Transparency - the clearness of the air can affect necessary exposure. Clear and transparent skies let more light from faint objects get through. Altitude - related to transparency is the altitude of the observing site. The higher we go, the more of the Earth's obscuring atmosphere we get above. This means that if we have two films with the same reciprocity failure and one is ISO 400, and the other is ISO 200, and the ISO 200 speed film needs 10 minutes to record an object, then the ISO 400 speed film will need only 5 minutes to record the same object.
Film speed can also vary to a certain extent depending on the development of the film.
With some films, increasing the development time can increase the speed a modest amount, but with most films we have to use exposure to record faint detail. If the faint shadow area detail is not exposed onto the film to start with, it cannot be developed in later. The contrast of the film can also be changed by varying the development time or developing chemistry with certain films, particularly black and white films such as Kodak's Technical Pan film. Color transparency films can be changed a lesser amount and color negative films the least. Reciprocity failure - Reciprocity describes the films ability to record the same detail when the exposure is varied depending on how much light hits the film.