International projectionist (Oct 1931-Sept 1933)

20 INTERNATIONAL PROJECTIONIST March 1932 Table A (Based on I . & L. Cinephor Lenses) Type Speed Focus Estimated Aperture Actual Aperture Series 0 f/3.5 5" 5/3.5 gives 1 3/7" 1 13/32" . Series I f/3 5" 5/3 " 1 2/3" 1 23/32" Series II f/2.4 5" 5/2.4 " 2 1/12" 2 3/32" Super Cinephor f/2.3 5" 5/2.3 " 2 4/23" 2 3/16" phor condensers have narrowed considerably the field of individual preference for projection lenses. The method generally used in the past in introducing newly developed projection lenses has been to try one out in a projector, with practically no consideration being given to the fact that the old lens is dirty and the new one clean. Naturally the new lens shows up well, by comparison, and sometimes the house buys it. I hope that this is no longer the practice here (Cleveland) , as an actual screen test is the only sensible method in which to determine the value of any projection lens. It is very important that we understand the "language" of lenses, in order that we may take advantage of the information on lenses which is disseminated through various channels. Much valuable information escapes that man who is not fully conversant with the "language" of lenses. The 'Working Diameter' One point often referred to is the "free aperture," or "working diameter" of a lens. Lens manufacturers endeavor to increase this aperture as much as pos^ sible. It might seem that in order to get a larger aperture all that is necessary is to make the glasses of larger diameter. While this is so, it must be remembered thatfrom the standpoint of the manufacturer every 1/16 inch added to the diameter of the lens very greatly increases the difficulty of securing a clear, sharply-focused picture over the entire screen. The advantage of the larger aperture is that it permits the passage through it of more light. The photographers among you understand the subject, but from experience 1 know that sometimes you do not correctly apply your photographic experience to a consideration of projection lenses. The speed of a camera lens is commonly rated as a fraction or ratio — the focal length divided by the so-called f-number, the quotient being the working diameter of the lens or of the diaphragm, if there is one. The smaller the opening in the diaphragm (that is, the larger the f-number), the clearer the picture and the greater the depth of focus, but the longer the time of exposure lequired. The larger the opening (that is, the smaller the f-number), the more difficult it is to build a lens that will give a clear picture all over, but the less time required for exposure. Photographic lens have now been developed rated at f/4.5, f/3.5, f/2, f/1.7 and even less, according to some manufacturers' claims. When we pass f/1.5, however, we may be pardoned for having a doubt as to accuracy of such claims. As noted previously, the projection lens started out on the Petzval formula with a claim of f/3.5 for portrait work. Most manufacturers of projection lenses did not claim better than f/4, however, and this size came toi be known as a "quartersize lens." It is possible that some of you have not known where that term originated. An f/4, five-inch focus lens would be 1^ inches in diameter, which you will remember to be about the size of the old quarter-size lens. Speed of Lenses Half-size projection lenses began to come in, that is, lenses rated at f/2, but in this case the f/2 was theory only, for it, is. only within the last year or so that even an f/2i5 lens of good quality has been put on the market; and f/2 is still in the distance. Lenses have now advanced in development beyond the projector, for in many cases better lenses could be provided if there was room for them in the projectors. This deficiency will be compensated for, we hope, in the newly developed projectors. In Table A is a representation of the dimensions of the various series of Cinephor lenses that are available, showing the free aperture and how it works out on the basis of the f-number. The Cinephor series is used because it is the only one on which I have aperture measurements. You will note the steady progression from f/3.5 to f/3, f/2.4 and now f/2.3 for the S'uper-Cinephor. I have no information as to the construction of the latter type, rated at f/2.3, but I have no doubt that it is a modification of the Petzval formula which permits of correction for spherical and chromatic aberration and for astigmatism. We touched briefly on these corrections in our talk last month. Referring to the illustration (Fig. 9), you will remember that our problem is to get s and t to focus together to correct for astigmatism, and also to get the two of them to lay back flat upon the screen to get flatness of field. In Figure 10 are the curves representing the results of the corrections. In curve C, s and t stay close together, but the field is badly curved. Curve B shows an earlier Cinephor in which 5 and t are kept. close together up to 7° with a fairly flat field. Anastigmats, or lenses which are corrected for astigmatism, are coiij[Please turn to page 29) EXCITER LAMP DEPRECIATION R. E. Farnham COMMERCIAL ENGINEER, GENERAL ELECTRIC COMPANY DEAR Mr. Fi];in: This refers to your letter of recent date inquiring as to the candlepower depreciation of the G.E. photocell exciter lamps. I should first like to correct one statement in your letter regarding the life of these lamps. Both the 8V2-volt, 4-ampere lamp designed for the Western Electric System, sound would be a slight reduction in sound volume necessitating advancing the gain control perhaps 1 point. There will be no effect, of course, on the quality of the sound due to bulb blackening. However, we do find that in some instances the filament coil sags slightly causing an uneven illumination of the small aper and the 5-ampere, 10-volt lamp designed ^^^^ ^^^^^^ ^f ^j^^ ^p^j^^i ^^^^^^^ ^his, for the Photophone System, as well as the 7%-ampere, 10-volt lamp formerly standard for the Photophone, have been designed for an average life of 50 hours when operated at their rated amperes, instead of 100 hours as mentioned in your letter. Life tests of these lamps show that the candlepower drops about 10 per cent in the 50-hours life. This depreciation is due to a tungsten deposit on the inside of the bulb coming from the filament. The effect of this blackening on the of course, would result in a considerable reduction in sound volume and undoubtedly an effect on sound quality, particularly where variable area sound films are used. However, the light sources of all three lamps mentioned have been so designed as to produce a somewhat wider solid band of light than called for by the optical system. Hence, some little sagging could take place before any effect on sound quality would be produced.