Jason Hoong , Dec 28, 2007; 01:07 a.m.
Hi All,
Has anyone experienced this strange phenomenon: when you set 2nd curtain sync
on the 20D (or the 580EX) the flash will always fire normally (1st curtain)
and then fire another time before the shutter closes (2nd curtain). Shouldn't
the flash fire only once during the 2nd curtain if it is set to fire at 2nd
curtain? When I use only the built-in flash and set 2nd curtain on the 20D
body the problem happens, it is the same if I attach the 580EX and set 2nd
curtain on the flash gun itself. Even setting 2nd curtain on both the 20D and
the 580EX resulted in the same issue, the flash fires twice: at 1st and 2nd
curtain...
Your opinion is greatly appreciated! Thanks!
Glen Flower , Dec 28, 2007; 01:18 a.m.
Not a problem, a feature. Read about preflash.
Colin Southern 
, Dec 28, 2007; 01:19 a.m.
The first flash is what's called the pre-flash - the camera uses it to work out how much umpfh it needs when it's time for the main flash.
Normally the two occus so close together you don't see them (1st curtain sync), but with 2nd curtain sync they become visable.
All quite normal :)
Jason Hoong , Dec 28, 2007; 01:46 a.m.
Hi Glen and Colin, many thanks for the help, the article is good!
I've been shooting with EOS5 and the 1N filmbacks, guess this proves I need to brush up on my digital photo skills :)
Thanks again!
Colin Southern , Dec 28, 2007; 03:16 a.m.
You're very welcome Jason :)
Jameel Khaja , Dec 29, 2007; 10:04 a.m.
Gents -- new wrinkle --
1) What about 2nd curtain sync on the 580EX using the flash off-camera, say using an ST-
E wireless controller?
2) What about 2nd curtain sync using the 580EX on-camera to trigger slave 430 flashes?
I can't make either situation work. The 580 EX seems only to allow 2nd curtain as a choice
when it is on the camera and the master/slave switch is set to "off".
This seems a pity unless I'm missing a relevant rule of physics that makes what I want
impossible. Any thoughts would be appreciated.
Cheers -- jk
Mark U 
, Dec 29, 2007; 10:15 a.m.
If you want to use 2nd curtain with off camera flash (beyond what you can achieve with an offshoe cord) then you need to have the primary flash on the camera operating in 2nd curtain sync with any slave optically triggered. The optical trigger can be a simple version provided you use flash exposure lock to dump the pre-flash - however you will not have E-TTL control over the slaves, so all units might as well be set with manual power settings.
The other solution is to use a sophisticated (and expensive) Pocketwizard setup - some of their gear can be set to use 1st curtain trigger from the body to send a delayed 2nd curtain trigger to a slave.
2nd curtain sync is disabled with wireless E-TTL.
Jameel Khaja , Dec 29, 2007; 10:38 a.m.
Thanks Mark,
So I take this to mean that an onboard 580EX cannot be used at 2nd curtain sync while
triggering a 430 at the same time by the usual Canon wireless master/slave relationship
(which I take you to mean is not "optical").
That seems silly on Canon's part. Any thoughts as to why?
Thanks for the time -- jk
Alistair Windsor , Dec 30, 2007; 08:58 p.m.
The pulsing wireless preflash is used to set the power on the slaves but does not communicate shutter speed information. The main trigger pulse terminates slightly after the opening of the shutter and then the slaves and, possibly, the main flash fire their metered flash amounts. If the shutter were open then the main flash would have to use it's metered pulse to fire the slaves - possible but the metered pulse may not be powerful enough to trigger the slaves and this eliminates the possibility of having the main flash not contribute to the exposure (OK it always contributes but if it is set to OFF then the contribution is very small).
Mark U , Jan 01, 2008; 02:25 p.m.
I think you would need a set of Metz 58 AF-1 flashes to achieve wireless 2nd curtain. Note the claim on p 113 of the manual:
(link)
and also the comment on p 116 that you need to tap the shutter release to cause flash and camera to communicate before 2nd curtain can be set.
