Owing to the number of variables to be taken into account in calculating the exposure time in aerial photography, an attempt was made to devise apparatus to determine the optimum exposure time automatically. It was found that ordinary, commercial photo‐electric exposure indicators gave results favourable only for oblique views from less than 2,000 m. A series of tests was carried out to determine the optimum exposure time as a function of all the conditions: emulsion sensitivity, aperture, altitude, light and atmospheric conditions. Four Lcica cameras, mounted in one unit so that any factor could be varied, were used to collect data over a period of twelve months. Based on this experience, a simple but robust, rotating indicator of the card type, constructed of two metal discs, was made and its construction, graduation and method of use arc described. Even with this indicator, successful photography is still too much dependent on the accuracy of the photographer's estimation of the light value, atmospheric purity, mist intensity, etc., and a more reliable exposure meter is described, consisting of a photo‐electric cell, galvanometer, variable resistance and a system of disc indicators The principle is as follows: the photo‐electric cell, of which the field is limited to that of the photographic lens emits a current which is a function of the luminosity of the subject to be photographed. This current causes the galvanometer needle to be deflected. The needle is brought back to a suitably selected graduation by turning one of the indicator discs which turns the cursor of the variable resistance in the photo‐electric cell—galvanometer circuit. The combinations of exposure period and aperture can then be read off. This apparatus and its components are described in detail, together with their graduation and method of use. A variation is also discussed for the case when it is desired to mount the indicator directly on the camera. A table gives correction factors for atmospheric conditions and altitude.
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