CHAPTER IX

SEMI-AUTOMATIC AERIAL PLATE CAMERAS

In the hand-operated camera the limit to progress is set when the number of operations is reduced to a minimum. In cameras using the larger sizes of plates a reduction in the number of operations almost inevitably results in inflicting considerable muscular labor upon the operator. Furthermore, distance operation becomes difficult to arrange for, because the common reliance—the Bowden wire—is unfitted for heavy loads. Consequently, for setting the shutter and changing the plates we must resort to some other source of power than the observer's arm. Air-driven turbines or propellers have been used on aerial cameras, as well as clock-work, and also electric power, the latter derived either from a generator or from storage batteries. The relative merits of these sources of power form the subject of a separate chapter. Mention only is here made of the form of drive actually employed in connection with the various cameras.

The term _semi-automatic camera_ is best used to designate that type in which the observer (or pilot) has merely to release the shutter, after which the mechanism performs all the operations necessary to prepare for the next exposure. There has been some difference of opinion as to whether it is ever advisable to go further than this with plate cameras. The English Service holds that completely automatic exposing, in addition to plate changing, is apt to encourage the making of many more pictures than necessary, involving carrying an excessive weight of plates. The French Service has rather generally favored entirely automatic cameras in theory, although during the war practically all the work of the French army was done by the hand-operated cameras already described.

=The English L Type Camera.=—The L, a modification of the earlier C and E models, differs from its predecessors chiefly in the addition of a mechanism which when connected with a suitable source of power can be used whenever desired for changing the plates and setting the shutter. As in the C and E types, all unexposed plates are carried in a magazine above the camera, while the exposed plates are shifted in a horizontal direction to one side and fall thence to a receiving magazine.

Fig. 50 shows the American model, which is a copy, with modifications, of the original English design. Its weight with one loaded magazine is about 35 pounds. Its manner of functioning may be studied from the picture of the mechanism (Fig. 51). The part of the mechanism to the left is inoperative during hand operation, and the large toothed wheel is locked by the removable pin shown hanging on its chain in Fig. 50. To change a plate and set the shutter the projecting lever (Fig. 50) is thrown over and back. This causes a sliding tray, in which the exposed plate rests, to travel to the right, over the receiving magazine, where the plate is dropped. After this the tray returns to the left exposing position. Simultaneously the shutter is wound up. Exposure is made either by pressing down upon the plunger, or better, by using a Bowden wire. Provision for both methods of exposing, one for the pilot and one for the observer, is shown in Fig. 81. The shutter is the variable-aperture type already described, provided in addition with a tension adjustment on the back of the camera. A flap behind the lens does the capping during the setting operation.

For power operation the camera is connected through a flexible shaft with a wind driven propeller (Figs. 50, 83 and 84). The locking pin is now moved over from the toothed wheel to the lever arm, so that the rotation of the worm driving the large toothed wheel forces the lever through its plate changing motion. To prevent repetition, a part of the periphery of the toothed wheel is cut out, so that it stops when its cycle is run. When the Bowden wire actuates the shutter release it forces the toothed wheel around into engagement (aided by one spring tooth) and so starts the cycle once more.

When connected with the air propeller the worm is rotated continuously. Other sources of power—an electric motor, for instance—can be attached through the same kind of flexible shaft. If an electric motor is employed it may be run continuously or it may be operated with an insulated sector introduced into the large toothed wheel so that the electric circuit is broken and the motor stops until the wheel is once more forced around by the exposing lever.

_Faults of the L camera._ The L camera was the mainstay of the English Air Service. In fact for the last two years of the war it was practically the only camera the English used, and they thought highly of it. It is, of course, subject to the limitation of small plate size and short focus lens. It is in many ways an inconvenient camera to handle. For instance, the upper magazine cannot be closed or removed until all the plates are passed through. Its dependence upon gravity for the plate changing operation is a fundamental weakness, responsible for its frequent tendency to jam in the air. Experience made the English observers very expert in relieving these jams. Sometimes they would turn the propeller backward (mounting it in an accessible position to provide for this contingency), sometimes they would shake or thump the camera. But while these makeshifts would serve to secure pictures—the chief object, of course, of the photographic service—they can scarcely be said to render the camera satisfactory.

Moreover, the propeller drive has not been universally approved, as it furnishes an additional mechanism to make trouble. Since it is not feasible to change from power to hand operation while in the air, the camera is put out of commission whenever the propeller or shaft is disabled. Bowden-wire controls for both plate changing lever and shutter release were common in the British service, which considered the extra operation or the extra muscular exertion unimportant when compared with the greater assurance of reliable action.

=The English LB and BM Cameras.=—During the closing months of the war an improved L type camera was constructed, the LB. This differs from the L in a number of detail changes, dictated by experience. The shutter is now made removable and self-capping. Pivoted lugs are provided to hold the exposed plate horizontal until the very instant it drops, in an effort to prevent jams caused by the plates piling up at an angle in the receiving magazine. The chief addition, however, is the provision of several interchangeable cones and cylinders, for carrying lenses of focal lengths from 4 to 20 inches. Fig. 95 shows the LB with 20 inch lens cylinder mounted on a bell crank support in the camera bay of an English plane.

The BM camera is but a larger edition of the LB, for 18 × 24 centimeter plates. It also carries several interchangeable lens cones.

=The American model deRam camera.=—The rotary changing box devised by Lieutenant deRam of the French army and incorporated in his entirely automatic plate camera, has been adapted by the American Air Service to a very successful semi-automatic camera. Fig. 52 shows the principle of this changing box. The pile of fifty plates, each in its sheath, is carried in a rectangular box open at top and bottom. The lower plate next the focal-plane shutter is first exposed; the pile then rotates about a horizontal axis through a complete turn. When the exposed plate arrives in a vertical position it is allowed to drop off, by the opening of cam actuated fingers, and lodges against the side of the enclosing camera box proper. Still further along in the cycle the plate is thrown off from its lodging place into a “scoop” on the top of the rotating container and laid on the top of the plate pile. Meanwhile the curtain of the focal-plane shutter winds up, at the same time that it is depressed out of the way of the revolving plate container. Although the plate changing operation depends on gravity, it nevertheless functions satisfactorily up to 30 degrees from the vertical.

The shutter in this model is the variable-aperture fixed-tension type, adjusted by pivoted idlers (Fig. 28). In the exposing position it runs within three millimeters of the plate surface, and is therefore of high efficiency for all openings. Capping during the operation of setting is performed by flaps at the bottom of the camera body. Interchangeable cones are supplied for lenses of various focal lengths.

For hand operation the changing box is turned over by means of a handle, which rotates four times for the complete cycle (Fig. 90). For semi-automatic operation an additional mechanism is provided on the side of the rectangular camera body, copied with some necessary modifications after the L camera power drive. From the observer's standpoint the operation of the whole camera is the same as in the L camera, with the important exception that power operation in no way interferes with hand operation. Indeed, the hand can help out if the power flags or fails.

This camera is most satisfactorily driven by a 12 volt ⅒ HP electric motor working through a flexible shaft attached to a swivel connection at the front of the semi-automatic drive box. A change once every four to five seconds is possible, but greater speed is apt to throw the changing plate too violently for safety.

The chief practical objection to this camera is its bulk. Its great height makes it impractical for many planes. Its weight of nearly a hundred pounds is a formidable load for a plane to carry, but this is no more and probably less than that of any other camera when taken up with the same number of plates in magazines. The price paid for economizing in magazine weight is that the whole camera body, excluding the lens cone, must be carried to and from the plane for both loading and unloading.