What does ISO speed measure in photography?

ISO speed refers to a film or camera sensor's sensitivity to light, making it a crucial aspect of photography. A higher ISO setting indicates increased sensitivity, allowing for better performance in low-light conditions. This capability helps photographers capture images without needing longer exposure times or wider apertures, which can be particularly advantageous in fast-moving or dimly lit environments.

When a photographer selects a higher ISO, they can achieve a brighter image, but it can also introduce noise or graininess, especially in film photography. Understanding ISO is essential for balancing exposure settings (aperture and shutter speed) to achieve the desired photographic results. Additionally, the relationship between ISO, aperture, and shutter speed is foundational in the exposure triangle, which is a critical concept for photographers at all levels.

This understanding is important in acknowledging how the sensitivity to light influences the overall quality and exposure of an image, which is why the correct choice focuses on film and sensor sensitivity.