Nikon D6 Reference Manual page 1111

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When an SC-series 17, 28, or 29 sync cable is used for off-camera flash
photography, correct exposure may not be achieved in i-TTL mode. We
recommend that you select standard i-TTL fill-flash. Take a test shot and
view the results in the monitor.
•
In i-TTL, do not use any form of flash panel (diffusion panel) other than
the flash unit's built-in flash panel or bounce adapters provided with the
flash unit. Using other panels may produce incorrect exposure.
D
Flash Compensation for Optional Flash Units
In i-TTL and auto aperture ( q A) flash control modes, the flash
compensation selected with the optional flash unit or the [Flash control]
item in the photo shooting menu is added to the flash compensation
selected with the W ( M ) button.
A
Modeling Illumination
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Optional CLS-compatible flash units emit a modeling flash when the
camera Pv button is pressed.
•
This feature can be used with Advanced Wireless Lighting to preview
the total lighting effect achieved with multiple flash units.
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Modeling illumination can be turned off by selecting [Off] for Custom
Setting e6 [Modeling flash].
Compatible Flash Units
1059