Nikon D500 - Manual (Page 368)

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4
Press J.
The message shown at right will be
displayed in the monitor and a row
of dashes will appear in the control
panel and viewfinder. To restore
normal operation without
inspecting the image sensor, turn
the camera off.
5
Raise the mirror.
Press the shutter-release
button all the way down.
The mirror will be raised
and the shutter curtain
will open, revealing the image sensor. The display in the
viewfinder will turn off and the row of dashes in the control
panel will flash.
6
Examine the image sensor.
Holding the camera so that light falls
on the image sensor, examine the
sensor for dust or lint. If no foreign
objects are present, proceed to Step 8.