AUTOMATIC AERIAL PLATE CAMERAS
=General Characteristics.=—The ideal in the automatic plate camera is to
provide a mechanism which will not only change the plates and set the
shutter, as does the semi-automatic, but make the exposures as well, at
regular intervals under the control of the operator. Such a wholly
automatic camera would leave the observer entirely free for other
activities than photography and it is to meet this tactically desirable
aim that the war-time striving for automatic cameras was due.
It is obvious that the one essential difference between the automatic
and semi-automatic types lies in the self-contained exposing mechanism
with its device for the timing of the exposures. There is no difficulty
in arranging for the driving power to trip the shutter, but it is no
easy matter to design apparatus which will space the exposures equally,
and at the same time permit of a variation of the interval. It is indeed
the crux of the problem of automatic camera design to provide for the
easy and certain variation of the interval from the two or three seconds
demanded for low stereoscopic views to the minute or more that high
altitude wide angle mapping may permit. This problem is one intimately
bound up with the question of means of power drive and its regulation,
and will be treated in part in that connection. It is to be noted,
however, that there are in general two modes of exposure interval
regulation. One is by variation in the speed at which the whole camera
mechanism is driven. The other is by the mere addition to a
semi-automatic camera of a time controlled release which affects in no
way the speed of the plate changing operation. In many respects the
latter is the best way to make an automatic camera.
While the advantages of automatic cameras are great it must not be
overlooked that a camera which can only be operated automatically is of
limited usefulness. It is not suited for “spotting” at any definite
instant, as, for illustration, at the moment of explosion of a bomb. It
should, therefore, be the aim of the automatic camera designer to so
build the apparatus that it can, at will, be used semi-automatically. In
addition, to meet the contingency of any break-down in the source of
power, the camera should be capable of hand operation, as in the case of
the American semi-automatic deRam. In short, the automatic camera should
not be a separate and different type; it should merely have an
additional method of operation.
Certain desirable mechanical features of all aerial cameras have already
been enumerated. Some of these may be repeated here with the addition of
others peculiar to automatic cameras. As a general caution, mechanical
motions depending on gravity or on springs should be avoided. Movements
adversely affected by low temperatures (20 to 30 degrees below zero,
Centigrade), are unsuitable. All adjustments called for in the air must
be operable by distance controls whose parts are large, rugged, and not
dependent on sound or delicate touch for their correct setting. The
center of gravity of the camera should not change during operation
(important in connection with the problem of suspension). The camera
should work in the oblique as well as in the vertical position. The
power required for operation must not exceed that available on the
plane. Electrical apparatus, for instance, should not demand more than
100 watts.
Any devices which diminish the weight of the camera are particularly
desirable in automatic plate cameras, because of the large number of
exposures which such cameras encourage. For instance, if the plates
could be handled without placing them in metal sheaths we should gain a
substantial reduction in weight (the sheaths weigh nearly as much as the
plates) as well as in the time necessary for handling.
=The Brock Automatic Plate Camera.=—This camera is somewhat similar to
the same designer's film camera, both in shape, in size, and in its
employment of a heavy spring motor for the driving power. It uses 4 × 5
inch plates, and carries a 10 to 12 inch lens.
The plate-changing operation is unique. As shown diagrammatically in
Fig. 52, the unexposed plates are carried in a magazine on top of the
camera, the exposed ones in a magazine inserted in the body of the
camera, directly below the unexposed magazine. The bottom plate of the
exposed pile drops into a sliding frame and is carried along the top of
the camera to the exposing position. After exposure, the plate is
carried back and drops into the receiving magazine. In order for the
plate to fall only the proper distance at each stage of the cycle,
special plate sheaths are necessary. These are cut away to form edge
patterns which clear or engage control fingers so as to ride or fall
through the sliding frame as required.
The camera is entirely automatic in operation. Regulation of the
exposure interval is by a special form of variable length escapement
controlled through a Bowden wire, in a manner parallel to that in the
Brock film camera, described elsewhere. These plate cameras were never
produced in quantity.
=Folmer 13 × 18 Centimeter Automatic Camera.=—This camera, also never
manufactured in quantity, is shown in Fig. 53, and a sketch of its
manner of operation is included in the _ensemble_ of automatic camera
diagrams (Fig. 52). Its most distinctive feature is perhaps the use of a
two compartment magazine. This is similar in form to the one already
described in connection with the hand-held cameras, but larger, to hold
eighteen 13 × 18 centimeter plates. The unexposed plates are placed in
one compartment, and after exposure are shifted to the other. The
transfer is effected by the motion of a rack, which is part of the
magazine and which is driven by a toothed pinion, also part of the
magazine, which in turn engages in a toothed wheel projecting upward
from the camera body. This toothed wheel is turned first in one
direction and then in the other by an arrangement of gears and levers
driven by the source of power, which as shown in Fig. 53 is a wind
turbine connected through a flexible shaft. Operation is either
automatic or semi-automatic as desired, and the camera can be put
through its cycle by hand if necessary.
[Illustration:
FIG. 53.—Folmer 13 × 18 centimeter automatic and semi-automatic plate
camera.]
[Illustration:
FIG. 54.—French model deRam automatic plate camera.]
As with several other designs, the completion of the working model of
this camera occurred after agreements had been reached by the Allies, as
to plate size, standard lens cones, and other features, not easily
incorporated in it, thus making manufacture inadvisable. The validity of
the design for peace-time work is, of course, not affected by this fact.
=The deRam Camera.=—The only completely automatic plate camera actually
produced commercially before the end of hostilities was the French model
deRam (Fig. 54). Its plate-changing action has already been described in
connection with the American semi-automatic model (Figs. 52, 90 and 91).
It differs from the American model in the shutter, which is of the
self-capping variety, carried on a rising and falling frame; and in the
exposing mechanism. The latter embodies a clutch whose point of
attachment to a uniformly rotating disc in the camera is governed
through a Bowden wire, whereby the interval between the plate-changing
operation and the shutter release is varied. The intervals are indicated
by figures on the dial to which the observer's end of the Bowden wire is
attached. The source of power for the camera is a constant speed
propeller. Complete semi-automatic operation is not possible, as an
interval of 1 to 2 seconds elapses between the time a single exposure is
called for and its occurrence. No arrangement is provided for hand
operation.
It will be noted that while this camera is a true automatic apparatus it
does not meet even a majority of the requirements listed above as found
desirable by experience. There exists a great opportunity for designing
and developing an entirely satisfactory automatic plate camera—provided
it is agreed that anything more than semi-automatic operation is ever
advisable for plates.