Nikon D810 User Manual page 224

A
Using FV Lock with the Built-in Flash
FV lock is only available with the built-in flash when TTL is selected for
Custom Setting e3 (Flash cntrl for built-in flash, 0 331).
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Using FV Lock with Optional Flash Units
FV lock is also available with optional flash units in TTL and (where
supported) monitor pre-flash AA and monitor pre-flash A flash control
modes. Note that when commander mode is selected for Custom
Setting e3 (Flash cntrl for built-in flash, 0 331), you will need to set
the flash control mode for the master or at least one remote group to
TTL or AA.
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Metering
The metering areas for FV lock when using optional flash unit are as
follows:
Flash unit
Stand-alone flash unit
Used with other flash
units (Advanced
Wireless Lighting)
200
Flash mode
i-TTL 6-mm circle in center of frame
Area metered by flash
AA
i-TTL
AA
Area metered by flash
A (master
flash)
Metered area
exposure meter
Entire frame
exposure meter