Proceedings of the British Kinematograph Society (1933)

SLIDES Nos. 4 AND 5 The test on that bench is an essential part of the routine of all Cooke Kinematograph lenses. After passing the visual test on the bench, the lens is next submitted to photographical determination of chemical focus. One of the objects used is a board inclined at 45° to the axis of the lens. SLIDES Nos. 6 AND 7 This board carries ruled parallel lines at a fixed distance apart, and the lens is focussed on the centre one. By having the board sloping we can photograph at the same setting not only the plane focussed on, but a depth of several inches, both before and behind. If a lens is right for chemical focus, it will show quite sharply the line focussed on, and one or two lines both in front and behind, thus forming a measure of the depth . of field. This test is made with incandescent light, then with mercury vapour light, and lastly with carbon arc light. Panchromatic film is, of course, used in each case. Focussing is done by taking the lens in its adapter and fitting to a special focussing microscope magnifying about 20 diameters, the photograph being taken with an Eyemo camera. As we know the distance apart of the lines on the board, it is quite an easy matter to reduce to a numerical measure any errors shown by the photographs. This, of course, is essential for us. Ina test with a 3 in. lens, made at 8 ft. distance, if instead of the central line focussed on the next line to it is sharp, the error in position of the focal plane is .0003 in. From what I have told you, I think you will see that Cooke lenses are designed to give sharp results with incandescent lighting, and that in doing so the results with arc lights have not been sacrificed. At this stage it might be well to discuss future developments in kinematography. As before mentioned, the trend of optical developments for the cinema industry has been towards providing greater transmissions of light in optical systems, and there is every likelihood of the demand continuing. This is particularly so in the case of lenses for photography, as it is desirable from many points of view to have more speed. Larger aperture lenses would enable the studios to work with less light and, consequently less heat, to the advantage of the artistes and the disadvantage of the electric supply companies. After speaking with many camera-men, I feel more than ever the need for the development of large aperture lenses, or, alternatively, faster films. Most photographers are agreed that for close-ups and for frontal lighting generally the tungsten filament lighting gives the most pleasing results. Since it has been necessary to expose at 24 frames per second, and due to the fact that the incandescent light has a lower actinic value, there has resulted the natural tendency to over-lighting, with flat results. This is especially so in cases where a photographer is afraid of his large aperture, and always keeps his lens stopped down slightly. I can assure you that with some photographers this doubt still exists, and they still employ more lighting than necessary by not opening full out. I would like those photographers to know that Cooke lenses are tested at full aperture, and that there is no reason why they should be afraid of opening out and getting good definition unless, of course, they are requiring extreme depth of field. Unfortunately, questions of depth of focus often restrict the extent to which the optician can push up the aperture of an optical system. Cinema camera makers are helping now by providing an accurate means of focussing on the film in the gate, but this cannot entirely eliminate the difficulties which arise from lack of depth. These difficulties are very real from both the camera-man’s and producer’s points of view. They were when the F/2 lenses replaced the F/3.5’s, but by realising the difficulties they were overcome. A development in which the restriction due to lack of depth with increased light transmission is notso serious is an optical system for sound-on-film recording or reproduction. Here the optical system works at a fixed focus, simplifying the problem. One of the latest developments which has found practical application is the introduction of super-speed panchromatic film. This together with the re-introduction of the arc lamp, has merely postponed the demand for abnormal aperture lenses—at 7