I'm fairly new to using flash in my photography, so I'm a bit of a beginner when it comes to the finer points.

I have an ST-E3-RT transmitter on my 5D Mark II, and a 600EX-RT Speedlite set as a slave. I have both the transmitter and the flash in ETTL, with everything (I think) set to auto. The flash is in HSS, as well as the transmitter.

It appears that whenever I have the flash speed set to over 1/100 of a second, an exclamation point with a TV appears on the transmitter LCD display.

I read the manual, but I'm still a little confused as to exactly why this happens. When the flash is mounted on the camera directly, I don't have this issue. There's no exclamation point or anything - even with very high shutter speeds (and HSS enabled).

Can anyone explain why this is happening and what I can do to correct it? It doesn't seem right that I'm unable to use even 1/200 shutter speed with the transmitter.


4 Answers 4


There is a fundamental timing issue involved with older cameras not designed with the radio trigger in mind. It does not affect direct connection or the IR/mounted-flash control of remote flashes, so a flash mounted to the camera directly or through an E-TTL cable, or remotely controlled by the pop-up flash or a camera-mounted flash will work at the normal sync speed. It is only in radio mode that there is the possibility of sync problems with older cameras.

The problem is the two-way nature of the communication between the radio controller. In IR (optical) slave mode, the camera/controller is not "aware" of the flashes at the other end. It is kind of obvious when nothing fires, but if you are using multiple flashes in multiple groups, the controller end is really pretty dumb. Note that with the radio system, there are only 16 available device slots (including the controller, so 15 flashes). Each of the flashes being used by the system is not just a receiver, it also sends data back to the controller. (The most obvious example of this is the "all units recycled" indication on the controller unit.) That includes an acknowledgement signal sent during the preflash event. Older cameras do not allow sufficient time for all possible ACK signals to be processed by the controller between the preflash and the shutter actuation, so it is possible that the flashes could fire after the second curtain begins to move.

The reason that you get a warning rather than a complete disabling of higher flash speeds is that if you are using a small number of flashes relatively close to the camera (so there are few return signals to process, and those signals will have a high signal-to-noise ratio), you may find that normal sync speeds, or a 1/3 or 2/3 stop down from normal sync, are possible. Canon is simply unwilling to guarantee that the sync will occur normally at full sync speed with older camera bodies not designed to allow for the return signals.

If you are using more than a couple of flashes at any significant distance (say more than 10m), there is a strong possibility that the flash trigger signal will not be sent before the second curtain starts to move unless the shutter is set to a slower speed than the normal sync. There are too many return signals to process, and they may be more difficult to decode. You will have to test that for yourself. Canon is willing to state categorically that it's unlikely that the flash trigger will occur simultaneously with the movement of the first curtain, which is why HSS is disabled on incompatible bodies.

  • \$\begingroup\$ Excellent answer. This clears everything up - thank you. \$\endgroup\$
    – Cody
    Commented Jan 6, 2013 at 21:50
  • \$\begingroup\$ Hmm. I never had such an issue using my YN-622 radio system with my 5D2 (2008) or 7D (2009) before I moved to the 5D3 (2012) and 7D2 (2014). Now I need to go back and see if it was because I never tried to use HSS or 1/200 second with those cameras. \$\endgroup\$
    – Michael C
    Commented May 31, 2017 at 18:56

Refer to page 19 of your ST-E3-RT manual. It specifies that when used with camera models released prior to 2012 (so, anything but the 1DX and 5D mark III at the time of this post) you lose high speed sync and your max sync speed is "one increment slower" than whatever it would normally be.

The flash sync speed is 1 increment slower

Check the flash sync speed (X = 1/*** sec.) of your camera, and shoot with a shutter speed up to a maximum of 1 stop slower than the flash sync speed (Example: When X = 1/250 sec. to 30 sec.). Also, high-speed sync shooting is not possible. When you set the shutter speed 1 increment slower than the flash sync speed, the warning icon will disappear.

Sucks that you gave Canon a bunch of your money for this feature and this is how they treat you. Could have purchased another flash or a PocketWizard and have been better off.

Edit: Right now the difference in price between the 600-RT and the ST-E3-RT is about $200 for which, if you (or anyone else reading this) bought the flash over the trigger, would gain the benefit of the full sync speed and high speed sync over IR and the benefit of a second flash. Also, a cheap $20 radio trigger would allow for the full sync speed, but at the expense of high speed sync and ETTL.


I don't have your exact setup, but Canon's full frame cameras have a lower flash sync speed than their crop sensor models (7D, 60D, etc.). Usually you can get about 1/180 but there are camera variations and some folks can't get better than 1/150.

The max flash sync speed is simply the speed in which the whole sensor is open at once. At higher speeds, say 1/500, the shutter is a slit that moves across the sensor. The moving slit doesn't work well for flash, as only the portion open in the slit gets exposed.

Sadly, adding radios or IR slaves slows down the process a bit. Which may mean that the 1/150 slows down to 1/120 or worse.

Try using it without the HSS setup. What the HSS does is sets the flash off in many fast, low powered sequences as the shutter slit crosses the sensor. And are you sure the 5Dm2 supports the HSS?

  • \$\begingroup\$ The shutter sync speed of a 5d mark II is 1/200 and it supports high speed sync. Most other Canon cameras (newish DSLRs anyway) have a max sync speed (non-HSS) of either 1/200 or 1/250. \$\endgroup\$
    – tenmiles
    Commented Dec 28, 2012 at 14:10
  • \$\begingroup\$ The quoted speed is 1/200, but variations in manufacturing cause some of them to be slower. \$\endgroup\$ Commented Dec 28, 2012 at 14:19
  • 1
    \$\begingroup\$ I did some poking around and I don't see that adding a trigger, IR or radio, actually slows anything down. As long as you have an intelligent trigger (such as the new ControlTL PocketWizards and the native Canon/Nikon ETTL systems or similar) then HSS is possible. I've only ever heard once, from one of Joe McNally's videos on KelbyTraining.com, that when using a third party triggering system the max sync speed goes from 1/250 to 1/200, though I've never heard that anywhere else or seen any tests on that (and I've used crappy triggers at 1/250 on my camera just fine). \$\endgroup\$
    – tenmiles
    Commented Dec 28, 2012 at 14:27
  • \$\begingroup\$ The Canon 5D and 6D do not support 1/250 sync. The recent APS-C bodies do. On a full frame, the shutter has to be bigger since the sensor is bigger. Hence the slower sync speed. \$\endgroup\$ Commented Dec 28, 2012 at 18:32
  • \$\begingroup\$ @PatFarrell slightly irrelevant here, but only the higher end Canon crop bodies (60D/7D) have 1/250s sync - the 1100D and 550D/600D/650D have 1/200s. \$\endgroup\$
    – Philip Kendall
    Commented Dec 28, 2012 at 22:26

Its a safety feature of the camera so that you won't get photographs with the shutter curtain still blocking the sensor. That's why most sports photographers dont use flash when taking high speed shots.

  • 3
    \$\begingroup\$ The max sync speed is faster than that on the 5dII and high speed sync enables faster shutter speeds as well. This particular problem has to do with an incompatibility between the trigger and the camera and not actually with the sync speed on the camera itself. \$\endgroup\$
    – tenmiles
    Commented Dec 28, 2012 at 14:20
  • \$\begingroup\$ Most sports photographers don't use flash because it doesn't make a difference. They're usually too far away... \$\endgroup\$
    – Cody
    Commented Jan 9, 2013 at 9:21
  • \$\begingroup\$ @Cody It actually can make a significant difference, even at several hundred feet away, when the camera is set to ISO 3200 or similar. It's just that the difference it makes is not a good one (it flattens the image). Many sports leagues don't allow flash photography by credentialed media shooting from the sideline, so for those it is a moot point. \$\endgroup\$
    – Michael C
    Commented May 31, 2017 at 18:48

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