Nikon D300 User Manual page 122

If [Off] (the default setting) is chosen for the [ISO sensitivity auto
control] option in the shooting menu, ISO sensitivity will remain
fixed at the value selected by the user (see page 94). If [On] is
chosen, ISO sensitivity will automatically be adjusted if optimal
exposure can not be achieved at the value selected by the user
(flash level is adjusted appropriately). The maximum value for
auto ISO sensitivity can be selected using the [Maximum
sensitivity] option in the [ISO sensitivity auto control] menu (the
minimum value for auto ISO sensitivity is automatically set to ISO
200; note that if the value selected for [Maximum sensitivity] is
lower than the value currently selected for [ISO sensitivity], the
value selected for [Maximum sensitivity] will be used). In exposure
modes P and R, sensitivity will only be adjusted if underexposure
would result at the shutter speed selected for [Minimum shutter
speed] (1/4000-1 s). Slower shutter speeds may be used if optimum
exposure can not be achieved at the ISO sensitivity value selected
for [Maximum sensitivity].
When [On] is selected, the control panel
and viewfinder show ISO-AUTO. W hen
sensitivity is altered from the value selected
by the user, these indicators blink and the
altered value is shown in the viewfinder.
96