ISO, or International Standards Organization, is the standard used to denote film and digital sensor sensitivity to light. It replaced the older ASA scale but retains the same numeric values. A higher ISO number indicates greater sensitivity, allowing photographers to shoot in lower light, while a lower ISO number produces finer grain or less digital noise.
Understanding ISO is essential for photographers to control exposure, image quality, and noise levels. Adjusting ISO appropriately helps achieve well-exposed images in a variety of lighting conditions, from bright sunlight to dim interiors, and is a fundamental concept in both film and digital photography.