Nikon Z9 Mirrorless Camera - Manual (Page 137)

137 Exposure
A Extended Shutter Speeds
For shutter speeds as slow as 900 s (15 minutes), select [ ON ] for Custom
Setting d5 [ Extended shutter speeds (M) ].
A Long Exposures
At speeds slower than 1 s, the shutter speed displayed by the camera may
differ from the actual exposure time. The actual exposure times at shutter
speeds of 15 and 30 seconds, for example, are respectively 16 and 32
seconds. The exposure time will again match the selected shutter speed at
speeds of 60 seconds and slower.
A Auto ISO Sensitivity Control (Mode M)
If auto ISO sensitivity control ( 0 156) is enabled, ISO sensitivity will
automatically be adjusted for optimal exposure at the selected shutter speed
and aperture.