How to Use the Nikon SB-900
Now that you've got your Nikon SB-900 professional-grade speedlight, make sure that you get the most out of it with this free photographer's guide, which will walk you through the flash unit's basic features and functionalities.
The DX and FX feature in the SB-900 allows you to use iTTL, iTTL-BL, iTTL-FP, or iTTL-BL-FP more accurately on FX and DX camera bodies. It will automatically switch to DX if you enter the DX Crop Mode on D3 / D3X / D700. SB-900 will automatically get the signal from the camera body and it will switch to either DX or FX. You can manually override that feature if you wish.
The SB-900 have SU-4 Mode, which allows the flash to be triggered wirelessly using onboard flash alone with non-CLS-Compatible digital SLR's. If one wish to prevent the onboard flash from reaching the subject, buy a $7 Nikon SG-3IR Panel and slide it into the flash hot shoe.
On the another hand, the SB-900 could be triggered wirelessly by having SB-400 and / or SB-600 onboard (SB-900 switched to SU-4 Mode) or having SB-800, SB-900, or SU-800 onboard (SB-800 switched to Remote Mode). The SB-900 could be connected and triggered with Pocketwizards (MultiMAX and / or Plus / Plus II) due to built-in external PC terminal.