Nikon D750 DSLR Camera (Body Only) - Manual (Page 476)

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The Low-Pass Filter
The image sensor that acts as the camera’s picture element is
fitted with a low-pass filter to prevent moiré. If you suspect that
dirt or dust on the filter is appearing in photographs, you can
clean the filter using the
Clean image sensor option in the
setup menu. The filter can be cleaned at any time using the
Clean now option, or cleaning can be performed automatically
when the camera is turned on or off.
❚❚
“Clean Now”
Holding the camera base down, select
Clean image sensor in the setup menu,
then highlight Clean now and press J .
The camera will check the image sensor
and then begin cleaning. Other
operations can not be performed while
cleaning is in progress. Do not remove or
disconnect the power source until
cleaning ends and the setup menu is
displayed.