Nikon D300 Complete Manual page 695

followed by the instructions in Step 6 repeated, the
image wasn't good enough; make sure that you've got
enough light and are only seeing the white card and
try again. Otherwise, you'll end up with a reference
photo, which shows up like this on playback:
8. When you edit your image using Capture NX, make
sure the Dust Off tool in the Raw Adjustments
section of the Base Adjustments step is enabled and
that the proper dust off photo is in the same directory
as the one you're trying to correct.
While not perfect, this function does work well enough to
keep your cloning and post-processing fixes to a minimum,
but it's not a replacement for sensor cleaning. You're
sacrificing some detail using this function and dust will
continue to build up on the sensor, which means that, short of
taking a reference photo for every image you make, it may not
correct every defect. Moreover, at some point there will be a
dust particle that resists being corrected in this fashion. I
should also point out that the Capture NX tool has a
maximum number of dust particles it can fix; once your
sensor gets past certain "dustiness," you'll find that Capture
NX refuses to correct images.
Assuming that you have dust on the sensor, there's not a lot
else you can do about it in the field (trying to remove the dust
in an environment where dust may still be present can prove
to be a very futile endeavor). So the dust reference photo
technique is worth using as a stop-gap measure until you can
get back to an environment that is more conducive to
cleaning.
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 695