Understand Slave Flash Control
Mar 16, 2015 10:28:25 #
Garyminor
Loc: Raleigh, NC
I need some help in understanding how the slave flash and the master work together.
I am using a Canon 70D with the built-in flash as master and a 430EX II Speedlite as detached slave.
1) The camera is set to full manual, including ISO.
2) The Menu => Flash control is set for
a. E-TTL II meter => Evaluative
b. Built-in flash settings => Flash mode => E-TTL II
c. Built-in flash settings => Wireless func => Slave:Built-in
d. Built-in flash settings => Slave:Built-in => 8:1
3) The detached 430EX flash is set for Slave, ETTL.
I know that under some conditions, the camera tells the flash to fire a pre-flash that the camera TTL sensor uses to determine the proper flash duration. Then the camera uses the master (built-in) flash to send a message to the slave that determines the slave flash duration.
If I use the flash directed to the subject, without any diffuser or bounce, everything works as expected.
However, I would like to bounce the flash off of the ceiling or adjacent wall, or use a diffuser. I am trying to understand how the flashes are controlled.
Questions:
1) Do both the built-in and the slave pre-fire to evaluate the scene or does only the built-in flash pre-fire? If its only the built-in flash, then a diffuser or bounce flash may not expose properly.
2) If the exposure requires more light than the slave flash can deliver, is the built-in flash power reduced to keep the 8:1 ratio, or does it increase the power to give the right exposure, at a different ratio?
3) Is there any sensor in the 430 EX that controls the exposure similar to the way the cameras TTL sensor works?
4) Is there any source of info that I can access? I have David Buschs Guide to Digital SLR Photography for the 70D.
I could do some experiments, but I havent figured them out yet. Any comments or suggestions are welcome.
Forever Learning, Gary
I am using a Canon 70D with the built-in flash as master and a 430EX II Speedlite as detached slave.
1) The camera is set to full manual, including ISO.
2) The Menu => Flash control is set for
a. E-TTL II meter => Evaluative
b. Built-in flash settings => Flash mode => E-TTL II
c. Built-in flash settings => Wireless func => Slave:Built-in
d. Built-in flash settings => Slave:Built-in => 8:1
3) The detached 430EX flash is set for Slave, ETTL.
I know that under some conditions, the camera tells the flash to fire a pre-flash that the camera TTL sensor uses to determine the proper flash duration. Then the camera uses the master (built-in) flash to send a message to the slave that determines the slave flash duration.
If I use the flash directed to the subject, without any diffuser or bounce, everything works as expected.
However, I would like to bounce the flash off of the ceiling or adjacent wall, or use a diffuser. I am trying to understand how the flashes are controlled.
Questions:
1) Do both the built-in and the slave pre-fire to evaluate the scene or does only the built-in flash pre-fire? If its only the built-in flash, then a diffuser or bounce flash may not expose properly.
2) If the exposure requires more light than the slave flash can deliver, is the built-in flash power reduced to keep the 8:1 ratio, or does it increase the power to give the right exposure, at a different ratio?
3) Is there any sensor in the 430 EX that controls the exposure similar to the way the cameras TTL sensor works?
4) Is there any source of info that I can access? I have David Buschs Guide to Digital SLR Photography for the 70D.
I could do some experiments, but I havent figured them out yet. Any comments or suggestions are welcome.
Forever Learning, Gary
Mar 17, 2015 13:34:41 #
I think the "Slave" function is using the on board flash as fire signal for the remote flash. The remote flash detects the flash of the "Master" on the camera. I think you would have to set up a wireless trigger/receiver for your setup to work in full TTL mode.
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