Nikon Auto-ISO Tips
The Nikon Auto-ISO allows the camera to adjust the ISO for the photographer.
To enable Auto-ISO sensitivity, either go into the shooting menu and select ISO sensitivity settings, or hold the ISO button down and rotate the front command dial. From the menu, the maximum ISO sensitivity can be set. Choose the highest ISO that can be tolerated. This is the highest the camera will go no matter what, even if the minimum shutter speed is set.
The minimum shutter speed is only a suggestion. It will select a shutter speed below this to prevent from underexposing the image. It will only go below this shutter speed as a last resort, i.e. when the aperture is at the maximum in shutter priority AND the maximum ISO sensitivity has been met. The minimum shutter speed can be set to AUTO. This means the camera will select the shutter speed based on focal length. The bias towards slower or faster shutter speeds can be selected.
Like the minimum shutter speed, the ISO setting is only a suggestion when using Auto-ISO. The camera can select a lower ISO, for example, when the shutter has been maxed out or the aperture is at its smallest (minimum). Typically, set the ISO to the minimum native ISO of the camera, i.e. 100 or 64.
If the camera is in aperture priority mode then it will adjust the shutter speed until the minimum shutter speed is met and then it will automatically adjust the ISO to get the correct exposure. The camera will go below the minimum shutter speed to avoid underexposing the image. If the camera is in shutter priority mode then it will automatically adjust the aperture until the maximum (largest) is reached and then it will automatically adjust the ISO to get the correct exposure. It will not go beyond the maximum ISO set and will underexpose if need be. Probably the most useful is in manual mode. If the camera is in manual mode then it can automatically adjust the ISO to match the camera settings and get the correct exposure. The camera will try to prevent over- and under-exposure by adjusting the ISO.
If using a tripod then Auto-ISO most likely is not needed unless trying to freeze action of some sort.