and Penn Law School. He rose from private to major while in service, on the staff of Gen. Omar Bradley. John K. West, formerly director of public relations for RCA Victor, has been appointed vice-president in charge of public relations. A native of Charleston, W. Va., he attended Wesleyan University and has been associated with RCA since 1930. Two of his "pet" projects have been the RCA exhibit at the Chicago World's Fair and the RCA Exhibition Hall at Radio City, N. Y. West was recently cited by the Navy Department for services in connection with the 1947 reserve recruiting campaign. * * * R. E. Warn, former managing director of Western Electric Company, Pty., Ltd. in Australia, has been named chief engineer, of Westrex, succeeding G. S. Applegate, recently deceased. Warn, with Western Electric over 20 years, has been "down under" since 1936 as manager of the company's Australian and New Zealand subsidiaries. He will be replaced in Australia by W. S. Tower, Jr., former Westrex commercial manager, who has been supervising the company's Caribbean operations. 34 INTERNATIONAL PROJECTIONIST December 1947