One of the questions I often get asked is how to make multiple Speedlites communicate reliably outdoors. Canon’s flash system uses pulses of light to transfer commands from the master flash to the slave flashes. The slaves don’t send communication back, but they do flash and pre-flash as requested by the master. Canon’s manuals give estimates as to how far these coded pulses of light can travel and still keep the communication going, however these can be difficult for photographers to achieve in bright light outdoors.
Pulsed light communication, not infrared
First let me kill one myth, the Canon system does not use infrared light to communicate. That red flashing light you may see is actually helping the camera focus on the subject. It works on the idea that if you are lighting something in the frame with a Speedlite then it might be good for the Speedlite to put a nice contrasty pattern on the subject for the AF system to see.
So what does the slave flash need to see, and where are it’s ‘eyes’. The shiny black plastic window that sits below the flash head and above the one that may flash red patterns is the receiving sensor on a slave flash. This sensor needs to be able to see the pulsed light commands from the master flash. Since most slave flashes have a bounce and swivel flash head you can point the flash head where it needs to go, and then point the receiving ‘eye’ back towards the master flash. This simple trick will get you some extra reliability over longer distances outdoors.
Getting more range and reliability
The big factor to get the maximum operating distance of your master is the strength of the pulsed light that comes from the master flash. All the flashguns that can be masters such as the Speedlite 550EX, 580EX & 580EX II have a zoom flash head. When you put one of these flashes in to master mode it will always zoom the flash head to 24mm as shown. Canon’s engineers thought that by spreading the pulsed light over a wide area there’s more chance for a slave flash to ‘see the light’. The downside is that the wide spread of light is less directional and more importantly less powerful which means outdoors in bright light it has trouble making itself ‘heard’ above the bright daylight. Press a few buttons and set the master flash to zoom it’s head to 105mm and now you have a narrower more concentrated beam of light that will be seen further away in brighter light. Then it’s a simple matter of aiming your new super powerful narrow beam of light towards the slaves. You can’t zoom the head of your on-camera built-in flash master on the EOS 7D and EOS 60D.
If the beam of light is too narrow and you have many slaves spread about then some will not see it. In this case back off the zoom to around 50mm or 70mm. If you can’t visualise how wide 50mm of flash beam is then check out how wide 50mm is on your lens, if you can see all your flashes through the camera viewfinder with the lens at 50mm then 50mm on the flash means they should all see the master flash.
One small drawback of this zoom and precise aim technique is that some photographers find it works when they are setting it and then when they go to take pictures it stops working. You might recall that the flash is on the top of the camera until you start taking portrait pictures then it’s on the side of the camera. In such cases the use of an off-camera cord like the OC-E3 or older OC-E2 and an assistant or light stand will help.