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I recall reading about A, B, and C groups where A:B is adjusted as a ratio but C (meant for backgrounds) is always fixed.

Trying wireless flash with my Canon 70D and two flashes, I don't see any C setting anywhere. Is that not in Canon optical wireless in general, or does it depend on other equipment and settings?

  • Under Built-in Flash Function setting, what Firing group is set? (A+B+C), (A:B), or (A:B C)? – inkista Oct 28 '14 at 23:59
  • I don't see any settings with C in them. "I don't see any C setting anywhere", thus I could not set (A+B+C) or (A:B C) because I don't see those as possibilities. – JDługosz Oct 29 '14 at 1:55
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The 70D unfortunately doesn't support controlling group C directly. It will however fire group C flashes when set to fire ALL. (Source: The manual)

The Canon optical wireless flash system still contains all three groups, it is simply a limitation of the camera body. You need something like the 580EXII to control group C.

  • I'll try using the Metz 58 AF2 as the master on the hot shoe. I wonder if flash units support it in general even as a slave group setting, or if it's rare/expensive? – JDługosz Oct 29 '14 at 21:22
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    @jdlugosz I'd imagine it's relatively common as a slave function, my 430EXII definitely allows you to assign it to group C. From a quick google the Yongnouo YN568EX also seems to support it as a slave. I suspect it is not that common as a master purely for product differentiation reasons. – Richard Smith Oct 30 '14 at 5:41
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The EOS 70D only has ratio control for 2 groups, A&B, however Canon Speedlites can be triggered if those flashes are set to individual slave mode.

Using the optical wireless system a slave can be set to manual mode by pressing and holding down the mode button on the flash for a few seconds. This will put the slave in manual and the group letter will not be displayed. If a Speedlite 600EX-RT or 600EX is used in optical wireless mode the display will show individual slave.

As an individual slave you set the manual power level on the flash. The flash will fire in sync with the other flashes being triggered even if they are using E-TTL automatic flash.

In short a slave set to individual slave mode will react to only one command from a master flash - the "fire now" command. All slaves in any mode, E-TTL or manual react to this command.

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