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