THE CAMERA—GENERAL CONSIDERATIONS
=Chief Uses of an Airplane Camera.=—The kinds of camera suitable for
airplane use and the manner in which they must differ from cameras for
use on the ground are determined by consideration of the nature of the
work they must do. Four kinds of pictures constitute the ordinary
demands upon the aerial photographer. These are single objectives or
_pin points_, _mosaic maps_ of strips of territory or large areas,
_oblique views_, and _stereoscopic views_. Each of these presents its
own peculiar problems influencing camera design.
_Pinpoints_ consist of such objects as gun emplacements, railway
stations, ammunition dumps, and other objects of which photographs of
considerable magnification are desired for study. Here the instrumental
requirements are _sufficient focal length of lens_ to secure an image of
adequate size; _means for pointing_ the camera accurately; enough
_shutter speed_ to counterbalance the speed of the plane; sufficiently
_wide lens aperture_ to give adequate exposure with the shutter speed
required; _means of supporting_ the camera to protect it from the
vibration of the plane.
_Mosaic maps_ are built up from a large number of photographs of
adjacent areas. In addition to the above requirements, mosaic maps
demand _lenses free from distortion_ and covering as large a plate as
possible, in order to keep to a minimum the number of pictures needed to
cover a given area; means for keeping the camera _accurately vertical_,
and means for _changing the plates or films_ and resetting the shutter
rapidly enough to avoid gaps between successive pictures. At low
altitudes and high ground speeds the interval between exposures becomes
a matter of only a few seconds.
_Oblique views_ are made at angles of from 12 to 35 degrees from the
horizontal, usually from comparatively low altitudes. They have been
found to be particularly suitable for the use of men who have no
training in photographic interpretation, being more like the pictures
with which the men are familiar. Distributed among the infantry before
an attack, they have proved indispensable aids to the proper knowledge
of the ground to be covered. The additional requirement here is for
_high shutter speed_ to eliminate the effect of the relatively very
rapid movement of the foreground.
_Stereoscopic views_ are among the most useful of all airplane pictures.
They are made from successive exposures, the separation of the points of
view being obtained not by two lenses at the distance of the eyes apart,
but by the motion of the plane. For this purpose the views should
overlap by at least 60 per cent; this, therefore, requires a very _short
interval between exposures_. For stereo-oblique views this may mean that
they are taken at intervals as short as one or two seconds.
=Chief Differences between Ground and Air Cameras.=—Certain definite
differences are thus seen to stand out between airplane cameras and the
ordinary kind. It is essential that the apparatus for use in the air
shall have high lens and shutter speed, means for rapid changing of
plates, and anti-vibration suspension. Without these features a camera
is of little use for aerial work. These requirements lead inevitably to
greater complexity of design. One simplification over ground cameras,
however, is brought about by the fact that all exposures are made on
objects beyond the practical infinity point of the lens; consequently,
all cameras are fixed focus. This fixed focus feature is a positive
advantage in construction, since it permits of the simple rigid box
form, desirable and necessary to withstand the strains due to the weight
of the lens and the stresses from the plane. But with the abandonment of
all provision for focussing in the air must go special care that the
material used in constructing the camera body is as little subject as
possible to expansion and contraction with temperature, since there is
often a drop of 30 to 40 degrees Centigrade from ground to upper air.
The effect of change of temperature on focus will be treated in the
discussion of lenses.
In addition to these differences, we must keep in mind certain
requirements which are conditioned by the nature and place of aerial
navigation. Thus all mechanical devices which will fail to function at
the low temperatures and pressures met at high altitudes are entirely
unsuitable. Experience has shown, too, that we must avoid all mechanism
depending primarily on _springs_ and on the action of _gravity_.
Vibration, and the motion of the plane in all three dimensions, conspire
to render mechanical motions unreliable when actuated by these agencies.
All plate changing, shutter setting, and exposing operations should be
as nearly as possible positively controlled motions. Because of the cold
of the upper air all knobs, levers and catches must be made extra large
and easy to handle with heavy gloves. Circular knurled heads to such
parts as shutter setting movements are to be avoided in favor of
bat-wing keys or levers. Grooves for the reception of magazines must be
as large and smooth as possible, and guides to facilitate the magazines'
introduction should be provided (Fig. 50). No releases or adjustments
which depend upon hearing or upon a delicate sense of touch are feasible
in airplane apparatus. Wherever possible, large _visible_ indicators of
the stage of the cycle of operations should be provided. Loose parts are
to be shunned, as they are invariably lost in service. Complete
operating instructions should be placed on the apparatus wherever
possible, to minimize the confusion due to changing and uninstructed
personnel.
=The Elements of the Airplane Camera.=—Disregarding its means of
suspension, the airplane camera proper consists essentially of _lens_,
_camera body_, _shutter_, and _plate or film holding and changing box_.
In certain of the aerial cameras developed early in the war all of these
elements were built together in a common enclosure. Later it was
generally recognized that a unit system of interchangeable parts is
preferable. In the case of the lens there arose various requirements for
focal length, from 25 to 120 centimeters, according to the work to be
done. Rather than use an entirely different camera for each different
kind of work, it is better to have lenses of various focal lengths,
mounted in tubes or cones, all built to attach to the same camera body.
In the case of the shutter it is desirable to be able to repair or
calibrate periodically. By making the shutter a removable unit, the
provision of a few spares does away with the need for putting the whole
camera out of commission. Similar considerations hold with reference to
other parts.
A further material advantage that comes from making airplane cameras in
sections is the greater ease with which they are inserted in the plane,
usually through the openings between diagonal cross-wires. It is in fact
only by virtue of this possibility of breaking up into small elements
that some of the larger cameras could be inserted in the common types of
reconnaissance plane. Illustrations of the building up of cameras from
separate removable elements are given in the detailed discussion of the
individual types.
=Types of Airplane Cameras.=—During the course of the war airplane
cameras have been classified on various bases, in different services. In
the French service, where the deMaria type of camera was standardized
early in the war, the usual classification was based on focal length;
thus the standard cameras were spoken of as the 26, the 50 and the 120
(centimeter). A further distinction was then made according to the size
of plate, this being originally 13 × 18 centimeters for the 26
centimeter, and 18 × 24 centimeters for the larger cameras. In the
English service the 4 × 5 inch plate was used almost exclusively, and
their various types of cameras were known by serial letters—C, E, L,
etc. Both these modes of classification became inadequate with the
ultimate agreement to standardize on the 18 × 24 centimeter size for all
plates, and to carry lenses of all focal lengths in interchangeable
elements.
For purposes of description and discussion, it is most convenient to
classify cameras according to their method of operation and the
sensitive material employed. On this basis we may distinguish among
cameras using _plates_ three kinds—_non-automatic cameras_,
_semi-automatic cameras_, and _automatic cameras_. We may similarly
discuss _film cameras_, but having treated the plate cameras
comprehensively, it will be found that the discussion of all types of
film camera can be handled most conveniently by studying the differences
in construction and operation introduced by the characteristics of film
as compared to plates.