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.


600EX-RT Speedlite Amazon B&H

ST-E3-RT Speedlite Transmitter Amazon B&H

430EX II Speedlite Amazon B&H

Extra-long ETTL Cords  Amazon  OCF Gear


25 Responses to Controlling Canon Speedlites as Slaves—Part Two: The Fundamentals of Optical Wireless

  1. jmt says:

    I have been completely stymied at learning to use off camera flash — this series is helping me start to edge into understanding some of the equipment I’ve invested in. Thank you so much. Your writing is clear and concise and makes the topic manageable. I can’t wait for the next installment.

  2. Jesse says:

    The 90EX also works as master (the small flash for the M). It is basically a rebel built in flash as an external one. It would probably work well as a small master for 5D or 6D if you didn’t want to carry too much extra weight.

  3. Dave says:

    This is great! The Speedlite 90EX can also be an optical master. Is its range sufficient to be useful as a master?

  4. Jacquelyn says:

    I have a Canon 7D, which works as a master to my 430ex (usually placed off-camera 45 degrees to the subject for the best lighting, but the 430ex isn’t firing when I set up in the evening sun (6pm). I face the sensor toward myself and face the flash toward the subject (obviously). 🙂 Do you have any recommendations for making the flash fire? (I’ve read to buy a cord because the flash isn’t registering due to the sun, or to use it on-camera, which I’d rather not do.)

    • Syl Arena says:

      @Jacquelyn – If, by turning the slave sensor to face the pop-up master, the slave is looking into the sun, then it will not see the master instructions. In this case, an off-camera cord will be a valuable too.

  5. Ron says:

    Great resource! I have a 600EX-RT (Master) and a 580EX II (slave). I can’t seem to get the slave to fire. Best case – when I test fire the master, the slave fires about half a second later. Worst case (and most often), it doesn’t fire at all. I think I’ve got all the settings right (set for optical, not radio; master and slave set accordingly; same channel). Any advice?

    • Syl Arena says:

      @Ron – If you are test firing via the Test button on the 600EX-RT, it is normal for the slaves to fire in delayed mode. When you have multiple slave groups firing, the master will fire all the Group A lights, then all of the Group B lights, and so on. When the shutter is fired, all of the slaves fire simultaneously.

    • Thierry says:

      Hello and thanks for this great information. I have a 580EX and an old 420EX (on a Canon EOS 6D). If I use the 580EX as the master, it triggers well the 420EX as the slave, but the master light happens before the picture is taken. I there a way to have both master and slave to fire at the same time?
      thanks in advance

      • Syl Arena says:

        @Thierry – Study up on the difference between Master Enabled and Master Disabled. See this post >

        You want Master Enabled for the master flash to send instructions to the slave and also fire when the shutter opens.

  6. Bob Sheridan says:

    I currently own an old 550EX speedlite and I was thinking about upgrading to the new 600EX-RT. If I place my new 600EX-RT as Master on my Canon 5D Mk II, will the 550EX slave recognize the master when in optical mode? Or is the 550EX too old?

    • Syl Arena says:

      @Bob – The 600EX-RT will work as an optical master for your 550EX. You’ll really enjoy the new menu system on the 600EX-RT. Much better that the 500-series.

  7. Dusan says:

    Hi. Can you help me to set up my speedlite 600ex-rt (step by step) so I can fire it with my pop up flash on my camera eos400d .
    Thank you.

  8. GaryB says:

    Great article! You mentioned a “Part Three: Setting Up Speedlites as Optical Slaves (coming)”. Is this available somewhere? If it’s here I’m missing it somehow.

    • Syl Arena says:

      Gary – You haven’t missed it…I never got around to getting it online. If you have an immediate question, check out and submit it there.

  9. Daniel Baerwald says:

    Hi Syl:
    I have one phottix strato TTL transmiter(on my Canon 70D) and one receiver with my Speedlite Canon 580EX II. Can I use a Speedlite 430EX II as a Slave? If yes, how do I configure?

  10. Gary G says:


    I tried to find the OCF Gear Smart Eye and it doesn’t seem to be available. However, here is the link to where the Sonia Optical Slave (with the green bottom that works with Canon EX) can be purchased. I ordered 2 of them today so I haven’t tried them yet. They cost $16.00 each plus shipping. Hope they work fine.
    Love your new ‘Speedliters Handbook’. Gary G

  11. Stuart S says:

    Syl, Can I use a PocketWizard Flex TT5 on my Canon 7D Mark II to fire a remotely mounted Canon 600 EX RT on top of another Flex TT5 and then have the 600 use optical triggering to fire an original 580 EX mounted without a radio? Just wondering since the 600 thinks it’s mounted on the camera hotshoe if it will allow me to use optical to fire that slave. If it won’t work I can order a TT1 and use the TT5’s as radio recievers for both flashes.

    • Syl Arena says:

      Stuart – I’ve not yet come upon a way to combine both radio and optical wireless in the same exposure.

  12. Joe Di Santo says:

    I have done a bit of research online but haven’t come up with anything definitive about this. I have the Canon Speedlites 199A and 420EX. Can one of these be used as slave to the other–the latter being mounted on my Canon 5D camera?

  13. John Wong says:

    In a B&H talk, you illustrated multiple speedlites inside an Apollo softbox, Was one set to master and the others on slave for optical triggering? I ask this as a way to set up HSS within the softbox to increase output without straining the resources. Are the slaves able to read the optical command from the bounced lighting within the box?

  14. Richard Hale says:

    I am using a MT-24EX as the master, and 5 other flashes as the slaves for studio “product” shots. One (600) is above front, 2 (580, 550) are either side of the subject and 2 (320s) are set up to flood the background and eliminate shadows. All are diffused. Setting the front above 600 to either channel A or B seems to make the other of the MT’s B or A adjust and fire too strongly. I usually set the other 4 to C but getting a consistent exposure is a problem (ETTL.) Setting the master to manual and forcing the ratio to include C does trigger the slaves, but there is no other adjustment possible. Thoughts? Abandon the MT and use the

    • Syl Arena says:

      Richard – Since your subject and lights are not in motion (meaning the distance between them remains fixed), I would shoot the entire set in Manual mode. ETTL (automatic flash) is helpful when the subject-to-flash distance varies from shot to shot. I think a 3-group setting with the Master in manual mode will be the best solution. When you get everything dialed in, the light will remain constant until you change it again. With Canon Speedlites, the master is always a member of Group A. If you also set the master to disabled (control the slaves, but not fire again when the shutter opens), then you will have three off-camera groups available. Switching the group setting on the 600 slave does not have an affect on the other slaves.

      • Richard Hale says:

        Thanks for the reply Syl, the 600 was effecting the MT-24 A or B side, not the other slaves on C. The manual setting (on the Mt-24) required me to also select the A, B, C setting through the ratio control, but then the +/- control had no further effect, nor did anything on the camera (5D3.)

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