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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 25 Jan 2011 (Tuesday) 20:04
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Master flash disabled , only slave firing --> why preflash req?

 
datadump
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Jan 25, 2011 20:04 |  #1

this could be a really basic question. i've always used generic flash with manual output modes and i have never owned two ETTL flashes until now so i'm playing around with ETTL master/slaves.

i have 580EXII as master, controlling a slave 430EX.

if in a case where i just want slave to fire, i disable master 580EX via disabling master per page 39 in the manual. but the master unit still fires... i dont get why?

it says in a small note in the page 39 that "it will still preflash to transmit wireless signals" to slave but i thought that was what the IR was for? so why does it need to send preflash?


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thaking
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Jan 25, 2011 20:06 |  #2

i don't have a setup like this, but my guess is to get ETTL for the slave the preflash is required...




  
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Hoppy1
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Jan 25, 2011 20:22 |  #3

It's not really IR, but semi-IR or near-IR. Which is what the ST-E2 master controller uses, but the 580EXII uses visible light (in multiple morse code like pulses fired at very high speed) that is filtered to near-IR by the slave unit's window.

If you like, in theory you can dim them down to near-IR with a filter but nobody bothers because they all go out before the shutter actually opens so don't effect the exposure.


5D2, 17-40L, 50/1.8, 24-105L, 70-200L 4 IS, 580/270EX, Strato II/RF-602, Elinchroms

  
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datadump
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Jan 25, 2011 20:38 |  #4

thanks for the info. so everything is working ok. surprisingly this wireless system works REALLY well. i thought it was line of site stuff so i always used cheap radio triggers for my sunpak manual flashes... but i finally collected two canon flashes and started working with these and i'm impressed.


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Master flash disabled , only slave firing --> why preflash req?
FORUMS General Gear Talk Flash and Studio Lighting 
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