. The American annual of photography. hey will merely needtwice as much light, but are soon disillusioned. As demon-strated in Figure 4, if the distance is doubled to get a pictureof twice the diameter, the same amount of light has to bespread over four times the former area, that is to say, anilluminant four times as powerful will be needed to get anequivalent lighting on the screen. A picture three times thediameter would require nine times as much light; one fourtimes the diameter would call for sixteen times the illumination,and so on. This law applies also when a lens of shorter focusis u
. The American annual of photography. hey will merely needtwice as much light, but are soon disillusioned. As demon-strated in Figure 4, if the distance is doubled to get a pictureof twice the diameter, the same amount of light has to bespread over four times the former area, that is to say, anilluminant four times as powerful will be needed to get anequivalent lighting on the screen. A picture three times thediameter would require nine times as much light; one fourtimes the diameter would call for sixteen times the illumination,and so on. This law applies also when a lens of shorter focusis used to obtain a bigger picture without taking the projectorfarther back. Cinema operators often involuntarily cover a larger screenthan their light justifies, simply because the hall or room islong and their objective is of short focus. The remedy, ofcourse, is to obtain a longer focus objective, and thus get asmaller and better-lit picture in spite of the extra is no necessity to have such an enormous screen as one 84. WESTWARD HO! THOMAS CARLYLE. 85 sometimes comes across. We do not see gigantic players onthe real stage; why, then, turn them into sons and daughtersof Anak on the simulated one? Yet another cause of poorly-lit cinema pictures is the useof a cheap small-aperture objective. Economy in this direc-tion is always bad policy, for the larger aperture of the better-grade objective will not only allow more light to pass, but thehigher quality of the lens is sure to signify improved defini-tion. Some of the less expensive home cinematographs andattachments will be found to give surprisingly better resultswith a change of objective. t»II ?-+1-
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