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made as light as is compatible with color rendering, and the whole aperture of the lens can be utilized. In a number of patented methods of taking the successive pictures, the light from the lens is split up into several parts, so that two or three pictures are taken simultaneously through the one lens. Assuming that any such process is perfect mechanically and optically, the loss of light entailed by using only one lens for several pictures is a very serious objection. In some cases where a complex reflecting system is used, it is probable that the loss of light will be so great that the method will be quite impracticable with any materials at present in sight.
When the pictures are taken successively through a single lens instead of simultaneously through two or three lenses, we have the disadvantage that the exposure must necessarily be shorter because the two pictures must be taken in the same period as that in which the one picture is taken in the other set. Sometimes an attempt is made to lessen this disadvantage by using very light filters, somewhat to the disadvantage of the resulting picture. Such light filters cannot be recommended as they are invariably detrimental to the color rendering.
The Exposure necessary for outdoor work may be worked out as follows : The exposure necessary for the production of a fully exposed picture under conditions of midsummer noonday lighting with an aperture of /:3.5, which is the largest aperture generally used for cinematographic work and negative film of the highest speed which can consistently be made, is approximately l-300th of a second. The best sensitizing which can be obtained at the present time gives an increase of exposure through the red filter of six times. The other filters can be adjusted to the red filter exposure. About l-50th of a second is therefore the shortest exposure with which we may hope to get fully exposed negatives, so that if we have a camera taking fifteen pictures a second, we should have no difficulty in making exposures in full summer sunlight in one-third of the time required for the shift of each picture, thus enabling us to take, for instance, two pictures in that time. It will be seen, however, that there is not very much margin in exposure, and that we cannot expect to take color photographs under other conditions than those of first-rate outdoor lighting.
When we turn to stage work, the conditions are somewhat different, because the amount of light which we can concentrate on the stage is limited mainly by the amount of electricity we are willing to use, though a second limitation must necessarily arise in the inability of the actors to face the light of such powerful stages. It may be reckoned that under the best conditions the light required for color work is twice or three times that required for black-andwhite motion-picture photography. For color work, naturally, the lamps must give white light, that is, they must be arcs or nitrogen tungsten lamps, the use of mercury vapor lamps being impossible. It may be considered that with suitably arranged stages the taking of dramas by artificial light in color is entirely possible.
2. Quality in the Negative. In most additive and subtr active processes the photographic quality of the resulting picture can
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