Film sensitivity standard — originally ASA, now mostly ISO. Determines light required; higher = more sensitive but grainier.
ASA Rating
Film sensitivity determines how strongly your film or sensor reacts to light—and thus, whether you can work in candlelight or in bright sunlight. The ASA rating was the American standard for this for a long time, but it was replaced by the ISO standard in 1987. Today we speak of ISO, but the logic has remained identical. An ASA 100 film is half as sensitive as ASA 200; ASA 400 requires four times less light than ASA 100. In practice, this means: higher value = wider aperture possible, shorter exposure time, less additional lighting needed.
On set, you notice this directly in the lighting setup. With ASA 100 (or ISO 100), you need massive HMI lights or diffusion flags for fill light in dim daylight; with ASA 800, your camera can manage with less light. This sounds practical—and it is. But: every doubling of ASA sensitivity is accompanied by an increase in grain (film) or noise (digital sensors). ASA 100 film material delivers fine, silky grain, ASA 800 becomes grainier, and ASA 1600 even more so. In digital cameras, the effect is similar: ISO 100 delivers clean images, ISO 6400 shows colored noise spots. This isn't always bad—sometimes you want the grain for a specific aesthetic—but it reduces flexibility in editing and color correction.
Your choice of ASA/ISO depends on the shooting context. Low-light scenes (interiors, night, mystical scenes) demand higher values, but cost you image clarity. Daylight shoots with ample controlled lighting can easily run at ISO 100 or 200. Modern cinema often switches mid-scene: you set ISO 400 for one scene, and ISO 200 for the next, which is brighter. Digital cameras do this without changing film; this is their advantage over analog material, where your magazine is set to one ASA.
Remember: ASA and aperture/exposure time are not independent. If your lighting setup is calibrated for ASA 400 and you subsequently switch to ISO 800, you will either need to stop down the aperture or halve the exposure time—otherwise, your image will be overexposed. This is not a theoretical problem; it happens constantly in everyday practice when the material is changed spontaneously or the sun emerges from behind a cloud.