Nikon D6 Reference Manual page 241

Shutter speed:
A
Extended Shutter Speeds
For shutter speeds as slow as 900 s (15 minutes), select [On] for Custom
Setting d7 [Extended shutter speeds (M)].
A
AF Micro NIKKOR Lenses
Users of external exposure meters need not take the exposure ratio into
account when adjusting aperture via the sub-command dial. Users will
however need to consider the exposure ratio when using the lens
aperture ring.
Exposure-Related Settings (Metering and Exposure Compensation)
189