Controlling Canon Speedlites as Slaves
• Part One: 600EX-RT as Radio Slave
• Part Two: Fundamentals of Optical Wireless (this part)
• Part Three: Setting Up Speedlites as Optical Slaves (coming)
[Note: In this series, I am writing about the master/slave system built into Canon Speedlites. There is another type of optical slave that is common in studio lighting. It fires the strobe when it senses a flash of bright light, but all changes to power, sync, etc. must be made manually on the unit.]
The Difference Between Canon Radio and Optical Slaves
Canon offers two ways for Speedlites to communicate—radio and optical wireless. Radio has the advantage of working over longer distances and the ability to communicate through opaque surfaces, like walls and softboxes. Radio wireless was introduced about a year ago with the launch of the 600EX-RT Speedlite and ST-E3-RT Transmitter.
Optical wireless must have a line-of-sight connection between the master and slave—although the master’s signal can bounce around objects and corners if the walls/ceiling are a light color. I once put a slave Speedlite in a shower stall knowing that I could bounce the master’s signal off of the bathroom mirror. Optical wireless works up to about 40’/13m. It cannot go through opaque surfaces–but is happy to fly through windows, which allows you to control slaves outside the room. Optical wireless will also go through translucent fabrics—like shoot-through umbrellas to control slave Speedlites nearby.
Canon’s optical wireless system is robust and reliable—even outdoors in noon sun. The key is to know how to set up the slaves so that they can see the master—which I will cover in a bit.
Speedliter, Know Thy Optical Master
The 600- and 500-series EX Speedlites can be optical masters (as can the macro ring and twin lights). The advantages of using a Speedlite as an optical master (over a pop-up flash as master) include:
- the Speedlite has greater range
- the Speedlite head can be tilted/panned directly at a slave
- the Speedlite can be used as an off-camera master (detailed at the end of this article)
[Note 1: The 600EX-RT can operate in radio OR optical wireless. For insights on radio wireless, read Part One of this series.]
Several Canon cameras now have pop-up flashes that can be used as masters—although their range and flash spread is more limited than a true Speedlite. Still, with a compatible pop-up, it can be very affordable to get started with off-camera Speedliting. Then, when you get the hang of the basics, you can add a second Speedlite and use it either as an on- or off-camera master (discussed below). For novice Speedliters with a pop-up master, I think that the 430EX II Speedlite is a great starter flash to use as a slave.
There is also an optical transmitter, the ST-E2, that was introduced some 20 years ago. It is essentially a pop-up flash tube hidden behind a thick piece of red plastic. There are many reasons why I do not recommend it—which I wrote about here. If you think that you need one, please read that article before you buy.
Speedliter, Know Thy Optical Slave
The Canon optical slave sensor is located on the front of the Speedlite. As shown in the opening photo, it is the black panel above the big, red panel—which is the auto-focus assist lamp. (For my Nikonian friends, your Speedlight’s slave sensor is located on the left side of the flash.)
One key to success with optical wireless is that the slave sensor must face the direction from which the master’s flash will come. So twist the body of your slave Speedlite so that it looks towards the master’s signal and twist/tilt the head so that it faces your subject. If you have only one slave, you can also pan the head of the master so that it fires towards the slave.
How Optical Wireless Works
With optical wireless, the master sends instructions to the slave(s) via a series of pre-flashes that come just before the main flash. These instructions tell the slave what mode to use (ETTL, Manual, or Multi), what power to fire at, and what sync speed to use.
When you make a change to the settings on the master, the instructions will be sent to the slaves when the master next fires.
The slaves will change their settings instantaneously and fire when the master tells them to fire. Once a Speedlite is set-up as a slave (which will be covered in part three), you change the slave’s operation via the master Speedlite. Remember, you won’t see a change on the slave’s settings until the master fires—an expectation that drives many novice Speedliters crazy.
Optimize Optical Wireless by Moving the Master Off-Camera
There are two solid reasons to move the master Speedlite away from the camera on an extra-long ETTL cord:
- to improve the geometry of the set-up so that the slaves can see the master
- to create valuable off-camera light with with master
In terms of geometry, when shooting optical slaves outdoors in bright sunlight, I turn the bodies of the slaves so that their sensors are looking away from the sun. Then I will run the master over to a spot where the slaves will see it and mount it onto a lightstand.
In terms of creating off-camera light with the master, this is largely a matter of economics. If you have more Speedlites than you need for a shoot, you can use one as a disabled master in the hotshoe—which I wrote about here. If you need to get good off-camera light from your master, then feel free to move it away from the camera.
To do this, you need to use an extra-long ETTL cord so that the full communication path is maintained between the camera and the master Speedlite. If you use an old-school PC-sync cord, the off-camera Speedlite will not work as an optical master.