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.
[Illustration:
FIG. 118.—Printing machine.]
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.”
[Illustration:
FIG. 119.—Interior of McIntire printer, showing lamps adjustable in
position for “dodging.”]
[Illustration:
FIG. 120.—Film printing machine.]
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.
[Illustration:
FIG. 121.—“Semperfocal” enlarging camera, with mechanism for holding
image in focus at any enlargement.]
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.
[Illustration:
FIG. 122.—Diagram showing enlarging with and without distortion: _A_,
enlarging without distortion; _B_, distortion for rectification of
print, by inclining printing surface; _C_, distortion, for
rectification of print, by inclining both negative and printing
surface.]
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