Nikon 4804 R1 Wireless Close-Up Speedlight System - Manual (Page 98)

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Flash Value Lock (FV Lock)
Available with compatible cameras. You cannot set the FV Lock function
on the SU-800 or SB-R200 directly. Set it on the camera.
Flash Value, or “FV,” is the amount of flash exposure for the subject.
Using FV Lock with compatible cameras, you can lock in the appropriate
flash exposure for the main subject. This flash exposure remains locked
in, even if you change the aperture or composition, or zoom the lens in
and out.
• Available flash modes are i-TTL and Auto Aperture flash.