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600 EX flash not firing at correct time




I've had two 600 ex flashes that I've had for about 1 1/2 years.  WIth one of them, I've had no problems.  With the second one, I have problems with.  It fires, but doesn't sync up with the shutter speed.  Almost like it's firing just before the image is captures.  When I test it out, the flash fires and then directly after I hear the shutter open and close. With my working flash, this isn't happening so I don't think it's a camera setting.  I've reset the flash back to factory settings to make sure I didn't inadvernently change a custom function setting, etc.  Does anyone have idea if I might be overlooking something or what I might do to remedy this?  Thought I would start here before sending it in for repair.


Thank you!



Compare every setting on both units and set the "not right" one to the same settings as the one that works the way you want. 

John Hoffman
Conway, NH

1D X Mark III, M200, Many lenses, Pixma PRO-100, Pixma TR8620a, Lr Classic

I was thinking of doing this...but reset to factory settings.  Thinking that would cover it instead of comparing to each other...but perhaps it's worth a try anyway.  Thanks for the suggestion.


Is the flash that you are having problems with located on the camera?  


The reason I ask is because many newer cameras have in-camera menu control over the external flash and I'm wondering if you set the flash to "disable".


While it seems counterintuitive, it's possible to use a flash attached to the camera body simply as a trigger for an off-camera flash (via optical) and if you set the camera's flash control menu to "disable" the flash, then the on-camera flash will still fire... but it fires before the shutter opens to tell the external flash when it should fire.  When the shutter actually does open, only the external flash fires again.


Tim Campbell
5D III, 5D IV, 60Da



It is on the camera.  I was hoping this might be the cause...but nope.  It was set to enable in the camera's speedlite menu.  When I'm experincing this problem, I have it on either ETTL or Manual.  Now, just a minute ago when I was checking your possible solution, I had it on Multi by mistake (on the flash) and it works together that way.  It also works with the Ext. A and M modes.  I just can't seem to figure out what I'm missing/overlooking.  I tried looking through the two menus of each flash and comparing.  Then my batteries died in my good flash (and I've been negligent on charging my other uncharged ones), so I'm just waiting a few minutes to get those back in and see if I can figure out something there.


Thank you for your suggestion!  

Which camera model is this?


Tim Campbell
5D III, 5D IV, 60Da

5d III.  🙂

In the menu system... First tab is "red camera" icon on the tab and there are 4 pages of this... You want page 1


Find "External Speedlite Control" -- select that (press 'set' to open the sub-menu)

Make sure "Flash firing" is set to "Enable"

I set my "Flash Function Settings"  to "ETTL", wireless mode to the radio icon (looks more like a lollipop icon)

You could set channel to "Auto" (I evaluate channels and pick one, but auto is the default and it should work.)

Critically... Make SURE that the "Master flash firing" is set to "enable"


Note that there is a menu option to "clear flash settings" and you may just want to use that just to clear things out.

There's also a "Clear all Speedlite C.Fn's" (custom functions)


My guess, however, is that "master flash firing" is set to disable or that "flash firing" it set to disable.


If you are in optical wireless (instead of radio wireless) the flash will fire to communicate with the off-camera flash before the shutter opens but won't fire when the shutter actually is open.  Since you have two 600EX-RTs (and you didn't mention anything else) then your preferred mode should be radio mode (the lollipop icon... not the lightning bolt icon.)


Sly Arena's book "The Speedliter's Handbook" is an excellent way to learn all the nuances of the Canon E-TTL system (and his update includes radio flash).  


Tim Campbell
5D III, 5D IV, 60Da

Thank you Tim for all of the info.  This is driving me crazy!


I've verified everything that you mentioned and everything is as you stated.  I also had previously cleared out all of the custom functions...both from the flash itself...and then from the camera's speedlite menu, just to be sure!  


So I should be clear also...that this is when I use them on camera.  Speedlight #1 (working) works great on camera.  Speedlight #2 (not "working" as desired) does not work on camera in ETTL or M.  If I set them up to work together...Speedlight #1 on camera and #2 off camera, they will work together (linked, #1 working as master) and if they are both off camera and triggered by the ST-RT-E3. I just can't get #2 to fire from my camera.  I am ususally using my flashes and these setups most at weddings... But...if I quickly pull out flash #2 to use just on camera, that's when I have the problem.  


Maybe I'm still missing something. I must be! I have benifitted from some of Sly Arena's videos before but never have checked out his book.  


Thanks again for your help thus far.


I once had a really mysterious problem with a speedlite.  The sync was off, and I was getting black bars in the photo from the shutter travel.


In my case, the speedlite had slid backwards in the hotshoe.  Just a fraction of an inch.  Which was enough to fowl up the contacts from making perfect contact.  It still worked, just didn't sync right.


Just something to check.


Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites

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