Q&A: How to Use Your Nikon SB-900 in Optical Slave Mode
A walk-thru on how to access and use the built-in slave in your Nikon SB-900 speedlight, and how to get better results from any slave outdoors, inside.
SU-4 Mode Button Sequence
SU-4 mode is a legacy TTL mode which existed long before Nikon's CLS system. Props to Nikon for continuing to include it. And the day they stop including it is the day I start hoarding old flashes.
By setting your flash into both manual and SU-4 modes, as they say, dis where da magic happen.
Here's how to do it.
1. Press the "OK" button until you see a custom function menu appear. Should take a couple of seconds.
2. Using the command dial, scroll until you get to "SU-4" mode, and press "OK".
3. Use the same dial. scroll to "on." Press "OK" again.
4. Press "Exit" to get out of the menu.
5. Set your selector switch to "Remote".
You are now in SU-4 slaved mode, but you flash may be in either Manual or Automatic firing mode. You almost certainly want Manual mode. Use the "mode" button to get to "M" (or manual) mode.
Now your flash is in manual mode, and set to fire when it sees another flash. You can adjust the power level in manual mode the same way you do it if you were not in SU-4 mode.
Tips on Usage
The slave eye is just below the battery door, and is seen as a small, bright circle in the photo at left. You need to position your flash to where the head points where you want, and the eye points toward the flash which will trip this flash.
(That's why the 180-degree rotation feature is valuable on the SB-900, and why you may want to add the same feature to your SB-800.)
If you read the SB-800 SU-4 tutorial, you know just how good this slave is. I count on them all the time, and they rarely let me down. If I am shooting in a room with all SB-800s, I will usually only take one set of PWs and slave the other flashes. They rock.
Here is how you can improve your already good results when shooting outside.
When you get a misfire, the problem is usually that the slave eye window was seeing bright sun in addition to your other flash's pop. This is easy to fix.
Once you get your lights set, use some gaffer's tape and make a little "porch roof" for your slave eye that will shade the sun but not block the view to the flash being used to trip this one. Your percentages will increase dramatically.
And, Speaking of Master Flash
One common mistake when using SB-800s or SB-900s in SU-4 mode (or any other flash with a good slave, for that matter) is not taking into account any "preflashes" that may be happening before your main exposure.
These slaves are so good they will almost certainly see and preflashes from the master flash and trip your slaved flash, too. And this all happens just a few milliseconds before your exposure, so the flash does not have time to recharge and pop again when you want it to.
The fix is easy -- make sure your "master" flash is not set to "master" mode, which enables a series of TTL-measuring TTL preflashes. You want all flashes in manual, where they will issue just one, pre-set pop. This way all of your flashes will sync together, and exactly when you want.
As long as everything is set to manual, your secondary flash is set to SU-4 mode and the slave eye can see your other flash, you will be amazed at how well it all works together.
And you might even feel good about how much coin you dropped on that spiffy new Nikon SB-900 speedlight.