Nikon D300 User Manual (en)

Auto ISO Sensitivity Control
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 
e and g, sensitivity will only be adjusted if underexposure 
would result at the shutter speed selected for [Minimum shutter 
speed] (
–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. When 
sensitivity is altered from the value selected 
by the user, these indicators blink and the 
altered value is shown in the viewfinder.