File:The American annual of photography (1922) (14595037969).jpg

Identifier: americanannualof36newy (find matches)
Title: The American annual of photography
Year: 1922 (1920s)
Authors:
Subjects: Photography
Publisher: New York : Tennant and Ward
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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the total mass of silver comprising that portion. It can bemeasured by chemical analysis but much more readily by anoptical instrument by comparison with a standard density. Inthe case of an average amateur negative the density of theshadows will average 0.25 to o 4 and the density of the high-lights from 1.2 to 1.8. One way of comparing the behavior oftwo developers is to expose two films in a camera on the samesubject, giving the same exposure, develop them side by side andcompare the densities of corresponding areas of the shadows,middle tones and high lights of the resultant negatives. Thismethod is not entirely satisfactory because very few areas ofuniform densityare large enough to permit of measurement andit is not possible to place any two areas which received the sameexposure side by side so as to compare the densities visually. (3) See How to Prepare Photographic Solutions, by J. I. Crabtree (Brit.Jour. Phot., 1919, p. 365); also, Chemical Fog, (Amer. Ann., 1919, p. 20) t88
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■^.^■Ci^*- *-i F. A. CHURCHILL.Artatone Print. DR. C. F. RODGERS. 189 It is much more simple, therefore, to expose a negative insteps of gradually increasing exposures so as to correspondwith the shadows, middle tones and high lights of a subjectand use these graded test strips in place of actual camera ex-posed films because the developed strips may be placed side byside and a comparison made of the densities of steps which re-ceived identical exposures. How to Expose a Step Negative. When measuring the characteristics of a developer in abso-lute units, it is necessary to use a standard light source anda special rotating disk or drop shutter for obtaining the gradedexposures, but for comparison purposes it is simply necessaryto give the test emulsion about 6 different steps in exposure,the lowest step being given such an exposure that after de-velopment the step will be just visible. The exact ratio of theexposures of the various steps is not important but should be,of the order

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