Nikon D90 Manual page 34

34 The Camera
At high ISOs noise can be noticeable in dark areas of a picture.
AUTO ISO
The D90 also offers an Auto ISO option. Auto ISO will automatically
raise or lower the ISO to help you achieve a correct exposure. This
can be a useful setting if you don't want to have to remember to
change the ISO when you go from bright light outdoors to dim light-
ing indoors.
Auto ISO is set through the Shooting Menu. Press the button and
use the multi selector to move down to the Shooting Menu.
There are two other options for auto control: " Maximum sensitivity "
and " Minimum shutter speed. " These settings give you the ability
to set some parameters on the ISO as well as shutter speed, which
makes Auto ISO more useful. Max sensitivity lets you choose the
highest ISO to be used. This is helpful if you don't want the camera
to go to ISO 3200 because of noise/quality issues, instead you can set
it where it works best for you.