British Kinematography (1950)

162 BRITISH KINEMATOGRAPHY Vol. 17, No. 5 which would give the same angle, or throw, (projector gate 3 x 2.2 ins.; 35 mm. camera sound aperture 22 x 16 mm.). Each lens is a highly-corrected anastigmat, and all surfaces are " bloomed " to reduce reflection losses. When projecting rectangular line test charts it has not been possible to detect any distortion of the picture. Quality of Image. The flatness of field and resolution of these lenses has been measured in accordance with the British and American Standards methods. High-contrast Bureau of Standards line test charts were photographed so that there was a chart at the centre of the slide, and others at 5 per cent, of the distance to the centre inwards from the edges and corners, and this negative was projected. First, the best focus for the centre chart was obtained and the resolution observed visually. Secondly, the best overall focus was obtained and the centre resolution measured, and also the tangential and radial resolutions at the corners. The figures are recorded in Table 2, in terms of lines per mm. on the slide. Consideration of these results shows that the definition is in all cases more than adequate. The best figure achieved with a normal motion picture camera is about 50 lines per mm. in the centre of the field, or on a 22 mm. frame the equivalent of about 1,100 lines in the picture width (in practice, of course, it is not as good as this because the definition falls off towards the edges of the picture). The slide is 3 ins. across or 75 mm., so that the number of resolvable lines in the projected picture will be, in most instances, more than 2,000 — or twice as many as the camera can resolve. This in turn means that one can " blow up " about one-quarter of the slide area to fill the camera gate without losing much definition, and since under normal conditions the camera is never focused sharply on the screen, the factor is even larger than these figures indicate. Under no normal operating conditions need one anticipate any falling-ofT in definition caused by this projector, when making process shots. These tests were made with a slide 3 x 2.2 ins., but it was noted that with a slide 3x3 ins. there was no significant reduction in corner definition, except slightly in the case of the 6.4-in. lens. TABLE I—] hocAL Length of Lenses Focal length, ins. ... 6.4 8 10 12.5 14 16 18 22 Aperture,// ... 2 2 2 4.5 4.5 4.5 4.5 5.6 Equivalent camera lens, mm. 38 48 60 75 84 96 108 132 TABLE : . — Resolution Best overall focus Best Centre ins. Lens Corners Resolution Centre Tangential Radial 6.4 fa 56 28 28 40 8 m 40 20 20 28 10 f/2 40 28 28 28 12.5 //4.5 40 40 40 40 16 //4.5 40 40 40 40 22 /75.6 40 40 28 40