Nikon SB700 on Nikon D5100 as Commander?
I have a D5100 and I know I can't use it as commander, but the SB700 can act as commander if I mount it on the hot shoe. I plan to buy three manual Yongnuo as slaves, can the SB700 control it remotely or not?
Yes you can use the SB700 as commander, but only with other Nikon commander enabled SB flash units.
Manual slaves will fire when they see another flash, even the built-in flash on your D5100, but you can't control Yongnuo flash output using an SB700 or any other CLS enabled Nikon flash.
No.
If the Yongnuo were iTTL and CLS remote compatible, then yes, the SB700 would control them. The problem is with the Yongnuo flashes not the SB700. For instance, if you used SB700s rather than the Yongnuo units, then the SB700 attached to your camera would control them.
The SB700 can only control flashes in iTTL mode that conform to the NIkon CLS system. Unfortunately, the Yongnuo manual flashes do not have that feature.
If the Yongnou systems truly have a slave mode, they should be able to detect the SB700 going off, and fire on their own. In that case, you don't even need a SB700 as the slaves should see the built-in flash as well. But there's no guarantee of that as I don't use Yongnou flash units - but if they are truly slaves, they should work that way… As that is what slave units are supposed to do.
However, having said all of that, slave units are manually controlled flash units, and you will have to adjust the output of each one on the flash control unit (if possible) to ensure the proper light hits the subject. In other words, the camera does not control the slave output.
Save the money and get 4 Yongnuo flashes instead. The YN flashes do have an optical slave, but that is all the control you get… When to flash. If you use a CLS flash, the YN flashes will go off when the CLS flash is doing the metering pre-flash part, and then not be able to go off when they are supposed to as they recharge.
The SB-700 would set them off properly if you put it in manual mode and set the power yourself, but that's a lot of money for a manual flash.