The most important variable for successful outdoor photography is correctly exposing the film to the available light. Thus, natural light varies with season, time of day, cloud cover, humidity, dust, and other ephemeral atmospheric conditions. To achieve gentle illumination and warm colors, professional photographers prefer the lighting conditions just after sunrise or before sunset, when a diffuse golden glow fills the sky. However, typical aerial photography is taken during mid-day hours when the sun is relatively high in the sky, in order to provide for full illumination or toplighting of objects as seen from above. The lens aperture and shutter speed are fundamental controls of how much light reaches the film.
Film speed determines how much light is required for correct exposure of the photographic emulsion.
Each of these factors will be examined in turn. F -stop is thus a fraction. The length of time the shutter remains open is called shutter speed. In other words, changing shutter speed by one interval either doubles or reduces by half the time interval. This factor is known as film speed, which is indicated by a standard ISO rating .