The images are formed through the change in the strength of brightness of the scene upon the film. At first glance, in aerial photography such changes are related only to the change in the type of the land feature. Of course, this hypothesis is an over-simplification of the issue, because there are many factors that are not related to the type or conditions of a land feature and can affect the measurements of the exposure value of the film. Since the above-mentioned factors affect the measurement of exposure but do not play a role in the true changes of the type of land cover due to conditions, they are called "external effects". Here, the major geometric effects that affect the exposure of the film are discussed.