The Nikon Speedlight SB900 can control three remote groups, designated as A, B, and C. Group M is the master flash. There are a total of four channels available.
Press the center of the on/off switch and rotate the switch to the Master position
Press the Select(SEL) button (marked by red arrow) and cycle through the groups until you highlight the M(master channel), You can either cycle through the groups by continuously pressing the SEL button ,or by pressing the SEL button once, then use the command dial to select the group.
In the picture above you can see the different flash modes that can be used. Any of the settings can be combined.
If you turn the master flash to Off / Disabled:
the SB-900 will still
make an small impact on the overall lighting of the shot. Especially if you shoot close up or if you are working with reflective surfaces. The only workaround for this is the Nikon SU-800.
Setting Flash Channel; To change the flash Channel, press the button underneath CH on the LCD(top arrow). You can either cycle through the channels by continuously pressing the button, or by highlighting the channel by pressing the Gh button once, then use the command dial(lower arrow) to select the channel.
The channel of the remote flash must be in the same channel as the master unit, or the remote flash will of course not fire.
Done your SB 900 is now set up as a master wireless flash.
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