Nikon D6 Reference Manual page 695

D
i-TTL Flash Control
When an optional flash unit that supports the Nikon Creative Lighting
System is attached and set to TTL, the camera uses monitor pre-flashes for
balanced or standard "i-TTL fill-flash" flash control. i-TTL flash control is
not available with flash units that do not support the Nikon Creative
Lighting System. The camera supports the following types of i-TTL flash
control:
Flash Control
i-TTL balanced
fill-flash
Standard i-TTL
fill-flash
•
Standard i-TTL fill-flash is activated automatically when [Spot
metering] is selected.
The camera uses "i-TTL balanced fill-flash" flash
control for a natural balance between the main
subject and ambient background lighting. After the
shutter-release button is pressed and immediately
before the main flash, the flash unit emits a series of
monitor pre-flashes which the camera uses to
optimize flash output for a balance between the main
subject and ambient background lighting.
Flash output is adjusted to bring lighting in the frame
to a standard level; the brightness of the background
is not taken into account. Recommended for shots in
which the main subject is emphasized at the expense
of background details, or when exposure
compensation is used.
Description
Using an On-Camera Flash
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