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.
[Illustration:
FIG. 50.—American model, English “L” type semi-automatic camera.]
[Illustration:
FIG. 51.—Mechanism of “L” camera.]
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.
[Illustration:
FIG. 52.—Diagram of automatic plate camera movements.]
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.