3.
General Factors.
a. Certain rules may be laid down for the shooting of all air photos.
Some
of these are:
(1) Selection of F/stop. The basic exposure: A bright sunny day with an
average subject would be F/16.
Dark subjects might be heavily wooded areas,
while brilliant or bright subjects would be deserts or bodies of water.
(2) Clear, sunny days are desirable for all air photographs in order to
get the greatest detail in the subject. Midmorning or midafternoon are the best
shooting times since the shadows from the subject will fall at good angles for
interpretation purposes. Shadows which are too long may actually obscure detail
or make interpretation confusing. If at all possible, the sun should be behind
and to one side of the camera.
(3) Selection of shutter speed. The fastest shutter speed possible under
the prevailing light conditions should be used. Since the camera will be focused
on infinity, depth of field is no problem, therefore small apertures should be
sacrificed to fast shutter speeds if necessary.
(4) These faster speeds are necessary for two reasons.
Not only is the
aircraft vibrating, causing camera movement, but the ground is also moving in
relation to the camera.
The lower the aircraft's altitude, the faster the
relative ground movement and the greater chance of subject blur.
(5) Panning the camera is another technique to help reduce blurring of the
object due to aircraft movement.
(6) No portion of the camera or the photographer's body from the waist up
should come in contact with the aircraft, since the vibrations of the plane will
be carried through to the film plane.
(7) Selection of lenses.
Normal lenses are best for most Signal Corps
type air photography.
Wide angle lenses produce images that are too small for
practical use, and telephoto lenses, which do give larger images, increase
relative subject movement, making objects is on the ground appear to move even
faster than when viewed with a normal lens.
(8) Filters.
It is a good practice to have a filter on the lens of the
aerial camera at all times, even if the day seems clear.
Even on bright,
cloudless days, atmospheric haze is present. You, as the photographer, may not
detect it, but film emulsions, being more sensitive to blue, will pick up this
detail-obscuring haze.
(a) For
black
and
white
film,
yellow
filters
will
eliminate
some haze, depending upon their degree of saturation.
A dark yellow
(No. 15), the most often-used filter in aerial work, will cut more haze
than the light yellow (No. 6 or f) filters since the No. 15 will eliminate
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