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Why 600EX II RT / ST-E3-RT Drops Wireless Link


I am posting this information in hopes it is easier to find. It was originally a reply to an earlier post but may not be easily discovered.

I believe I have proven the theory that the Canon ST-E3-RT (ver. 2) / 600EX II RT wireless system is dropping wireless links because of external electromagnetic interference. As I mentioned in an earlier post I ran a number of tests yesterday with the ST-E3-RT sender and three 600EX II RT flashes, trying different channels and ID's. This testing was done in my home office not far from my Wi-Fi router. This home office has a single router and the normal complement of connected wireless devices (a couple of TVs, an Echo Show, one Samsung and one Apple cell phone). In this environment I experienced drops in less than a minute to at most 12 minutes. Some channel/ID combinations were better than others.

This morning I conducted a similar test on the same units. This time, I created a Faraday cage of sorts by placing all units (sender and Flashes) in a metal filing cabinet in my garage. This was in hopes of shielding them from most of whatever electromagnetic interference I have in my home office.

They ran flawlessly for over an hour before the flashes powered down as they were set to do. I then cycled the power on the remote, the three flashes woke up and reconnected and flashed when the test button was pressed. It's been over 30 minutes since the system was powered back up and still no drops. Each flash is in its own group; A, B, and C. A & B are in ETTL mode, and C is in manual mode at 1/128 power.

I believe this series of tests and the experiences that many of you have shared on this forum proves that the Canon flash system is unreliable where low to moderate electromagnetic interference exists. The system appears to be unsuitable for most venues in which it would likely be used where Wi-Fi and cell phones are in the vicinity.

This is an old design, but it's still being sold by Canon. The ST-E3-RT (ver. 2) was introduced a year or so ago and the EL1 & EL5, which use the same wireless protocol and have been reported to also have the same issues, were also introduced a little over a year ago. 

Now the question is, how do we get this information and test results in front of Canon so something might be done about this issue. At the very least they should stop selling this system until the problem is resolved in current production units. It's simply an unreliable system for real-world applications.

PS: As I post this note, it's now been 1 hour and 55 minutes, with no dropped links.

PPS: The flashes went through another auto power off as programmed and were again powered up when the ST-E3-RT was power cycled. Their wireless links restored as expected. I concluded the test after 3-1/2 hours with no dropped wireless links. I then removed the units from the Faraday cage (metal filing cabinet) and returned them to my office where they all lost their links in less than 5 minutes. Again, this points to external interference as the cause in a fairly common home Wi-Fi environment. This system appears not suited for its intended wireless use.



Short summary:

I have exactly the same gear as JLCKJC and exactly the same link problem. I went through a next level step in debug by buying an EL5 and ST E10 to pair up with my R6 mkii to see if the latest greatest had a fix for this obvious and “in your face” deal killer problem. Turns out it fails EXACTLY like the older gear. So, I returned it and then came upon this post. Details of my journey below. 



Thanks for your excellent experiment and write up. I’ve been struggling with my formerly reliable set of three 600ex RT flashes and an ST-E3-RT: precisely your kit. I read a bunch of other posts and had a super unsatisfactory call with Canon wherein they told me that 1. They would not give or SELL me the factory firmware upgrade because the 609EX-RT is no longer supported. 2. They probably would not have done it before either because I only have three units and not 5 like their bulletin about the problem states. They also did not have a Canon repair authorized sevice company or shop to refer me to. This made me think they were just trying to force us to all re-buy all our speedlites. This would make sense if they just wanted to cut complexity in their service (not cost - I’m willing to pay for a fix) and sell more gear (why would we do that given this issue?). 

I called B&H and the person who advised me was unable to sort it as well, but pointed out that pro level solutions from other vendors with wireless absolutely don’t have the issue, which I believe and have witnessed first hand. There are only so many wireless frequency bands allotted to this kind of work and a limited number of semiconductor chips that most companies buy, but Canon seems to have the problem while others don’t. Also, the link initially works and then drops.

The conclusion is that there maybe RFI but others have software that deals with it. Canon could too if they paid attention or effort. I’ve managed large software teams for embedded applications processors (the kind of engineers that do this sort of stuff). The face that the link comes back when power cycles means at the very least the system could poll status every 20 seconds and just trigger the reboot sequence (or part of it) internally. Flush the stacks, clear the flags and go forward - or whatever. Instead, Canon has chosen to not only fail to address this deal killer issue on the legacy gear but even on their newest gear. 

I took my (extensive and boring) debug process one step further by buying an EL5 and an ST E10 to see if Canon had made progress. Since I have an R6 MKii which has flash compatibility issues in its history up until the recent 1.30 firmware update, I thought that this kit, which is listed as compatible with my R6 MKii by Canon) may be better. 

I used the ST E10 as master and the EL5 as slave. It worked for 3 minutes and then dropped. Pretty much like the legacy gear. I used the EL5 as master to trigger my 600EX RT flashes. Link still dropped in three or less minutes. I changed channels and all the other moves we have been doing to exorcise this issue and got no joy. 

I called tech support. No joy. Returned the new gear the day after getting it.


I believe the new gear has the same issue as the old gear. 

I did one last thing to recover some of my $2000 of sunk cost over the years of these Canon speedlites.  Since I typically use them indoors and only have three groups, I reverted to optical trigger. It’s very limiting in function by comparison, but the settings still properly display both in the camera flash function screen (camera:tab3:external speedlite control:flash function settings) and on the Speedlite’s screen. Of course, this is a set back and highlights the reasons I went with RF triggered in the first place. 1. This does mot work well outdoors on a sunny day. 2. The range is limited. Like 90’ specified or so vs almost 300’ if the radio link worked. 3. Need line of sight between the master and slaves. 4. 3 groups vs 5 groups. 5. You lose direct setting if each flash and have to go with ratios of A:B and then power for C. There are other limitations - these are just off the top of my head. Oh- here’s something: the ST E3 RT does not have an optical transmitter so I have to use one of my 600EX RT as the master. I’d not bother getting a used ST E2 as I’m not convinced it will play with the R6 or other newer cams. I don’t wanna even try. 

Anyways, hope this is helpful, and hoping Canon fixes this issue pronto because support long term is one of my main reasons for shooting with Canon vs something else. 

Thanks for the detailed feedback. I hope others will chime in with their experiences now that it's pretty clear what the issue is here. The user manual for the STE-10 states that it's compatible with the following Canon Speedlites: EL-1600EX II-RT600EX-RT, and 430EX III-RT. The 600 series is now out of production, but the other units are relatively new. It is safe to say that other than repackaging, the STE-10 uses the same suspect design (HW and/or FW) and therefore the system still has the same issue of dropping Wi-Fi links when used in the vicinity of other Wi-Fi equipment and cell phones as your additional testing showed. The current Canon flash system is not suitable for its intended use.

Also, thanks for repeating my experiment. It's easy enough to do. I hope others do the same and post their results too. Now, I need to figure out how to design a portable EMI shielded tent to use as a portable photo studio! (patent pending).

As I told the people at Canon CPS…. “Stop it with Optical as a solution.. what year is this?” Laughter in background….


BTW, I repeated your Faraday cage experiment using a microwave with proper safeguards to make sure it didn’t turn on. It’s a pretty great RF cage. Got similar results - the units stay linked in a microwave with the door closed…


I tried another experiment that others have done in other threads - use a Westcott FJ-X3M universal master RF trigger to control the Canon RT flashes. It duplicates the Canon RT protocol on 2.4GHz. Unfortunately, it also duplicated the Canon performance and dropped out after just a couple minutes. Could not make it to 10 minutes - not even close. 

I have a new suspicion about the cause. I was using 2.4GHz wifi a LOT at home and know it was prevalent at many venues in the form of WiFi for laptop and phone use in buildings. I had no noticeable issues for years. Then, the last two or three years, the issues we have documented here rose up to make the gear useless. I don’t think it is the routers, phones and computers as much as it is all the IOT stuff. Think Google, Amazon, Wejo, etc. The Alexa, google, smart switches, etc. They all run on 2.4GHz and there are a LOT more of them. I have maybe 50 in my home. I know folks who have more. Additionally, the wifi modules in those devices tend to be less well built than the ones in our more expensive laptops and phones; certainly not as good as the routers. Specifically, the big expensive network stuff has very sharp spectrum banding - that is, they are within their allocation of the 2.4GHz band (there are a bunch of bands dividing up use of the few hundred megahertz around the 2.4GHz central frequency). All these “mushy” little radio stations tend to be turning on and sending all sorts of messages all the time. 


My guess is that IOT is the cause because of the timing of the proliferation and the issues we have been seeing sort of coinciding when loosely viewed. Because I’m that kind of guy, I’m getting a buddy to loan me a 20GHz spectrum analyzer and a suitable antenna probe. If I find out anything, I’ll post here. Hopefully, if it is something actionable by Canon, I can find a way to show them. 

There's something definitely going on with the Canon flashes.   I also have a large amount of devices (typically 12) in my house connected at any given time to our WiFi network (dual 2.4 GHz and 5 GHz mesh).  No issues with the 600 EX series flashes and first gen ST-E3-RT from years 2014 through around 2021 or so.   But problems did start in the past year or two as others have found.

Back in 2014, I didn't have as many devices as today, however, most of them connected at 2.4 GHz.  Over the years, more and more devices connected instead at 5 GHz.  Thus, the amount of devices in my household connecting today at 2.4 GHz is actually quite low.

I've since moved on to Profoto gear and so far have not experienced any drops in communication.


Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers

Hi, Ricky,

I am sure all your phones, pads, laptops, desktops and your entertainment center gear uses 5GHz WiFi. The thing that I was referring to that causes most of us to have a stunning amount of 2.4GHz are all the home automation toys. These include Ring, Arlo, TP-Link, Blink, Amazon, and many others. If you have a home security system that has WiFi, it is likely using 2.4GHz as well. This is because 2.4GHz penetrates walls better than 5GHz. Once again, your case may be different if you don’t have all the home automation stuff, but if you do, my guess is that you have a ton more 2.4GHz than anything. Also, Bluetooth uses 2.4GHz as well. 

In any case, you seem to be getting the same bad results as the rest of us here, and Canon is notably absent in the discussion ☹️

We don't have much home automation.  Just a few Apple Homepod minis.  Doesn't explain though why all the dropped signals in my back yard.

Hope you're able to figure out the root cause with your various experiments.

Still haven't experienced any drops with the Profoto gear.  So hopefully that is smooth sailing going forward.


Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers

If you still have your canon flash gear would you be prepared to turn off the Apple Minis and repeat the tests? I‘d be very interested in the results🙃