IISO Control
Selected Nikon D-SLRs offer auto ISO control, a feature that will maintain a selected shutter speed range. Here's how it works: In aperture-priority operation, for instance, choose ISO control and set a base shutter speed of, for example, 1/250 second—meaning that you don't want the shutter speed to go below that setting. When the light level in the scene requires a shutter speed slower than 1/250 second, the camera will automatically kick up the ISO to maintain that shutter speed.
Along with f/stop and shutter speed, ISO is an important element of exposure control.