ISO is a standardised measure of a camera sensor's sensitivity to light, determining how much the captured signal is amplified during exposure. A low ISO value ( such as 100 or 200 ) means the sensor is less sensitive, requiring more light for a correct exposure but producing a clean image with minimal noise. A high ISO value ( such as 3200, 6400, or above ) amplifies the signal significantly, allowing shooting in darker conditions but introducing visible grain or digital noise into the resulting image.

The term derives from the International Organisation for Standardisation, which established the standard scales that unified the previously separate ASA and DIN film speed ratings into a single system. In film photography, ISO corresponds to the film stock's light sensitivity: slower films with low ISO values are fine-grained and suited to bright conditions, while faster high-ISO films are more sensitive but grainier. In digital photography and cinematography, ISO controls the electronic amplification of the sensor signal rather than the physical chemistry of film, but the relationship between sensitivity, noise, and exposure remains the same conceptually. The three elements of the exposure triangle ( aperture, shutter speed, and ISO ) each contribute to how much light reaches or is registered by the sensor, and balancing them is fundamental to achieving correct exposure with the desired visual characteristics.

In AI image and video generation, ISO is not a direct parameter but its visual effects are highly relevant to prompt writing. Describing high-ISO characteristics: visible grain, reduced colour saturation, softer shadow detail: communicates an aesthetic of low-light documentary or cinéma vérité. Describing clean, low-ISO qualities evokes daylight shooting or studio-controlled conditions. Referencing film grain as a stylistic quality, or specifying low-light conditions that would require high ISO in practical camera work, helps AI models produce outputs with the intended photographic character.