CHAPTER XXIV

PRINTING AND ENLARGING

=Contact Printing.=—Single prints are made most simply in a printing frame held at a short distance from a light source. When any quantity must be made, as in turning out prints at high speed for distribution to an army before an attack, _printing machines_ are employed. These consist essentially of a light box, a printing frame of plate glass, and a pressure pad. In the commercial models, such as the Crown and the Ansco, which are equipped with electric light, merely bringing the pressure pad down and clamping it automatically turns on the light, while release of pressure terminates the exposure.

The question of regulating the distribution of light is of considerable importance with negatives taken by focal-plane shutters of non-uniform rate of travel. In the McIntire printer (Fig. 119), the separate electric bulbs are on long necks in ball and socket joints, so that they can be brought individually closer to the printing surface or farther away from it, thus permitting a wide range of “dodging.” This printer also has an automatic time control for the light, a valuable device where many prints from the same negative are desired.

These machines are well suited for printing aerial negatives, either plate or cut film, if used where a source of electric current is available. The chief defect, which may be caused by faulty construction, is imperfect contact between paper and negative, a cause of serious unsharpness on prints destined for minute study in interpretation.

The printing of aerial negatives may be done either on roll or cut paper, and if films are used, a further alternative is offered of handling it either in the roll or in cut form. Where many prints are to be made from one negative roll paper has some advantages, particularly if a developing and drying machine is available. But for moderate numbers the advantage is small, since cut prints can be developed quite conveniently in goodly numbers in the ordinary trays. But the advantages of keeping film in the roll form are very great, both in respect to storage and in respect to handling during printing, as the rollers provide the necessary tension and prevent the film “getting away.”

For the American Air Service, cut paper has been used exclusively. For film printing, the Ansco machine has been equipped with roll pivots to take film 24 centimeters wide which may be advanced in either direction by turning large milled heads (Fig. 120). If we put rollers on the two remaining sides of the box to handle paper we transform the printer into the same form as a French machine, in which paper and film are moved at right angles to each other. A disadvantage of this modification, however, is the difficulty of examining the negative to be printed.

=Stereo Printing.=—To make separate prints from the two elements of a stereoscopic pair and mount them side by side after proper orientation is too slow a process if quantities of prints are needed. One method of multiple production is to make a master stereogram, and then produce photographic copies of it, but there is inevitable loss of quality in this copying process. An intermediate method is to print from both negatives on the same sheet of paper. In order to do this the negatives must be placed in rather large frames, with mats properly located to guide the placing of the paper. The Richard double printing frame is a practical device which simplifies the necessary manipulations. It consists essentially of a platform pierced with three illuminated openings. The two negatives are compared, superposed, and orientated over the central opening and then shifted laterally, one to each of the two side openings, which serve both as printing frames and masks. The printing back slides on a rod, permitting the paper to be lifted up and moved between exposures. Once the negatives are properly placed, stereo prints can be turned out quickly and easily.

=Enlarging.=—In the French service contact printing was the rule during the war. The English practice, on the other hand, was to take small negatives—4 × 5 inches, with 8 to 12 inch lenses—and enlarge them, usually to 6½ × 8½ inches. For this purpose a regular part of the English photo section equipment was the _enlarging camera_ (Fig. 115). This may be briefly described as a short focus camera in which the subject to be photographed is a negative, illuminated by transmitted light, whose image is thrown by the camera lens on the paper or other sensitive surface. By making the distance between negative and lens less than that between lens and paper, the resulting print is an enlargement, and _vice versa_. The scale of enlargement or of reduction is varied over limits set only by the length of the camera and the amount of light available.

The lens employed must of course possess sufficiently high quality to preserve all the sharpness of the negative, and focussing must be done with accuracy. Next to the lens the most important element is the light source. This may be of the point form, such as a concentrated filament electric lamp, an oxy-acetylene lime light, or an acetylene flame. The latter was extensively used in the English service, while acetylene generators for emergency purposes formed part of each American photo truck equipment. With point light sources we must use _condensers_ to focus the light into the projecting lens. Much less efficient, but the only recourse where large condensers are not available, is a diffusing glass behind the negative, illuminated either by a bank of electric lamps with mirrors or by a U tube mercury vapor lamp, where proper current can be got.

The device for holding the printing paper must permit quick changing, but insure good contact. We may use either a spring plate to hold the paper against plate glass from behind, or else a weight acting on a lever arm of sufficient length.

The need for some automatic means of focussing an enlarging camera has been very generally felt. An illustration of such an enlarging camera is that put out by Williams, Brown & Earle, of Philadelphia, known as the “Semperfocal” (Fig. 121). In this camera the movements of the lens, paper easel and negative are so inter-related and actuated with respect to each other that the correct focus of the instrument is maintained for any degree of enlargement or reduction. This feature is a great help in making up mosaic maps, where prints of continuously varying scale ordinarily occasion serious delay for individual focussing.

Determining the correct enlargement for each negative of a mosaic is perhaps the most important problem in the use of the enlarging camera for aerial work. The correct setting of the camera may be found by either of two methods: the negative may be previously scaled and marked with a line on its edge, which must be projected to a definite size; or the true location of several points in the picture as obtained from an accurate map may be marked on the enlarging camera easel according to the desired scale, and the negative image projected to coincide with these. In either case, if an exact scale is desired, allowance must be made for paper shrinkage, a matter which must be determined by previous experiment.

=Rectifying.=—Negatives taken when the plane is not flying level will be distorted (Figs. 134 and 135). Contact prints from these will not fit into a mosaic, and no mere enlargement or reduction will make them available. It is necessary with these negatives to resort to a _rectifying camera_. This is an enlarging camera built so that the negative and print easel may be inclined about vertical and horizontal axes, thereby purposely introducing a distortion sufficient to offset the distortion of the negative. Thus, if the bottom of the printing surface is moved away from the lens, that part of the picture will be enlarged; if moved toward the lens, reduced.

For small rectifications the common practice is to tilt the printing surface alone, a method that is practical as long as this tilting does not affect the focus so much as to require prohibitive stopping down of the lens. For great distortions, such as that inherent in the principle of the Bagley camera, it is necessary to tilt both negative and print in order to preserve an approximate focus, a given portion of the negative moving toward the lens as the corresponding portion of the print is moved away. Both schemes for rectification are shown diagrammatically in Fig. 122.

=Developing and Drying Prints.=—The developing of prints follows closely that of cut or roll film, and so need not be treated separately.

The drying of emulsions on paper is more easily accomplished than the drying of emulsions on glass, for two reasons: the emulsions on paper are much more thinly coated, and there is diffusion of moisture into the atmosphere from front and back of the printing medium. In the field a common method has been to soak the prints in water-free alcohol and then burn off the alcohol, thus securing a dry print within two or three minutes after the conclusion of washing. A later method very generally employed is to cover wooden frames three or four feet above the ground with chicken wire or muslin, and on these lay the prints after soaking them in alcohol. Below the frames currents of warm air rise from pans of burning alcohol, previously used to soak the prints and now useless as alcohol because of their high water content.

Before putting them in alcohol it is advisable to squeegee all the surface water from the prints. This may be expeditiously done by removing them in mass from the final wash water upon a large ferrotype plate, and either running the plate and prints together through a wash wringer with light pressure, or covering the whole with a sheet of blotting paper and pressing out the water underneath by means of a rubber squeegee vigorously applied.

For base work one of the modern automatic print-drying machines used in commercial photography would be desirable. Glossy surfaces are given prints by the usual ferrotype plate method. But this is too time-consuming for war practice, and besides has but doubtful advantage where papers of the glossy type are chosen.

VI PRACTICAL PROBLEMS AND DATA