Nikon D300 User Manual page 475

too much brightness, use a gray surface instead of white.
Most of the time this error is caused by insufficient light.
Figure 5-23. Incorrect exposure
4. If you don't see the screen in Figure 5-23, and the shutter
fires, you have successfully created an Image Dust Off
ref photo. You will find the image shown in Figure 5-24
on your camera's Monitor. A 2-megabyte file is created
on your camera's memory card with an ending of .NDF
instead of the normal .NEF, .TIF, or .JPG. This NDF file
is basically a small database of the millions of clean
pixels in your imaging sensor and a few dirty ones.
(Example filename: DSC_1234.NDF.)
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