Nikon D90 - Manual (Page 227)

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Acquire dust off reference data.
Press the shutter-release button the rest of the way down to acquire Image Dust
Off reference data. The monitor turns off when the shutter-release button is
pressed. Note that noise reduction will be performed if the subject is poorly lit,
increasing recording times.
If the reference object is too bright or too dark, the camera
may be unable to acquire Image Dust Off reference data
and the message shown at right will be displayed. Choose
another reference object and repeat the process from
step 1.
D Image Dust Off Reference Data
The same reference data can be used for photographs taken with
different lenses or at different apertures. Reference images can not be
viewed using computer imaging software. A grid pattern is displayed
when reference images are viewed on the camera; histograms and
highlights are not displayed.