Proper exposure of backlit subjects can be difficult without using flash. Remember most built in flashes have a working distance of 25 feet or less. Film and Light. Controlling the amount of light that strikes film is critical for good pictures. SLR cameras do this two ways, by modifying the duration and intensity.
Intensity is controlled by a diaphragm inside the lens.
This is called the F stop setting. The main difference between each is the films sensitivity to light. The higher the ISO number, the less light is required to properly expose the film. Therefore the higher the ISO number, the faster the film. 100 speed film is adequate for most situations and it's usually less expensive than faster films.