Nikon D300 User Manual page 219

These scenarios present excellent environments for ISO
sensitivity auto control. The camera will use your normal
settings, such as your normal ISO, shutter speed, and
aperture, until the light will not allow those settings to
provide an accurate exposure. Only then will the camera raise
the ISO or lower the shutter speed to keep functioning within
the shutter/aperture parameters you have set.
Look at ISO sensitivity auto control as a failsafe for times
when you must get the shot but have little time to deal with
camera settings, or when you don't want to vary the shutter/
aperture settings but still want to be assured of a well-exposed
image.
Unless you're a private detective shooting handheld telephoto
images from your car or a photojournalist or sports
photographer who must get the shot every time regardless of
maximum quality, I personally would not recommend leaving
ISO sensitivity auto control set to On. Use it only when you
really need to get the shot under any circumstances!
Of course, if you are unsure of how to use the correct ISO for
the light level due to lack of experience, don't be afraid to
experiment with this mode. At the very worst, you might get
noisier than normal images. However, it may not be a good
idea to depend on this mode over the long term because noisy
images are not very nice.
Are there any drawbacks to using ISO
sensitivity auto control?
Maybe! It really depends on how widely the light conditions
will vary when you are shooting. Most of the time, your
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