down for depth of field, since the entire scene iseffectively at infinity. (You may prefer to stop downone or two f/stops to obtain the critical aperture of thelens for the sharpest image.) Make necessary exposureadjustments by varying the shutter speed. With yourlens wide open, you can use the fastest shutter speed thatconditions will allow. Thus you are better able to reduceimage motion.For air-to-air photography, depth of field may be afactor you have to contend with, especially when youare making close-ups. You may have to stop down anduse a slower shutter speed to get the required depth offield. 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 speedand very little movement of the subject is apparent. Youshould use the fastest shutter speed possible tocompensate for vibration of the aircraft in which youare flying.AERIAL MISSION CALCULATIONSIn the planning stage of an aerial mission, you mustperform several calculations. Careful planning isimportant before preflight, so you, the pilot, and theflight crew know exactly what steps are required tofulfill mission requirements. The facts andrequirements for a mapping mission consist of therequired scale of the photography and the area to bephotographed. Using this information, you can makeseveral calculations to determine such factors asaltitude, number of frames per flight line, number offlight lines, film requirements, and so forth. There area number of methods and mathematical formulas thatyou can use to arrive at these conclusions. Throughcareful calculations, you can determine the followingfactors:AltitudeFocal lengthGround coverage in feetNumber of passes requiredScaleGround-gained forwardExposure interval or picture frequencyGround speedShutter speedAltitude above the terrainGround coverageExposure intervalFilm usageIn vertical photography, the area covered by aphotograph may be limited, particularly when thephotograph is taken from a low altitude. From a highaltitude, a larger area is included, but each part of thearea is recorded smaller. To increase the area coveredat a low altitude, you should make a series ofoverlapping photographs, then splice them together toform one large photograph (a strip or a mosaic). Bydoing so, you can cover a large area and the objects onthe ground are reproduced in a relatively large imagesize.NOTE:When working on planning calculations,recheck your work for accuracy. Certain steps of eachproblem are dependent upon other sections of theproblem. An error made early in the calculations causeserrors in the steps that follow.IFGA FORMULAOne of the most useful formulas for aerialphotography calculations is the IFGA formula. Evenwhen only one frame is required to capture your subject,the IFGA formula should be used. For example, whenassigned to take aerial photographs of a new ship, youshould use the IFGA formula to determine the minimumaltitude or distance that is required to fit the ship intothe frame. By knowing the length of the ship, the lensfocal length, and the size of the negative, you candetermine the distance or altitude from the ship. Whenthe aircraft must fly at a specified altitude, you candetermine the ground coverage by substituting the othervariables in the IFGA formula. Refer to the trainingmanual Photography (Basic), NAVEDTRA 12700, forstep-by-step application of the IFGA formula.AERIAL PHOTOGRAPHIC DATACOMPUTERAn aerial photographic coverage and flightplanning computer, such as the Aerial Photographic4-18