ISO

ISO is the sensor of the camera that controls how sensitive the camera is to light and commonly its range is from 100 to 6400. In Program mode, you control the ISO, and the camera compensates automatically with Aperture and Sutter-speed. In addition ISO affects image noise because the higher the ISO level is, the grainier the image is.

On the left part of the pictures, we can see two photos with an ISO of 100, while on the right part we can see the same photo with an ISO of 6400.