ISO 100 is a standard for measuring the sensitivity of a camera's sensor to light. It is part of the ISO (International Organization for Standardization) system, which helps photographers determine how well their camera can capture images in various lighting conditions. A lower ISO number, like 100, indicates less sensitivity to light, making it ideal for bright environments. Using ISO 100 can result in high-quality images with minimal noise, which is the grainy effect that can occur at higher ISO settings. Photographers often choose this setting for outdoor photography or well-lit indoor scenes to achieve clear and detailed pictures.