In theory, one should set the ISO/ASA of the camera to match the ISO/ASA of the film one loads into the camera. This is what those just learning how to shoot film should start with.

In practice there are times when one might alter the ISO/ASA setting to be different from the loaded film.

• If a film camera does not have an Exposure Compensation dial, one can use the ISO/ASA setting to effectively dial in EC. If, for example, one has loaded ISO 200 speed film and one desires to underexpose by one stop, it's as simple as changing the ISO/ASA setting to ISO 400, which is one stop faster than ISO 200. For the same film, if we wanted to overexpose by 2/3 stop, we could set the film speed to 125 and the meter would give us a reading that would expose 2/3-stop over for 200 speed film.
• Contrast of B&W film can be increased or decreased by over or underexposing when shooting and then compensating by doing the opposite with development times. For example, if one shoots ISO 400 B&W film at ISO 800, the film will be underexposed by one stop. When the film is developed, increasing the amount of time the film is immersed in developer can compensate, but at the expense of less contrast and more grain. Sometimes this look is desirable. Sometimes it is not desired, but the subjects being photographed and the lighting conditions under which they are photographed necessitate "pushing" film. Sports at night, for instance.

Most film manufacturers publish data sheets for each of their films that outline development times for shooting the film at different speeds as well as for developing the film when it is shot at the advertised sensitivity. They also include data regarding exposures longer than about one second (for most films) that are affected by the Schwarzschild effect, also known as reciprocity failure. Each film has different characteristics, and how much compensation must be made for long exposures can vary significantly from one film to the next.

The thing to remember for "pushing" roll film is that the entire roll will be developed at once, so one is committed with shooting the entire roll at a specific ISO/ASA value.

On the other hand, using the ISO/ASA dial for exposure compensation can be done on a per-shot basis. If I take three or four shots of a snow covered landscape, I can increase the exposure so that the camera's meter is not fooled and the white snow doesn't turn out a shade of medium gray, then set the film speed back to its normal setting for subsequent shots taken later under more normal shooting conditions. One must be careful to remember to return the ISO/ASA dial back to the actual speed of the film after taking the shot, though!

Before 135 film started using DX encoding to allow the camera to automatically set the ISO, many 35mm SLRs had a small holder on the back of the film door that was just the right size to hold the square end of a film box. The ends of film boxes almost always had the type of film and speed printed on it. These were often called "film reminders."