2. Built-In Optical SlaveThis off-camera technique doesn't require any additional gear, as well, but your flash must have slave-eye capability. Optical slaves operate by triggering the flash when the slave eye "sees" another flash go off. Most studio packs have a slave eye built into the unit. This allows you to use one transmitter to trigger the flash, and the rest will fire once they see the first flash go off. Output is controlled manually.
Many speedlights also have a slave-eye function. My Nikon SB-900s have a setting called SU-4 mode. This setting puts the flash into a slave-eye mode. You can use either Manual or Auto flash modes with the slave eye. Auto mode enables a flash sensor to meter output for correct exposure; in Manual mode, the flash fires at the output you set.
3. Dedicated CordThe first way I ever used flash off-camera was with a dedicated cable. These cables are inexpensive and maintain complete functionality of the flash as if it was still on the camera hot-shoe. One end of the cable attaches to the hot-shoe, the other end to the flash, and you're ready to go.
One advantage of cables is that you never have to worry about signal interference. I've covered many events walking around with my flash attached to a cable shooting spontaneous moments. When shooting alone, I hold the flash in one hand and the camera in the other. I put my camera on autofocus and in Aperture-priority mode so all I need to do is aim at the subject for correct exposure and focus. It takes a little practice to get your aim down holding the flash in your other hand. When I first started using this technique, my aim was bad and I flashed (blinded) a lot of unsuspecting bystanders.
My Nikon SC-28 cable is nine feet long. I can attach three of these together to shoot further away from my subject. Attach your speedlight to a weighted stand to keep the flash in place when the cables are stretched.
4. Optical TransmitterWhen Nikon first introduced the SU-800 optical transmitter in 2005, my photography world was rocked. This was a revolutionary device that allowed me to trigger my speedlights from the camera, but even more exciting, I could control the output of multiple speedlight groups. Now I could increase or decrease the flash output right from my camera without taking a step. Complex lighting solutions became simple affairs.
Optical transmitters work by sending an infrared signal to the flash at the moment of exposure. Their range varies. Optical transmitters work really well in dark areas and in rooms, but are less effective in bright, sunny conditions where the signal can get interference. Technically, this type of transmitter needs line of sight to work, but I've found my SU-800 works around corners and behind furniture in rooms. Most likely, the signal is bounced around and still hits the flashes.
A big advantage of using a dedicated optical transmitter for your brand of flash is that the speedlights have a receiver installed. Just attach the optical transmitter to your camera, put your flashes in "remote" mode, and you're ready to shoot. No extra receiver accessories are needed. And if your camera has a pop-up flash, you may be able to set it to "commander" mode and use this flash as your optical transmitter. Check your camera manual to see what features your pop-up flash offers. Also, speedlights like the Nikon SB-900 can be set to "commander" mode and used to trigger other speedlights.
Another great feature of a dedicated optical transmitter like the SU-800 is that I can shoot in different flash modes. I often use high-speed sync to darken my backgrounds or help freeze action. Other times, I'll use Manual mode to keep my flash output consistent during a shoot.