American cinematographer (Jan-Dec 1926)

December, 1926 AMERICAN C1NEMATOGRAPHER Nine Duplication of Motion Picture Negatives Fine Grain and High Resolving By J. G. Capstaff High Maximum Contrast Not Power Are among Requirements and Desired for Duplicating Emul for Material Used. M. W. Seymour* sion; Many Defects Involved. The making of a first class duplicate negative calls for greater skill and makes greater demands upon the materials than appears at first sight. A perfect duplicate negative would be one which would give prints identical in every respect with those obtainable from the original. This means that the duplicate negative should have perfect tone reproduction and definition or sharpness and should appear no more grainy than the original. The essential requirements thus placed on the printing material are: sufficient latitude to reproduce correctly scale of tones likely to be met wTith in an original negative; extremely high resolving power; and fine grain. To these must be added the practical requirement of sufficient speed for contact printing. It may be said that no one emulsion excels in all of these characteristics. If an emulsion has the finest posble grain it cannot also possess the greatest latitude obtainable combined with the maximum speed, and so on. Particular Purpose Each type of emulsion is made for a particular purpose and consequently has the qualities most essential for that purpose even at the expense of other desirable, but less important qualities. Motion picture negative film is especially designed for use in the camera. It has high speed to permit taking pictures when the light is not brilliant, great latitude to cover errors in exposure, and a medium value for its maximum contrast. It also has sufficiently high resolving power and fine grain to serve its intended purpose. Although it is an excellent negative material, it is not the best for making duplicates. Positive film, on the other hand, is intended for making prints for projection; it has latitude to cover the range of tones in a normal negative and the speed necessary for cuiitact printing. It also has fine grain, high resolving power, and sufficiently high contrast *Research Laboratory of the Eastman Kodak Company; this paper was read before the recent meeting of the Society of Motion Picture Engineers. to give good prints from flat negatives. The best duplicating material, however, should have even higher resolving power and a lower maximum contrast. The reasons for this will now be considered. The Negative A motion picture negative under the microscope is seen to be made up of black silver particles with clear interstices between. Whereas the function of the printing emulsion is to image these particles and interstices, no emulsion made has high enough resolving power to do so perfectly. The image of the granular structure always appears more ill-designed and coarse than the original, with the result that the picture when enlarged on the projection screen appears more grainy than the negative from which it was printed. The increase in graininess is not serious in positive prints from original negatives, but unfortunately it can become painfully evident in prints from duplicate negatives, because in the operation of making a master positive, then a negative from this, and finally a positive print, the grain structure is coarsened three times. It is essential then that if the graininess of the screen picture is to be kept at a minimum, the emulsion used in making both the master positive and the duplicate negative should have the highest possible resolving power. No High Contrast It is desirable that a duplicating emulsion should not have a high maximum contrast, not only because high contrast is unnecessary, but because of development defects that occur when development is not carried to completion, as would be the case were a high contrast emulsion used for duplicating. The defects produced with low development are termed the "Eberhard" effect and the uMackie" line. Eberhard, a Danish astronomer, showed that the density of small exposed areas in a film differed from that of large areas which had received the same exposure, and that the inequality was greatest when development was incomplete. He found that under these conditions a small ex (Continued on Page 21)