So, how do you get a bunch of Canon Speedlites to fire…
at the same time that you fire a bunch of other types of flash? Say you have a Quantum Qflash, a Metz 58 AF-1, a LumoPro LP160, and a Calumet Travelite. Then say that you need to take a group portrait of these guys as they are firing and light it with Speedlites. How do you get everyone to fire at once? You use optical slaves…a whole bunch of them. [If you’ve heard the myth that Canon Speedlites don’t work with optical slaves, mark that person down on your holiday card list as a misinformed Speedliter.]
An optical slave is an electronic eye that will trigger a Speedlite when it sees a large burst of light. Since an optical slave literally works at the speed of light, it will fire an off-camera flash at essentially the same time that your on-camera Speedlite fires. Many brands of flash, other than Canon, have optical slaves built in. Since ours do not, we have to add them externally.
The upside of optical slaves:
+ Relatively inexpensive
+ Eliminates cords from running through the frame
+ Enables Speedlite to be concealed within the scene
+ No batteries required
+ Can mix Speedlites and other flashes
The downside of optical slaves:
– Most optical slaves are not Canon-compatible
– Range limited to about 30’/9m
– Must have clear line-of-sight between on-camera flash/trigger and Slave
– Can be fired by other nearby cameras
– Speedlites must be fired in Manual mode, unless you have a $pecial digital slave
Optical Slaves: The Basics
The triggering burst for an optical slave can come from any flash or an infrared trigger mounted to your camera. So, think of the optical slave as a receiver and the flash or infrared trigger as the transmitter.
An optical slave is an inexpensive way to trigger an off-camera Speedlite when you want to avoid the hassle of cords and connections. Keep in mind that the connection is line-of-sight—meaning that you cannot fire optical slaves hidden on the other side of a wall. For that, you have to use a radio trigger. Also, optical slaves won’t set the power of your Speedlite. You’ll have to do that yourself.
Optical Slaves & E-TTL Don’t Mix
There are two other caveats to using optical slaves with Canon Speedlites. The first is that, unless you use a $pecial digital slave, you’ll have to operate your Canon Speedlites in Manual mode. This is because optical slaves don’t think. They just connect the circuit and trigger the flash when a burst of light comes along. When you are shooting E-TTL or with red-eye reduction activated, there is a pre-flash before the actual exposure. The pre-flash causes the optical slave to fire the Speedlite prematurely. All Canon pop-up flashes always emit a pre-flash—except the 7D when in Manual flash mode. So, to get rid of the pre-flash, switch your Speedlite(s) from E-TTL to Manual mode — and don’t use a pop-up flash.
Canon Speedlites Need A Special Optical Slave
The other issue (and this is the reason for the myth) has to do with the working voltage of Canon Speedlites. An optical slave gets its power from the Speedlite rather than from a battery. With Canon Speedlites there is the peculiarity that the voltage does not drop far enough after the flash exposure to release the typical slave circuit. Essentially, the slave thinks that the Speedlite is still firing. With ordinary optical slaves, the result is that they will fire a Speedlite one time and then lock up. Fortunately, there is an optical slave with an added circuit that is made just for Canon Speedlites. And, luckily, this technology adds less than the price of a venti cup of fancy coffee to the cost of the optical slave (about $16 as opposed to $12).
Sonia’s Canon EX-Compatible Optical Slaves
Sonia is a leading manufacturer of optical slaves. As Canonistas, the key for us is to buy the green ones. The standard yellow and orange versions are not for us. The secret to the green slaves is that they have the additional circuit that makes them compatible with the power cycle of Canon Speedlites.
Sonia offers Canon EX-compatible slaves in both mono miniphone and PC-male. You can buy Canon-compatible Sonia slaves here on FlashZebra.com.
Connecting Optical Slaves To Canon Speedlites
580EX II: An optical slave with a PC-male jack can be plugged directly into the PC socket on a 580EX II. This direct connection is the only reason I have slaves with the sometimes-it-works-and-sometimes-it-doesn’t, old-school PC-sync connection. The key to obtaining PC-sync happiness is to carry a little wrench-thingie (a “PC tip conditioner“) that you can use to periodically tighten the flanges on the PC-male fitting.
All other Speedlites: You will need a hotshoe adapter for the flash into which you plug the optical slave. Be sure to get an adapter that has a threaded socket in the bottom so that you can connect it securely to your lightstand (these are called “flash hotshoe adapters” as opposed to “camera hotshoe adapters”). I prefer adapters that have a miniphone socket over those that have just a PC-sync port. The miniphone is a much more reliable connection than the ancient PC-sync. Flash Zebra has a good selection of miniphone hotshoe adapters here.
An infrared trigger is an alternative to firing optical slaves with an on-camera flash. Infrared waves are just below that part of the electromagnetic spectrum that we see as red. While we can’t see infrared, optical slaves can. Infrared triggers work much like a manual flash in your camera’s hotshoe—except that the flash is not seen—which is great if you want to avoid the pitfalls of on-camera flash.
The two downsides to infrared are that infrared is not reliable outdoors on bright / hot days and that infrared has a limited range—approximately 15 to 30 feet. At least the infrared triggers themselves are relatively inexpensive—when compared to the cost of radio triggers.
Flashpoint and Wein are two manufacturers of infrared triggers. The Wein (about $60 here) is roughly twice the price of the Flashpoint and offers more robust construction. If you are just getting started, the Flashpoint (about $30 here) will get the job done nicely. Both activate optical slaves with reliability.
This article excerpted from my to-be-published ‘Speedliter’s Handbook‘, coming November, 2010.