This section is from the book "Aerial Cameras, Aerial Films, And Film Processing", by Richard W. Underwood.
Aerial cameras are available with focal lengths from about 25 mm to more than a meter. The focal length governs the scale relationship of the actual size of the subject to its image size on the exposed film. For example, a camera with a focal length of 500 mm at an altitude of 1 km would yield an image scale of 1:2000. A ground distance of 100 meters would be 5 centimeters (or one part in 2000) in the photographic image. Longer focal lengths do indeed increase your image size, but not necessarily your ability to detect information.
If you wish to secure photographic coverage of large areas but are not greatly interested in absolute detail, you should use a short-focal-length lens. Because fewer flightlines will be required to obtain the desired coverage, savings can be made in costly aircraft operation hours. Additional small savings result in costs of film and processing. Conversely, if you need to obtain fine resolution, you will require a camera with a longer focal length. In many cases you may be attempting to secure various types of information simultaneously, and therefore both types of cameras will be required.
The conventional aerial camera (150-mm focal length with a 230-mm square format) is considered a wide-angle camera. The cone of light passing through the lens is approximately 90 degrees. The so-called superwide-angle aerial camera with a field of view of about 120 degrees uses the conventional 230-mm square format but has a lens with a focal length of approximately 88 mm. The user should also consider cameras with interchangeable lenses, which provide the ability to use various focal lengths with one camera body.