If ISO sensitivity is fixed, you may find it difficult to achieve the desired exposure without varying shutter speed or aperture, particularly in settings with a mix of sun and shade, such as a soccer stadium or baseball diamond during a day game. Enabling auto ISO sensitivity control allows the camera to adjust ISO sensitivity for optimal exposure.
Auto ISO sensitivity control can be turned on or off by holding the ISO button and rotating the sub-command dial. ISO AUTO is displayed when auto ISO sensitivity control is on and ISO when it is off.
To prevent ISO sensitivity going too high, you can choose an upper limit of from ISO 200 to Hi 2 using ISO sensitivity settings > Maximum sensitivity in the photo shooting menu.
ISO sensitivity settings > Minimum shutter speed is used to choose the shutter speed below which auto ISO sensitivity control will kick in to prevent underexposure in modes P and A. Choose from speeds of 1/4000 to 30 s, or select Auto to have the camera choose the minimum shutter speed based on the focal length of the lens.