CHAPTER XIII

CAMERA AUXILIARIES

=Distance Controls and Indicators.=—All operations connected with the exposing and changing of plates (except the changing of whole magazines) should be arranged for accomplishment at a distance. Other operations, such as changing the shutter speed or the interval between exposures in an automatic camera, which are usually done on the ground, may sometimes be satisfactorily left for performance at the camera. Conditions of extreme inaccessibility may, however, make it necessary to carry even these controls to a distance. Indicators of the number of exposures already made, and of the readiness of the camera for the next exposure, may be attached to the camera, but often are more profitably placed at a distance. Distance control and indication are especially necessary if the pilot makes the exposures—a common English practice in two seaters, and the only recourse in single seaters.

When electric power is available, electrical distance control devices are perhaps the simplest kind, as they transmit motive power without displacing or jarring the camera. Solenoids suffice for the simple pressing of releases or for counting mechanisms, while small service motors may be utilized for operations involving more work. A standing practical objection to electrical control lies in the necessity for using contacts, which are apt to be uncertain under conditions that involve vibration.

_The Bowden wire_—a wire cable carried inside a heavy non-extensible but flexible sheath—constitutes the most satisfactory mechanical means for transmitting straight pulls. By means of “the Bowden” a pull may be transmitted so as to be made entirely relative to two parts of the same body, calling forth no tendency of the body as a whole to move. Thus in the L camera shutter release (Fig. 50), the releasing lever with its attached counter is several feet distant from the camera. If the plate bearing the lever and sheath end is rigidly fastened down, the pressure exerted on moving the lever acts between the lever and the end of the sheath. This pressure passes immediately to the other end of the sheath, while the pull on the wire is transmitted to its farther end on the camera. In this way the conditions at the lever are reproduced, but with the advantage that, due to the flexible cable and sheath, any vibration of the lever support is damped out.

Due to its stretching, there is a pretty definite limitation to the feasible length of the Bowden wire. This length is about four feet. Where according to English practice the pilot makes the exposure, a considerably longer wire and sheath are called for. In this case the effective length of the release is increased by giving the pilot a second releasing lever, connected to the first by a rigid rod (Fig. 69). The releasing lever, wire, and all mechanical parts of the Bowden release should be made much stronger than would be indicated by bench tests of the camera. In the air it is impossible to decide either by sound or by delicacy of touch whether the mechanism has acted, so that the observer is apt to pull much harder than necessary and to strain or break the release if it is weak.

The Bowden wire is used in the American service only for shutter release. In the English service it has been used for plate changing with the L camera.

=Sights.=—In airplane photography the need for a finder or sight is fully as great as in everyday work. A new condition, however, prevails, for except with hand-held cameras, and even to some extent with them, the operation of pointing the camera involves pointing the whole vehicle that carries the camera. The pointing of airplane cameras is therefore akin to the sighting of great guns. While the observer may perform the actual operation of taking the picture, the responsibility for covering the objective rests with the pilot. Teamwork counts equally with tools. Airplane camera sights may accordingly be divided into two classes: sights attached to the camera, for use principally with hand-held apparatus, and sights attached to the plane, for the use of pilot, of observer, or of both.

=Sights for Hand-held Cameras.=—The simplest form of sight attached directly to the camera is modeled on the _gun sight_, consisting of a forward point or bead and a rear V. This sight of course serves merely to place the objective in the center of the plate and gives no indication of the size of field covered. Another simple sight of rather better type is the _tube sight_—a metal tube of approximately one inch diameter and three inches length, carrying at each end pairs of wires crossed at right angles. The camera is in alignment when the front and back cross wires both exactly match on the object to be photographed. The best way to mount the cross-wires is with one pair turned through 45 degrees with respect to the other, so that it is at once apparent which is the front and which the rear pair (Figs. 31 and 39).

_Sights to indicate the size of the field_ are usually less needed on hand cameras than on fixed vertical cameras. Yet certain circumstances make them most desirable, for instance in naval work where a complete convoy must be included on the plate. A sight of this kind can be made up of two wire or stamped metal rectangles, a large one in front and a smaller one behind, of such relative sizes and separations that the true camera field is outlined when the eye is placed in position to see the two rectangles just cover each other. The dimensions should be so chosen that the correct position of the eye is approximately its natural location with respect to the camera when this is held in the hands in the plane. It is usual to provide the rectangular sights with cross-wires to indicate the center of the field. Alternative rear sights are simple beads or peep-holes—the use of the bead assuming that the camera is held at about the right distance from the eye for the rectangle to indicate the field. The peep-sight is not a desirable form, as it is hard to hold the camera as near the face as is necessary. These various types of rectangle sights are well illustrated in the cameras shown in Figs. 38, 40 and 186. They are all made so as to fold down flat on the camera and to snap quickly open when needed. The springs to support the sights must be fairly strong, and the surface presented to the wind as small as possible. Wire frames give very little from the pressure of the wind, but flat metal frames are apt to be bent back.

The position of the sight on the camera is important. If the observer can stand, or if he sits up well above the edge of the cockpit, the conventional position of the sight on a pistol, namely, on top, is unobjectionable. But if the observer sits very low, as he usually does, then the sight should be on the bottom of the camera, thereby avoiding any need for the observer to raise his head unduly into the slip stream. Similarly, if the camera is used over the side for verticals, as it is in flying boats, a sight on the top is impractical, since it requires the observer to lean out dangerously far (Fig. 185).

=Sights Attached to the Plane.=—Any of the sights just described can be attached to cameras fixed in the plane, but they would be useless in the positions ordinarily occupied by the camera. It has therefore become common practice to attach the camera sight to some accessible part of the plane. The most primitive method of sighting is merely to look downward over the side—a method in general use to the very end of the Great War. One step in advance of this is to mark a large inverted V on the side, with its vertex at a point where the observer can place his eye and so see the fore and aft extension of the field of view covered by the camera. This kind of sight was common on the French “photo” planes. On some of the English planes the tube sight was carried on the outside of the cockpit. Any of the sights described can be carried on the inside of the fuselage, provided a hole is cut in the floor. For satisfactory sighting a hole in the floor is really necessary, as it enables the terrain on both sides of the vertical to be seen. One drawback to the simple hole, however, is that it cannot be made large enough to show the whole field from the ordinary height of the observer's eye, thus forcing him to bring his head down near the floor. This difficulty is gotten over in a very beautiful way by the use of the _negative lens sight_ shown diagrammatically in Fig. 71.

Let _F_{1}_ be the distance at which the edge of the hole (or a rectangle marked on the lens) appears the size of the camera field (if the hole is the size of the plate, _F_{1}_ is the focal length of the camera lens). Let _F_{2}_ be the distance from the floor to the observer's eye. What is desired is a concave lens which will diverge the rays from their normal meeting point at _F_{1}_ to a new meeting point, _F_{2}_. The focal length of lens required is given at once by the simple lens formula—

1 1 1 ——————— - ——————— = ——— _F_{1}_ _F_{2}_ _F_

Thus if _F_{1}_ is 12 inches, and _F_{2}_ is 36 inches, _F_ will be 18 inches. The lens is to be marked with a rectangle showing the shape and size of the camera field, and a central mark such as a cross. An upper rectangle, or a bead, or a pair of cross wires a few inches below the lens, may be used for the other sight. For precision work the sight above or below the lens should be adjustable in position, especially where the camera suspension permits the camera to be adjusted for the angle of incidence of the plane.

A negative lens sight should be placed in the observer's cockpit, if he takes the pictures, and also in the forward cockpit, so that the pilot may be accurately guided in his part of the task. In addition, it is advisable to place a negative lens well forward in the pilot's cockpit, to enable him to see the country some distance ahead. The lenses should be planoconcave with the flat side upward; otherwise, all the loose dirt in the airplane settles in the middle of the concave depression. A negative lens sight in a metal frame forming a completely self-contained unit ready for mounting in the plane is shown in Figs. 72 and 73.

=Devices for Recording Data on Plates.=—_Numbering devices._ The number of the camera is impressed on negatives taken with the American L camera through the agency of a small transparent corner of celluloid. It would be entirely possible to incorporate a rotating disc which should turn by the operation of plate changing and carry a series of numbers, so that each exposure could be numbered serially. Numbering of individual plates is more commonly done by holes, notches, or even numerals, in the turned over portion of the sheaths, which are then recorded photographically when a picture is taken (Fig. 75). The chief objection to this method is the difficulty of keeping the sheaths together in sets, especially as individual ones become damaged or lost. In practice there is also danger of the sheaths being carelessly loaded in wrong order.

The more ambitious idea of recording on the plate all the information given by the instrument board of the plane occurs independently and spontaneously to all aerial photographic map makers. These ideas vary from attempts to photograph the actual instrument board on every plate—a difficult task indeed with the instruments and camera placed as they are in the ordinary plane—to the incorporation of compass, altimeter, and inclinometer in the camera itself.

Figure 58 shows the plan adopted in the English F type film mapping camera already described, for photographing a compass and an altimeter on the film. Here the combined compass and altimeter dial is above the camera, and is mounted in a cell with a glass bottom. Below it is a lens focussing the needles and compass points on the plane of the film. The light for photography is furnished by a diffusely reflecting white surface on top of the camera, illuminated by the sky. (The camera was carried outboard.) In Fig. 56 is shown a picture with the compass image impressed upon it.

Figure 74 shows a type of inclination indicator found in some captured German cameras. It consists essentially of two small pendulums or plumb-bobs; one to indicate lateral, the other longitudinal inclination, arranged to be photographed in silhouette on the plate, as shown in the lower part of the diagram and in the print from a captured negative (Fig. 75).

Both these devices suffer from the deficiencies of the instruments they photograph. The compass and the inclinometer, as already mentioned in the discussion of airplane instruments, only behave normally in straight-away flying, failing to indicate correctly when the plane is subject to accelerations in any direction. In general all attempts to record directional data in the camera are of little promise, unless either the instruments or the camera are automatically held level by some gyroscopic device. If the instruments are so controlled, rather elaborate means for photographing them are necessary. If the camera is stabilized, the inclinometers are unnecessary, and the compass behaves rationally.

Another scheme for indicating inclinations, which is not subject to the above objections, is to photograph the horizon either on a separate film or on the same sensitive surface, simultaneously with the principal exposure. The difficulty here is the practical one that it is only feasible in localities of great atmospheric clearness. Ordinarily, especially anywhere near the sea-coast, the horizon is too rarely seen to be a reliable mark (Fig. 4). It is possible, however, that this objection could be overcome by the use of specially red sensitive plates and suitable color filters, as discussed in the chapter on “Filters.” The method would in any case be useless in mountainous country.

The difficulties discussed with reference to direction indicating instruments of course do not hold with the altimeter. Ordinarily, though, the altitude changes slowly enough to permit of sufficiently accurate records being made by pencil and pad. For high precision map making a photographic record of altimeter readings has a legitimate claim. As we have seen, a small altimeter is incorporated in the English F camera, but the bulk which a really precision altimeter would assume would be a bar to its use in this way. A time or serial number record on the plate or film, synchronized with a similar record on the film of an auxiliary camera which photographs the altimeter and other instruments, may be the simplest way to preserve the majority of the desired data.

=Devices for Heating the Camera.=—Parts of the camera mechanism which depend on the uniformity of action of springs or upon adequate lubrication are susceptible to change with variation of temperature. At high altitudes low temperatures are met which may freeze ordinary machine oils or may cause springs to seriously alter their tension, even to break. To meet this difficulty, and probably also to dispel the occasional condensation of moisture on the optical parts, the German cameras are equipped with an electrical heating coil placed just below the shutter, and arranged to connect with the general heating and lighting current of the plane. Two contacts are ordinarily provided, for offsetting the effects of temperatures of -15 and -30 degrees centigrade. An additional function of this heating coil is perhaps to maintain the sensitiveness of the plates or film.

III THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS