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.
[Illustration:
FIG. 69.—Bowden wire release in rear cockpit, with rod connected to
similar release for the pilot.]
[Illustration:
FIG. 70.—Bowden wire release with stop watch attached, for use in
timing for overlaps.]
=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.
[Illustration:
FIG. 71.—Diagram of negative lens sight.]
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.
[Illustration:
FIG. 72.—Negative lens and mount, viewed from above.]
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.
[Illustration:
FIG. 73.—Negative lens and mount, side view.]
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).
[Illustration:
FIG. 74.—Diagram of inclinometer used in some German cameras.]
[Illustration:
FIG. 75.—Photograph made with German camera, showing inclinometer
record, four points for locating diameters and center of plate, and
(upper right-hand corner) number of the plate sheath.]
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