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Introduction to Slave Flashes
Have you ever found yourself shooting in a situation where you wish you had another light to use or you had one but had no way of triggering it? By making one of your flashes an optical slave, you can free up one of your wireless receivers and add that extra light to your lighting kit. How it works is either your flash has a built in optical slave function or you need to buy a cheap adapter that fits on the bottom of the flash so that it knows when to fire. When optical slaves detect a flash of light from another source (pop-up flash or another strobe), they fire at nearly the same instant. The good news is that if you are within your camera's flash sync speed (x-sync), you won't notice the difference and you are free to use modifiers and diffusers with it! This is usually anywhere from 1/160 to 1/300 of a second depending on your camera.
Photo by PhyrePh0x.
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How to Use a Slave Flash
Now that you know what a slave flash is, integrating one into your workflow should be pretty simple. The main caveat is that to fire, the optical slave must maintain line of sight (LOS) with another flash (probably one that does have your wireless receiver attached). Sometimes this may not always be possible, but often times you can work around this and position your flashes accordingly. The good thing is you can still be shooting from 100 feet away and all your lights will reliably trigger, granted of course your slaves maintain LOS with another light.
Slave flashes work best indoors or outdoors in close proximity. They are best put on either side of your subject when lighting it from both sides so they catch the burst of light. This way, you guarantee the best reliability of firing. Another way to get around this would be to turn your flash head in the direction you want it to light while keeping the optical eye in LOS with another flash.
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Making Your Flash into an Optical Slave
If you have a high-end Nikon flash, the good news is that you already have one built in. These models are the Nikon Speedlight SB-900, SB-800, SB-26, SB-80DX and the SB-50DX. Some of the Sigma and Lumopro flashes have optical slaves too. Sony and Canon Speedlite flashes have slaves, but they only fire with certain bodies or same branded flashes acting as a master. If you know of any other flashes, let us know by commenting below!
If your flash is not listed above, then your other option is to pick up one or two optical slaves adapters that fit on the bottom of the flash hot shoe. They work the same way, by triggering the flash to fire when they see a pulse or burst of light. You can find these cheaper on eBay, but B&H has them for $35.00. Again, reliability is not top notch, but for $35, it beats buying another PocketWizard if you don't need one!