Film is able to record images because it is sensitive to light. This sensitivity is expressed as an ISO film "speed" number. The higher the number, the "faster" the film, which means the less light it requires to take a picture. Selecting a film speed Slower films in the ISO 100 range are fine for outdoor use in sunlight. But for maximum all-around versatility — fast action, low light outdoors, and indoor photography — we recommend ISO 400 or 800 film. Film speed and Recommended Picture Situations