AERIAL FILM CAMERAS
The weight of the glass and the sheaths in the plate camera forms its
most serious drawback. This weight must be reckoned at least three
quarters of a pound for each 18 × 24 centimeter plate. Consequently,
with the use of these large plates, and with the demands for ever
increasing numbers of pictures to be taken on long reconnaissance
flights, a serious conflict arises between the weight of the
photographic equipment and the carrying capacity of the plane. Among
plate cameras probably the most economical in weight is the deRam. It
carries fifty 18 × 24 centimeter plates, and has a total weight of
approximately 100 pounds. An advance to 100 or 200 plates—not feasible
in the deRam construction—even if we assume the lightest possible
magazines, would bring the weight of camera and plates to 150 or 200
pounds, which would be detrimental to the balance and would seriously
infringe on the fuel carrying capacity and ceiling of any ordinary
reconnaissance plane.
Early and persistent attention was therefore paid to the possibilities
of celluloid film in rolls, as used so widely in hand cameras and in
moving picture work. The two great advantages of film would be its
practically negligible weight (approximately one-tenth that of plates,
not including sheaths) and its small bulk, which would permit the
greatest freedom in the development of entirely automatic cameras to
make exposures by the hundreds instead of by the dozens. Certain
disadvantages were foreseen at the outset: the difficulty of holding the
film flat and immune from vibration in the larger sizes; the difficulty
of quickly developing and drying large rolls; the question whether as
good speed or color sensitiveness could be obtained in sensitive
emulsions when flowed on a celluloid base as on glass. Early trials
revealed a further problem to solve: how to eliminate the discharge of
static electricity occurring at high altitudes, especially when the
weather is cold.
As far as camera construction is concerned the chief problems are to
hold the film flat, and to eliminate static.
=Methods of Holding Film Flat.=—Several means have been proposed and
used for holding the film flat. Disregarding mere pressure guides at the
side, which are suitable only for small area films (up to 4 × 5 inch),
the successful means have taken three forms: _pressure of a glass
plate_, _pressure of the shutter curtain_, and _suction_. A glass
pressure plate can be used in either of two ways; the film may be in
continuous contact with it or may be pressed against its surface only at
the moment of exposure. The advantage of this first method lies solely
in its mechanical simplicity; its disadvantage in the likelihood of
scratches or pressure markings on the film. Where a glass plate is used
there is always the chance of a dust or dirt film accumulating, or of
the condensation of moisture, to impair the quality of the negative.
There is, moreover, an inevitable loss of light (about 10%), together
with some slight distortion, due to the bending of the marginal oblique
rays through the thickness of the glass. In cases where a filter would
normally be employed, the loss of light is minimized by using yellow
glass for the plate, so that it serves for filter and film holder as
well.
_Pressure of the shutter curtain_ is utilized in the Duchatellier film
camera by furnishing the edges of the curtain aperture with heavy velvet
strips, whose light and gentle pressure during the passage of the
shutter holds the film against a metal back. In many ways this is the
simplest film-holding device; it occasions no loss of light, and needs
no mechanical movements or external accessories, such as are called for
in the suction devices next described. There is always danger of
markings on the film, if the velvet is not of the right thickness and
softness, and the operation and speed control of the shutter are
necessarily complicated by the additional frictional load.
_Suction_ of the film against a perforated back plate has been found a
very successful means of securing flatness. Suction at the moment of
exposure may be produced by the action of a bellows, which has been
compressed beforehand by the camera-driving mechanism. Continuous
suction can be produced either by a continuously driven pump, or by a
Venturi tube placed outside the fuselage. The Venturi tube (Fig. 55)
consists of a pipe built up of two cones, placed vertex to vertex, to
form a constriction. When air is forced through this at high velocity
suction is produced in a small diameter tube taken off at the
constriction. A suction of two centimeters of mercury, acting through
holes about one centimeter equidistant from each other in the back
plate, has been found adequate to hold flat a film 18 × 24 centimeters.
One merit of suction applied only at the moment of exposure is that the
film-driving mechanism does not have to work against the drag of the
suction. Continuous suction, on the other hand, gives a longer
opportunity for flattening out kinks in the celluloid, and easily
permits movement of the film during the exposure, either for the purpose
of permitting a longer exposure or for the purpose of preventing
distortion due to the focal-plane shutter. A disadvantage of continuous
suction is the production of minute scratches on the celluloid surface
as it drags over the suction plate. These are ordinarily too small to
cause trouble, but may show up when printing is done in an enlarging
camera.
[Illustration:
FIG. 55.—Venturi tube on side of plane.]
=Static discharges= are produced by the friction of the celluloid
against the pressure back or other surfaces with which it comes into
contact. They show in the developed film as branching tree-like streaks
(Fig. 56) and in cold dry weather may be numerous enough to ruin a
picture. The discharges are noticeably less frequent with film coated on
the back with gelatine (“N.C.”), but the extra gelatine surface is
extremely undesirable. When handled by developing machines, as large
rolls must be, this back gelatine surface becomes scratched and bruised
in a serious manner. Plain unbacked film is much to be preferred if the
static can be obviated.
To avoid static, it is necessary to provide for the immediate
dissipation of all acquired electrical charges. Experiments made by the
United States Air Service have shown that nothing is so good as rather
rough cloth, thoroughly impregnated with graphite, held in close contact
with the celluloid during as great a portion of its travel as possible.
In the United States Air Service film camera which uses suction through
a perforated back plate, the plate has been covered with thin graphited
cloth, and similar cloth sheets are pressed against the film rolls by
sheets of spring metal (Fig. 65). In cameras with this equipment no
trouble has been experienced with static.
[Illustration:
FIG. 56.—Print from film camera negative, showing static discharge,
and (upper left-hand corner) record of altitude and compass
direction made by Williamson film camera auxiliary lens (Fig. 58).]
=Representative Film Cameras.=—_The English F type_ (Williamson). This
is one of the earliest cameras designed for film, as is indicated by the
nature of the power drive, which presupposes that the camera is to be
carried on the outside of the fuselage. Its essential features are shown
in Figs. 57 and 58. It consists of a rectangular box with a cone at the
front on which is mounted a propeller, intended to be rotated by the
wind made by the motion of the plane. This drives, through a governor
controlled friction clutch, a train of gears which draws the (5 × 4
inch) film across the focal plane, sets and exposes the shutter at
regular intervals.
[Illustration:
FIG. 57.—English type “F” (Williamson) automatic film camera.]
Above the camera, supported on a tripod, are a compass and altimeter,
both recording on a single dial, illuminated from below by the light
reflected from a circular white disc painted on top of the camera. An
image of the dial is thrown on a corner of the film by a lens, whose
shutter is actuated in synchronism with the main focal-plane shutter. No
special means are provided for holding the film flat. Special film with
perforated edges is used.
The camera was designed for mapping work on the Mesopotamian and other
fronts where no maps at all existed.
_The Duchatellier_ camera is essentially a film magazine to fit on the
standard French deMaria camera bodies, of the 18 × 24 centimeter size.
In its simplest form it embodies a shutter (the regular focal-plane
shutter of the camera being removed) and a film-moving mechanism, both
actuated by a single motion of the hand. Automatic and semi-automatic
operation are accomplished by an auxiliary mechanism to which Bowden
wires from the hand lever are attached. The motive power is an air
propeller. Variation of speed is obtained by changing the point of
contact of a roller on a friction disc, the disc being directly
connected to the propeller shaft, the roller to the camera drive shaft.
[Illustration:
FIG. 58.—Interior of type “F” camera, showing lens for photographing
compass and altitude readings.]
The most distinctive features of the Duchatellier camera is its use of
the focal-plane shutter to hold the film flat during the exposure. As
already explained, this is accomplished by pressure, velvet strips on
the shutter edges keeping the film close against the back plate. The
return of the shutter curtain to the “set” position is accomplished by
locking it to the film by perforating points, so that it is pulled
across as the film is wound. This introduces between each pair of
pictures a strip of tremendous over exposure, as wide as the curtain
opening. A fixed-aperture variable-tension shutter is used. The magazine
carries a roll of film long enough for 200 exposures, feeding the long
way of the picture. When film needs to be changed in the air, this is
done by changing the entire magazine, including its shutter.
[Illustration:
FIG. 59.—G. E. M. automatic film camera.]
_The G. E. M. camera_ (Fig. 59) is a very light self-contained
clock-work-drive camera taking 36 pictures six inches square. The film
is unrolled from a small-diameter feeding roller on to a large-diameter
receiving roller to which the driving mechanism is attached. By this
means approximately equal spacing of pictures on the film is assured.
The film is held flat by continuous contact with a glass plate, which is
made of yellow glass, so that it serves at the same time as a color
filter. The lens—of 8 to 12 inch focus—is equipped with a single speed
between-the-lens shutter. The operation of the camera is entirely
automatic. The interval between pictures is controlled by varying the
clock-work speed, through a lever on the outside of the camera box.
Protection of the camera from vibration is sought by supporting it on
four spring cushions mounted on a solid frame, to which the camera is
held by spiral springs attached to its sides.
[Illustration:
FIG. 60.—Brock automatic film camera.]
_The Brock Film camera_ (Fig. 60) is an entirely automatic, very compact
self-contained camera, taking one hundred 4 × 5 inch pictures. The
motive power is clock-work, regulated in speed by an escapement
controlled by a flexible shaft carried to a dial which may be fastened
to the instrument board or to some other convenient part of the plane.
The lens is 6, 12, or 18 inch focus. The shutter is of the
fixed-aperture variable-tension type, of long travel, and with a flap
behind the lens for covering during the setting period. None of the
special means above described for holding the film flat are provided. A
metal plate resting on the back, and a flat metal frame in front with a
4 × 5 inch aperture, are considered sufficient check on the excursions
of the small-sized film. A ball bearing double pivoted frame serves to
support the camera in a pendulous manner, permitting it to assume a
vertical position after tilting. Damping of oscillations and vibration
is arranged for by two pneumatic dash pots.
The _German film mapping camera_, shown in Fig. 61, is distinguished by
a number of special features. The size of the pictures, 6 × 24
centimeters, is unusual. It has its advantages, however. Since the short
dimension is in the line of flight, the maximum width of field covered
by the lens is utilized (Fig. 17). This of course necessitates a larger
number of exposures to complete a strip, which is perhaps an added
advantage, since the narrower the individual pictures the better the
junctions will be, especially if large overlaps are made. This proved to
be the case with captured German mosaics. Difficulty is experienced in
making overlaps on a turn (Fig. 62), but this is not a vital objection.
The shutter has a fixed aperture, narrower at the center than at the
ends, to compensate for the falling off in illumination away from the
center of the lens. No safety flap is needed because the curtain moves
in opposite directions on successive exposures, thereby also
compensating for shutter distortion, as has already been discussed.
Shutter speed is controlled by varying the tension of the actuating
spring.
[Illustration:
FIG. 61.—German automatic film camera.]
The camera is driven by an electric motor, connected to a set of gears,
whose shifting provides for speed variation. The film is moved by rubber
rollers which are cut away for part of the circumference, allowing the
film to stand still until they bite again. A yellow glass pressure plate
holds the film during the exposure and serves as color filter also (Fig.
63). An electric heater is provided near the shutter, as in all the
later German cameras.
[Illustration:
FIG. 62.—Method of joining and printing film from German camera.]
_United States Air Service automatic film camera_—_Type K_ (Figs. 64,
65, 92, 93, 98, 99). This is an entirely automatic camera, manufactured
by the Folmer and Schwing Division of the Eastman Kodak Co., taking 100
pictures of 18 × 24 centimeter size at one loading. As with all the
American cameras of this size, it uses the standard lens cones of any
desired focal length. The camera proper consists of a compact chamber in
which the film rollers are carried at each end forward of the focal
plane, the shutter lying between. In consequence of this arrangement the
vertical depth of the camera is the absolute minimum—short of decreasing
the length of the optical path by mirror arrangements—making it possible
to suspend the camera diagonally in the American and British planes, for
taking oblique pictures.
Flatness of the film is secured by a suction plate covered with
graphited cloth and connected with a Venturi tube. The top cover is
removed for re-loading. The shutters on the first cameras of this type
are of the variable-tension fixed-aperture design, though later ones
have the variable-aperture curtain controlled by an idler, as used in
the American deRam. An auxiliary curtain shutter serves to cap the true
shutter during setting.
The operation of the film driving mechanism is comparatively simple. It
consists of a train of gears, driven by a flexible revolving shaft
attached to some separate source of power capable of speed variation.
The action of the gears is to move the film, set the shutter and then
expose it; in the earlier cameras with the film continuously moving. In
the first cameras constructed the space between the pictures varies as
the film rolls up, due to the increasing diameter of the roll. In later
cameras the film roller is disengaged from the gears just before the
shutter is tripped, so that the film stands still during the exposure,
and is then re-engaged at a new point on a ratchet wheel governed by the
diameter of the receiving roll, whereby the pictures are equally spaced.
In all the cameras, punch marks made at the time of exposure enable the
limits of the picture to be detected in the dark room by touch.
[Illustration:
FIG. 63.—Film winding and exposing mechanism in German film camera.]
Variable speed is arranged for in any one of several ways. For
peace-time uses a turbine attached to the side of the plane is simple
and positive, and, provided it is made of sufficient size—which is not
the case with the one shown in the Figure—will give adequate speed
regulation upon varying the aperture through which the air enters. The
Venturi tube may be carried upon the same mount, or a small rotary pump
can be attached on the same shaft. Where the high wind resistance of the
turbine is an objection the camera is driven electrically, by a motor
acting through the intermediary of a variable speed control described in
the next chapter (Fig. 68).
The camera weighs complete about forty pounds, and the film rolls about
four pounds. The latter can be changed in the air without great
difficulty provided the camera is mounted accessibly and so that the top
may be opened.