Nikon D300 User Manual page 217

Note: The other exposure modes, S – Shutter priority and M –
Manual, allow you to control the camera in a way that
overrides certain parts of the ISO sensitivity auto control.
In M – Manual mode the camera completely relinquishes all
control of the shutter and aperture. It can adjust only the ISO
sensitivity by itself, so it can obey the Maximum sensitivity
but the Minimum shutter speed is overridden and does not
apply.
In S – Shutter priority mode the camera can control the
aperture but the shutter speed is controlled only by the camera
user. So, ISO sensitivity auto control can still control the
Maximum sensitivity but has lost control over Minimum
shutter speed.
Also, it may be a good idea to enable High ISO NR—as
discussed a few pages back—when you enable ISO sensitivity
auto control. This is especially true if you leave the camera
set to the default Maximum sensitivity value of 3200.
Otherwise, you may have excessive noise when the light
drops.
When and why should I use ISO
sensitivity auto control?
How much automation do you need to produce consistently
excellent images? Let's explore how and when automatic,
self-adjusting ISO might improve or degrade your images.
What is this feature all about? When and why should I use it?
Are there any compromises in image quality when using this
mode?
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