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MOTION PICTURE PHOTO G R A P H Y
phragm aperture suitable for the given exposure. Other scales show the distances that will be in focus with the different stops used, arranged so that the focal depth of four different lenses can be found. Several other exposure meters are made on the principle of the slide rule, with scale corresponding to the factors of "plate speed," "diaphragm number," "light," "exposure," and the exposure is found by simple inspection without an actinometer. They are designed for use with particular brands of plates, but can be used for others of similar speeds.
The last types of meters described depend for their light measurement upon matching a tint or shade, a rather difficult matter for most persons. A new instrument based on the same principle, but which does not require the tint to be matched, is the Steadman Aabameter. It may be obtained from any photo supply dealer. It consists of a series of graduated openings which give a ratio of exposure upon a strip of sensitized paper in the progression of i, 2, 4, 6, 8, 16. The number of gradations recorded in a given time gives the light strength and reference to a simple chart tabulated on a card, and gives the proper exposure at a glance.
Another class of exposure meters comprises those in which the intensity of the light is estimated visually by extinction through a semi-transparent medium of increasing intensity, such as J. Decoudin's, in which the exposure is judged by the disappearance of a series of small clear openings on a graduated scale of densities when laid on the most important part of the image as seen on the ground-glass. Its indications are not very definite, and the proper scale changes in density after a time. A better form is "E. Degen's Normal Photometer," consisting of two sliding violet glass prisms, one adjusted for the diaphragm ap^ertures, the other for the actinic illumination of the object. They are mounted with their outer faces parallel.
In use the upper slide with prism is drawn out so that the pointer coincides with the division indicating the diaphragm aperture to be used: the object to be photographed is then viewed directly through openings at one end of the instrument, and the lower slide is drawn out and pushed back slowly till the object viewed is almost obscured. The attached pointer will then indicate the exposure required, or, reversing the order, the diaphragm aperture for a given exposure can be found. Auxiliary
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