Film speed is a numerical rating used to indicate the sensitivity of a photographic film to light, expressed as an ISO value. The higher the ISO number, the more sensitive the film is to light, and the less exposure it requires to produce a correctly exposed image. Conversely, a lower ISO number indicates a less sensitive film that requires more light to achieve the same result.
Film speed ratings follow a standardised scale established by the International Organisation for Standardisation (ISO), which replaced earlier systems such as ASA and DIN. Common film speed ratings range from ISO 25 or 50 at the slow end, through ISO 100 and 200 for general purpose use, up to ISO 800, 1600, and beyond for low-light and action photography. Each doubling of the ISO value represents a doubling of the film's sensitivity to light, equivalent to one stop of exposure.
The choice of film speed involves a trade-off between sensitivity and image quality. Slower, lower ISO films produce finer grain, richer colours, and greater tonal detail, making them well suited to landscape, studio, and portrait photography where lighting conditions can be controlled. Faster, higher ISO films are more versatile in low light but typically exhibit more visible grain and slightly reduced sharpness.
In digital photography, the ISO rating serves the same fundamental purpose, indicating the sensitivity of the camera's image sensor - typically a CCD or CMOS sensor. Increasing the ISO on a digital camera amplifies the signal from the sensor, allowing shooting in lower light conditions, but introduces digital noise in a similar way to the grain seen in fast film stocks. Understanding and controlling ISO remains one of the three pillars of photographic exposure, alongside aperture and shutter speed.