Take it away

The ability to fire Speedlights off-camera with a wireless trigger and optical slave mode is always useful…

Go wireless

You need a wireless trigger, with the transmitter on the camera’s hotshoe and the receiver attached to the Speedlight. If you don’t have a wireless trigger you could use your camera’s pop-up flash in Commander mode, but this relies on line-of-sight communication.

Set Slave mode

A wireless trigger isn’t necessary to fire the second flash. Instead you can set the flash to Optical Slave mode so it’ll fire when it detects the burst from the first flash. On our Nikon SB-910, first we set the switch to Remote, then we set it to the Optical Slave mode, which is called SU-4.