SB-900 SU-4 Optical Slave
SU-4 Mode
Like the SB-800, the SB-900 includes a handy SU-4 mode. This allows the SB-900 to be optically triggered by seeing any other flash, even a non CLS flash. Using the SB-900 in SU-4 / Manual mode allows you to control the output just like any studio flash. However, unless my units are faulty Nikon seems to have forgotten the audio beep function to notify you when the SB-900 has completed the recycle and is ready to fire again. I always liked this on the SB-800 and it's one more backwards step that I hope they will address in a future firmware update.
Access to the SU-4 mode is via the custom functions menu, but given the extra time normally associated with setting up manual mode remotes, that's not such a big deal.
To enable SU-4 mode hold the OK button for 2 seconds, or until the custom functions menu appears. Using the command dial select the SU-4 menu item then press the OK button. Use the command dial to move the cursor to the 'ON' position then press OK. Press EXIT to exit the custom functions menu and enter SU-4 Optical Slave mode.
At this point I recommend setting the SB-900 to manual firing, because TTL is no longer active. Manual mode will provide the most consistent results over the entire power range. Using the MODE button to change the firing mode to Manual (M), and set the output as required. The optical sensor is on the left side of the SB-900, by the battery door, so make sure that the main body is arranged so that this is positioned to receive the light from any other flash.
To exit SU-4 mode simply return to the Custom functions menu (hold down the OK button for 2 seconds), find the SU-4 mode, press OK, select 'OFF', press OK then press EXIT.
Unfortunately, unlike the SB-800, the SB-900 does not emit an audio beep when recycled in SU-4 mode.
To see how to set CLS modes, see the following links:
Setting the SB-900 as a CLS MASTER
Setting the SB-900 as a CLS REMOTE