Nikon D810 - Manual (Page 224)

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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).
A 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.
A Metering
The metering areas for FV lock when using optional flash unit are as
follows:
Flash unit Flash mode Metered area
Stand-alone flash unit
i-TTL 6-mm circle in center of frame
AA
Area metered by flash
exposure meter
Used with other flash
units (Advanced
Wireless Lighting)
i-TTL Entire frame
AA
Area metered by flash
exposure meter
A (master
flash)