Definition:
Three things control exposure in a camera: aperture, shutter speed, and ISO. ISO is the third one: the setting that controls the sensitivity of the image sensor to light. Raise it, and you can shoot in darker conditions. Raise it too far, and the image starts to break down into digital noise.
What It Means in Movies:
ISO is one of the fundamental technical controls the camera department works with on every setup. In low light, a DP might push the ISO to capture enough exposure without flooding the space with artificial light, accepting a grainier image in exchange for a more naturalistic look. The relationship between ISO, aperture, and shutter speed is the exposure triangle: change one, and the others need to compensate.
Why It Matters:
ISO is not just a technical number. It has aesthetic consequences. High-ISO grain has been embraced by filmmakers as a texture that adds rawness and authenticity to an image. The decision shapes what the image looks and feels like, which makes it a creative choice as much as a technical one.
Example:
Shooting a night interior scene with minimal practical lighting, a DP pushes the camera ISO to capture enough exposure without introducing artificial sources. The resulting grain is visible but intentional. It makes the scene feel like it is actually happening in a dark room.