down for depth of field, since the entire scene is
effectively at infinity. (You may prefer to stop down
one or two f/stops to obtain the critical aperture of the
lens for the sharpest image.) Make necessary exposure
adjustments by varying the shutter speed. With your
lens wide open, you can use the fastest shutter speed that
conditions will allow. Thus you are better able to reduce
image motion.
For air-to-air photography, depth of field may be a
factor you have to contend with, especially when you
are making close-ups. You may have to stop down and
use a slower shutter speed to get the required depth of
field. This is not much of a problem because the photo
"bird" (aircraft in which you are present) and the target,
usually another plane, are flying at about the same speed
and very little movement of the subject is apparent. You
should use the fastest shutter speed possible to
compensate for vibration of the aircraft in which you
are flying.
AERIAL MISSION CALCULATIONS
In the planning stage of an aerial mission, you must
perform several calculations. Careful planning is
important before preflight, so you, the pilot, and the
flight crew know exactly what steps are required to
fulfill mission requirements. The facts and
requirements for a mapping mission consist of the
required scale of the photography and the area to be
photographed. Using this information, you can make
several calculations to determine such factors as
altitude, number of frames per flight line, number of
flight lines, film requirements, and so forth. There are
a number of methods and mathematical formulas that
you can use to arrive at these conclusions. Through
careful calculations, you can determine the following
factors:
Altitude
Focal length
Ground coverage in feet
Number of passes required
Scale
Ground-gained forward
Exposure interval or picture frequency
Ground speed
Shutter speed
Altitude above the terrain
Ground coverage
Exposure interval
Film usage
In vertical photography, the area covered by a
photograph may be limited, particularly when the
photograph is taken from a low altitude. From a high
altitude, a larger area is included, but each part of the
area is recorded smaller. To increase the area covered
at a low altitude, you should make a series of
overlapping photographs, then splice them together to
form one large photograph (a strip or a mosaic). By
doing so, you can cover a large area and the objects on
the ground are reproduced in a relatively large image
size.
NOTE: When working on planning calculations,
recheck your work for accuracy. Certain steps of each
problem are dependent upon other sections of the
problem. An error made early in the calculations causes
errors in the steps that follow.
IFGA FORMULA
One of the most useful formulas for aerial
photography calculations is the IFGA formula. Even
when only one frame is required to capture your subject,
the IFGA formula should be used. For example, when
assigned to take aerial photographs of a new ship, you
should use the IFGA formula to determine the minimum
altitude or distance that is required to fit the ship into
the frame. By knowing the length of the ship, the lens
focal length, and the size of the negative, you can
determine the distance or altitude from the ship. When
the aircraft must fly at a specified altitude, you can
determine the ground coverage by substituting the other
variables in the IFGA formula. Refer to the training
manual Photography (Basic), NAVEDTRA 12700, for
step-by-step application of the IFGA formula.
AERIAL PHOTOGRAPHIC DATA
COMPUTER
An aerial photographic coverage and flight
planning computer, such as the Aerial Photographic
4-18
