CHAPTER XI

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.

=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.

=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.

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.

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.

_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.

_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.

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.

_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.

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.