EXPOSURE OF AERIAL NEGATIVES
The principal factors governing the length of exposure in the airplane
camera have already been discussed under various headings. These are
briefly, the nature of the aerial landscape, the practically attainable
lens apertures, the form of the camera support, the speed of the plane,
and the characteristics of plates, films and filters. It is convenient
however, to re-assemble this information in one place, in such form as
to apply to the practical problem of determining the exposure to be
given in any specific case.
=Limitations to Exposure.=—In the ordinary photography of stationary
objects, exposure is a variable entirely at the operator's command.
Plates of any speed may be selected, so that attention may be focussed
on latitude, color sensitiveness, and other tone rendering
characteristics. The exposure may be made of a length sufficient to
insure all the useful photographic action lying in the “correct
exposure” portion of the sensitometric curve. The exposure ratio of any
filter it is desired to use is a matter of indifference—its effect on
color rendering need alone be considered.
Airplane photography is sharply distinguished from ground “still”
photography by its severe limitations as to the amount of the exposure.
The actual duration is definitely restricted by the high speed of the
plane. In peace work this can be offset in part by using slower planes
or by flying against the wind. The practical limitation to 1/100 second,
set by war-time requirements as to definition of fine detail, may be
increased to 1/50 of a second, or even more, where mapping of grosser
features is the object. A common, but entirely avoidable limitation, is
that due to vibration of the camera. By proper mounting this may be
entirely overcome, leaving the ground speed of the plane the only source
of exposure-limiting movement. The amount of light reaching the plate
constitutes a primary factor in exposure, and this is a matter of lens
aperture. Generally, lens aperture is smaller the larger the plate
required to be covered, and the greater the focal length. Because of
their larger aperture, the short-focus lenses which will be favored for
peace-time large-area mapping will permit more and longer working days
than have been the rule in long-focus war photography. The necessary use
of filters, particularly at the high altitudes which would be chosen in
mapping, in order to economize in the number of flights needed to cover
a given area, introduces an inevitable decrease in the amount of light
available at the plate, as compared with surface photography under the
same illuminations.
Broadly speaking, it may be said that all the demands made in reference
to aerial photographic exposure work are to _decrease_ the amount of
light reaching the plate. Any surplus offered, as by the midsummer
noon-day sun, must be immediately snapped up, either by decreasing the
exposure to get greater sharpness, or by introducing filters to get
greater photographic contrast. The absolute exposure of the plate tends
to be kept at the irreducible minimum. As already stated, it lies, with
present photographic materials, on the “toe” of the “H & D” curve, just
reaching up into the straight line portion.
=Estimation of Exposure.=—According to the foregoing argument the
problem of estimating an aerial exposure resolves itself largely into
one of deciding how short this may be made. Or, if the light is strong,
whether it is sufficient so that a filter may be introduced without
demanding more than the 1/100 second or thereabouts which is dictated by
the motion of the plane.
Deciding upon exposures in the field has been largely a matter of
experience and judgment. A majority of the cameras in use during the war
were not furnished with shutters calibrated in definite speeds.
Consequently, the sergeant upon whom the decision usually devolved
became a storehouse of knowledge as to the slit widths and tensions
appropriate to each individual camera. This knowledge had to be acquired
from the results of actual photographic reconnaissances, or from special
test flights, both of them wasteful methods. But the chief objection to
this state of affairs lies in the fact that the knowledge thus acquired
is of no use to anyone else, nor is it applicable to other types of
camera.
The first essential to placing exposure estimation upon a sound basis is
therefore an accurate knowledge of shutter performances. Either the
shutter speeds should be placed upon the camera by the manufacturer and
periodically checked, or a regular practice should be followed of
calibrating shutters, either at a base laboratory or even in the field.
Assuming that the speeds of all shutters are accurately known, the
process of estimating the requisite exposure becomes less a matter of
mere guesswork and more nearly a matter of precision. For this purpose
data on the variation of light intensity during the day and during the
year (Figs. 101 and 102) should be taken as a guide. These data refer of
course to visual and not to photographic light, but since it is always
necessary to use color filters, which make the active light of
approximately visual quality, this is no valid objection. The effects of
clouds and mist must of course be learned largely by experience, but
with the above daylight data at hand, anyone in possession of definite
information on the correct exposure with a given plate for a known day
and hour need not go far wrong in estimating exposures at any other time
in definite fractions of a second.
[Illustration:
FIG. 109.—Chart showing aerial exposures for all times of the day and
year. Data on basis of F/5.6 lens, Jougla orthochromatic plate, and
clear sunlight, no filter. Exposures to be doubled and tripled for
overcast and cloudy weather.]
_Exposure data charts._ Fig. 109 shows a chart, prepared in the French
service, indicating aerial exposures for all hours of the day throughout
the year. These are for clear sunlight, for a lens of aperture F/5.6 and
for “ortho” plates without a filter. They are based on what is probably
an over-estimate of the actual speeds given by the French shutters. For
“light” clouds the exposures are to be doubled, for “heavy” clouds
quadrupled, and for forests and dark ground “lengthened.” Charts of this
form should be extremely useful, but they were actually not of great
service because of the prevalent lack of knowledge of true shutter
speeds.
_Exposure meters._ Aerial photography offers an excellent opportunity
for the use of exposure meters, particularly those of the type in which
a sensitive surface is exposed to the light for a measured time
sufficient to darken a predetermined amount. The sensitive paper of the
meter may either be exposed from the ground to the direct light of sun
and sky, or from the plane to the light reflected from the ground. The
first method will give figures subject to some correction for the
character of the ground to be photographed—whether fields, forests, or
snow. The second method is to be preferred where the shutter speed can
be adjusted in the air, according to the indications of the meter, or
where the filter can be selected and put in place during flight. Trials
with a commercial Wynne exposure meter, used in the latter manner, give
as a working figure an exposure of .001 second for each 4½ seconds taken
to darken the sensitometer strip to match the darker comparison patch.
This relation applies to a lens of aperture F/4.5, on Cramer Commercial
Isonon plates without filter.