a. Any photographer who has tried to duplicate or reshoot a landscape
photograph knows that an exact duplication of the scene is next to impossible.
Factors entering into the difficulty of such an attempt include seasonal and
climatic variations, lighting differences depending on the time of day and/or
natural or man-made changes in vegetation and landmarks. Two photos taken within
minutes of one another will appear different upon close examination...photos
taken hours apart will be so different that even a casual observer will note
(1) Knowing that such a difference will occur in two photos of the same
scene, the Army photographer does all within his power to exaggerate this effect.
Such exaggerated differences result in a class of photographs which the Army
terms "comparative photography."
(2) By definition, comparative photographs are merely two photos taken
with a time lapse between exposures, the amount of time elapsed depending on the
results desired. The amount of elapsed time might be days, weeks or even months.
However, it might be no more than the few seconds required for the photographer
to change over from a panoramic to an infrared emulsion or from a skylight to a G