The ISO setting on the Nikon D3100 adjusts the camera’s sensitivity to light. At a higher ISO, you can use a faster shutter speed or a smaller aperture (higher f-stop number) because less light is needed to expose the image.
You can't adjust ISO in Auto and Auto Flash Off exposure modes, and in Guide mode, you can access the ISO setting only if you select certain options as you work through the Guided Menu screens. If you want to play with ISO, switch out of Guide mode.
In any other exposure mode, adjust ISO as follows:
Press the Fn (Function) button while rotating the Command dial. By default, the Fn button on the side of the camera is set to provide quick access to the ISO setting. Just press and hold the button to highlight the ISO setting in the Shooting Info display and then rotate the Command dial to adjust the setting.
Quick Settings screen: Press the Info Edit button to shift from the Shooting Info display to the Quick Settings display. Highlight the ISO setting and press OK to display the options. Choose the desired ISO setting and press OK.
Shooting menu: You also can adjust the ISO through the Shooting menu.
Keep these additional ISO factoids in mind:
Auto ISO in the fully automatic exposure modes: In Auto and Auto Flash Off mode, the camera uses the Auto ISO setting and selects the ISO setting for you. In the Scene modes, you can stick with Auto ISO (the default setting) or select a specific ISO value.
Auto ISO in P, S, A, and M modes: Auto ISO doesn’t appear on the ISO settings list, but you still can enable Auto ISO as sort of a safety net. Here’s how it works: Dial in a specific ISO setting. If the camera decides that it can’t properly expose the image at that ISO given your current aperture and shutter speed, it automatically adjusts ISO as necessary.
To get to this option, select ISO Sensitivity Settings on the Shooting menu and press OK. On the next screen, turn the Auto ISO Sensitivity option to On. The camera will now override your ISO choice when it thinks a proper exposure is not possible with the settings you’ve specified.
Next, use the two options under the Auto ISO Sensitivity setting to tell the camera exactly when it should step in and offer ISO assistance.
To disable Auto ISO override, just reset the Auto ISO Sensitivity option to Off.
ISO value display: Although the Shooting Info screen always displays the current ISO setting, the viewfinder reports the ISO value only when the option is set to Auto. Otherwise, the ISO area of the viewfinder is empty.
Hi 1 and Hi 2: The specific ISO values presented to you range from 100 to 3200. But if you scroll past 3200, you discover two additional settings, Hi 1 and Hi 2, which translate to ISO values of 6400 and 12800, respectively. Choosing ISO 3200 pretty much ensures a noisy image, and shifting to the Hi settings just makes things worse.