What is meant by focal length?
Focal length is the distance from the middle of the camera lens to the focal plane (i.e. the film). As focal length increases, image distortion decreases. The focal length is precisely measured when the camera is calibrated. Knowledge of the focal length used, along with the altitude of the photographic aircraft, makes it possible to determine the scale of the aerial photograph (assuming land uniformity).
For example, with a camera focal length of six inches, a flight altitude of 6,500 feet above mean sea level, and an average ground elevation of 1,500 feet, the representative fraction would be computed as follows: 0.5 ft. divided by (6,500 ft. - 1,500 ft.) = 1:10,000 scale.