What does ISO measure in photography?

ISO measures the sensitivity of the camera's sensor to light, which is a critical aspect of photography. A higher ISO setting increases the sensor's sensitivity, allowing the camera to capture images in lower light conditions without needing longer exposure times or wider apertures. This means that when you increase the ISO setting, you can take photos in darker environments while still achieving a correctly exposed image.

Understanding ISO is essential for photographers, as it directly influences the final image quality. While higher ISO values can help in low-light situations, they may introduce digital noise, which can degrade the overall image quality. In contrast, lower ISO values produce cleaner images with less noise but require more light for proper exposure.

The other choices do not accurately represent what ISO measures. Focal length pertains to the characteristics of the lens, the distance of the subject is related to composition, and the amount of light used during exposure relates to aperture and shutter speed rather than the sensor's sensitivity. Thus, the definition of ISO as the sensitivity of the camera's sensor to light is key to understanding exposure settings in photography.