03-14-2017 02:32 AM - edited 03-14-2017 02:32 AM
I don't know how exactly the 5D senses the flash but most canon DSLRs have a microswitch built into the hot shoe, this is activated by sliding the foot of the flash into the shoe. It doesn't even have to be a flashgun, a flat piece of plastic the right size will activate the switch and as far as the camera is concerned that switch being activated means an external flash is fitted. There is not normally any intelligent sensing for this facility.
The 5D does not work that way. If the flash is mounted and turned off (with charged batteries) it ignores the flash. The odd thing is that in the exact same situation, if the batteries are dead the camera acts as if the flash is on and limits shutter speed to 1/200.
While some people in this thread think that makes perfectly good sense, it does not make ANY sense whatsoever.
03-14-2017 02:50 AM
Strange...I need to test this with my 5DIII tonight. Last night I was shooting with a mounted flash on my 7DII and it didn't behave this way. With the flash mounted but turned off, there was no speed restriction.
I can understand that with the flash turned on - even with dead batteries, the camera still sees it as flash on. The batteries may be low enough not to fire but there is still some power in the system. What I don't get is with it turned off and the camera still thinks the flash is on.
In many cases I mount the flash on my 5DIII camera all the time without turning it on. I can't recall running into this problem. I'll report tonight or later when I have the camera to test it with.
Test this with dead batteries if you have any. If I'm using charged batteries in my flash then my 5DmkIII does not limite shutter speed if the flash is turned of. Just like your 7D. The strange thing is with completely dead batteries my camera limits shutter speed even when the flash is on or off.
I do not have any more dead batteries but I just tried to replicate this behavior with no batteries in the flash and the camera acted as one would expect - no shutter speed limitations whether the flash was on or off. So there is something specific to having dead batteries in there.
On the day in question, when I had dead batteries in my flash, I was able to replicate this several times by taking the flash off my body and putting it back on. Each time I put the flash on my camera with the flash turned off, my shutter speed was limited to 1/200. It's not like this was some bizarre one-time ocurrance.
03-14-2017 03:01 AM
I tested on my 5DIII and with the flash mounted but turned off, it does not get limited. I had 1/500 and it stayed there. As soon as I turned the flash on, Tv went to 1/200. Turned it off, the Tv went back up to 1/500. This was how I remembered it and that how it worked on mine. Not sure what happened to yours.
I don't have any dead batteries sorry so no test but as I stated earlier...dead batteries are relative...there may not be sufficient charge to power the flash but there is still enough juice to tell the camera that the flash is on...no surprise there.