Your tutorials are the best I have found in 12 years of using a camera! Bert Fedor - Birmingham, AL
Now we're going to look at how you put the SB900 in master mode to use it as the commander--the master flash--in a multi-flash setup where we're using through the lens metering. In our case we're going to use one remote flash and one master flash with through the lens metering in the shooting you'll see us doing in a little while.
So to start out, you want to first put it into master mode by holding down the button in the center of the power button and turning it to the right until it gets to the master position. And now you can see you've got several flashes listed in the column on the left. M at the top is "master," which is this flash, which is going to be on the camera.
And we have A, B and C flashes potentially out there. We're just going to talk about using one, which is our A flash. And you can see right now they're set in TTL mode, which is what we want. But if one of these is not in the mode you want, the way you select them and change their modes, you use the function button one here, at the left side, to switch between them and you can see it just steps from one to the next. I'm going to select the A. And to change its mode, you use the mode button here. So let's say if your A was set in manual mode and you wanted it in TTL, you just select it and you use the mode button until you've got it in TTL mode and then you can push "OK" to lock it in. And in the shooting that we're going to do in a while, we're going to have a master on camera and a remote flash off camera in TTL mode, like this.
Okay, here's one more important thing to know about setting up your master flash. In a lot of situations you don't want the master flash that's on your camera to be putting out light. You only want light to be coming from your remote flash. And the way you turn off the master as a light source is by selecting it, using function button one here, until you have the M--master flash--at the top selected and then you use the mode button to change its mode until it shows these little dashes, which show that it's turned off as a light source. And you push "OK" to lock that in. Now the master flash that's on your camera will not be putting out light. But the remote flash--A--will be working in through the lens metering mode.