Nikon D6 Reference Manual page 419

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Enable or disable first-frame exposure lock.
Highlight [First-frame
exposure lock] and press 2 .
- [Off] is recommended if lighting and other conditions will
not change during shooting, [On] when photographing
landscapes and the like under variable lighting.
- Selecting [On] locks exposure at the value for the first
shot, ensuring that all photos have the same exposure.
Large changes in subject brightness during shooting may
however result in apparent variations in exposure. This
can be addressed by selecting [Off].
Varying Focus over a Series of Photos (Focus Shift Shooting)
Highlight an option and
press J .
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