This might be interesting for people using the EOS 5D with speedlites on or off-camera.
While doing some ‘frozen flash’ images I discovered some unexpected behavior of the EOS 5D:
Some background first:
FP Flash, also known as High Speed Sync and denoted with the (Bolt)H icon is normally used for fill-in flash in bright conditions where your shutter speed exceeds the XSync limit (1/200 on the 5D). That way you can keep using wide apertures to achieve a nice background blur and still flash your subject for fill-in. It actually works by firing a seemingly continuous beam of light for the length of time that the shutter is open (avoiding in that way the banding that you would have otherwise). This feature requires a much higher energy output than normal flash and therefore your flash range is dramatically reduced.
Well, all this background info is to be able to explain the bug in simple terms:
Even when FP Flash mode is active on the flash (H[bolt] icon active on the flash LCD or red led lighted up on the ST-E2), the FP mode should only activate when your shutter speed exceeds XSync speed.
The problem I found is that the 5D is activating the FP mode also when used at XSync speed (1/200). This behaviour is unexpected and certainly undesired, given that it will dramatically affect flash exposures. It reduces the flash range, increases recycle times and consumes batteries faster.
To make it very concrete: If you have FP flash active on the flash and your shutter speed is 1/200, the flash will fire in FP flash mode (also called ‘long burning’), so instead of having a typical flash duration of 1/10000, the flash will fire for 1/200, an exposure 5x longer!
The tricky side of this bug is that there’s no apparent visible difference between normal flash and FP flash. You can only observe this bug by photographing a rapidly moving subject, like running water. In my case I was photographing an hourglass running and noticed the bug when I was unable to freeze the sand grains falling, even with the lowest flash power setting (that is supposed to have a duration of about 1/35000 of a second)
This bug is more likely to annoy you when you less expect it: when you are in a (studio like) controlled environment and your camera is set to manual (1/200 and f-stop to match the desired effect)… the result is that you won’t have enough ‘juice’ from your flash and recycle times will be painstakingly long.
(From my own painful experience…!)
The workaround is pretty straightforward: turn off FP-mode on your flash when you don’t need it!
I reported the bug to Canon support and they confirmed the bug, but have not provided a solution. My guess is that it should be solved with a firmware upgrade.
You see some sample images on these posts (below):
I hope this post saves some other people from the pain of the unknown…