October 25, 2014, 10:32:16 AM

Author Topic: 7D and rear curtain flash :(  (Read 8080 times)

Forceflow

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Re: 7D and rear curtain flash :(
« Reply #15 on: May 16, 2013, 01:19:42 AM »
Well, the flash must support second or rear curtain sync too, or it isn't there to use.

And that is the thing I really don't understand. What great magic must the flash know in order to be able to rear-synch? I mean the camera sends the signal to fire the flash before the shutter closes and that's all the flash needs to do. This is especially true for bulb mode since in that case Canon cannot even do it's famed 'I'll send the shutter speed setting to the flash' since the camera doesn't know the shutter length anyway. So it should just send a dumb 'fire now' command.
I have done the combining of flashes that you described but that is pretty much useless for me. I need all my flashes to fire on the rear curtain and not have some of them fire on first curtain and some on the rear curtain. (Well, I guess I could see one or two applications were this would be a fun thing to do, but not for the majority of my pictures)
This is turning out to be much more annoying and certainly a lot more expensive than I thought it would :(
Canon 7D - Canon 50mm 1.8 - Canon 24-70mm 2.8 L - Canon 100-400mm 4.5-5.6 L IS - SIGMA 85mm 1.4 - SIGMA 150mm 2.8 OS Macro - SIGMA 10-20mm 3,5

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Re: 7D and rear curtain flash :(
« Reply #15 on: May 16, 2013, 01:19:42 AM »

Marsu42

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Re: 7D and rear curtain flash :(
« Reply #16 on: May 16, 2013, 04:59:38 AM »
I mean the camera sends the signal to fire the flash before the shutter closes and that's all the flash needs to do.

In ettl mode, the flash sends the command signals with the low-power pre-flash before the shutter *opens* - it probably tells the slave flashes "wait this time, then fire with this strength". The pre-flash isn't only for flash power calculation, that's why you need to have a pre-flash in optical mode even if all flashes are set to manual - and the new rt mode seems to be a 1:1 copy but with radio transmission.

But it is indeed a bit mysterious why slave flashes would need a dedicated "2nd curtain sync" capability for this, probably their fw doesn't allow them to wait for a longer time than 1st curtain sync (mere speculation)

Forceflow

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Re: 7D and rear curtain flash :(
« Reply #17 on: May 16, 2013, 05:14:49 AM »
I mean the camera sends the signal to fire the flash before the shutter closes and that's all the flash needs to do.

In ettl mode, the flash sends the command signals with the low-power pre-flash before the shutter *opens* - it probably tells the slave flashes "wait this time, then fire with this strength". The pre-flash isn't only for flash power calculation, that's why you need to have a pre-flash in optical mode even if all flashes are set to manual - and the new rt mode seems to be a 1:1 copy but with radio transmission.

But it is indeed a bit mysterious why slave flashes would need a dedicated "2nd curtain sync" capability for this, probably their fw doesn't allow them to wait for a longer time than 1st curtain sync (mere speculation)

In non-bulb mode this is true, but in bulb mode it's not. (Since the camera cannot know for how long I will press the shutter) Hence the camera must send a simple 'fire now' command.
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Marsu42

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Re: 7D and rear curtain flash :(
« Reply #18 on: May 16, 2013, 05:19:19 AM »
In non-bulb mode this is true, but in bulb mode it's not. (Since the camera cannot know for how long I will press the shutter) Hence the camera must send a simple 'fire now' command.

Ok, very valid point, the only idea I have about that is that it sends the flash power setting via pre-flash, and then the actual fire signal via dumb flash trigger (slaves fire when the main flash fires). Probably this two-stage version only works for long bulb and not for short x-sync, so it cannot be used for 2nd curtain remote...

bycostello

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Re: 7D and rear curtain flash :(
« Reply #19 on: May 16, 2013, 05:49:58 AM »
big failing on canon the 2nd curtain issues

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Re: 7D and rear curtain flash :(
« Reply #19 on: May 16, 2013, 05:49:58 AM »