Nikon D300 User Manual page 477

If you hold your camera too far away from the white surface
you are photographing, you will get a warning that says, The
selected Dust Off reference photo contains too much dust and
could cause image details to be lost. Do you want to use it
anyway?
When your lens is a distance away from the surface, you
might actually be photographing small dirt on the surface of
your white subject. Those spots can mix in with the dust seen
on the sensor and make an inaccurate Image Dust Off ref
photo. Later, when you use it to remove dust from an image,
it removes the dust found on the sensor as well as all the other
spots that were on the white surface. Those extra spots were
not really on the image, so Capture NX2 will be removing
spots it shouldn't. The image quality may go down from all
the unnecessary re-interpolation.
If you get the warning screen shown in Figure 5-23 that says,
Exposure settings are not appropriate. Change exposure
settings and try again, just go shoot another Image Dust Off
ref photo with a nice bright, clean, white surface. Put the lens
very close to the surface, and make sure it is not in focus. You
might even want to manually set the lens to infinity if you are
having problems with this often.
Once you've found your favorite white or gray surface for
Image Dust Off ref photos, keep it safe and use it
consistently.
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