Below is a collection of frequently asked questions regarding the 7D's new wireless flash control functionality.
I'll try to update this list with any new frequently asked questions down the road to provide a comprehensive resource for the new wireless flash control feature.
Please post any additional questions you may have or revisions to the existing answers.
Last update: 2009/10/16 - V.3 Re-worded to indicate flash always fires.
1. What is wireless ETTL flash control?
From the 7D camera manual:
"The camera's built-in flash can work as a master unit with Canon Speedlites having wireless slave feature and wirelessly trigger the Speedlite to fire".
In a nutshell, the 7D can control your Canon flashes as well as some Canon compatible 3rd party flashes.
2. What external flashes can I use?
Currently, the list of confirmed flashes includes:
- Canon 580 EX II
- Canon 580 EX
- Canon 550 EX
- Canon 430 EX II
- Canon 430 EX
- Canon 420 EX
- Sigma EF 500 DG Super
- Sigma EF 530 DG Super
Flash compatibility is also discussed in this thread:
3. How does it work?
The built-in flash emits a burst of flashes before the cameras shutter is opened to instruct the remote flashes to fire and to communicate the requested flash ratios and measure flash exposure. When the shutter actually opens, the built-in flash will fire again in one of two modes:
1) If the built-in flash iss enabled for the actual exposure, it will fire at a predetermined output level to contribute to the exposure.
2) If the built-in flash is only used to control the external Speedlites, it will fire at a very low power just to finally trigger the external flashes and not to contribute to the final exposure.
4. I only want the external flashes to fire, but my built-in flash still fires?
When you set the camera to only control the external Speedlites, the built-in flash will still fire it's pre-flash to communicate with the external Speedlites. After the preflash, the built-in flash will emit a very low power flash when the shutter opens to signal the external flashes to fire. This sync-flash will not contribute to the final exposure (or minimally so).
5. I thought the flash control uses IR, why do I see the flash?
The built-in flash is not capable of only emitting IR light - unlike Canon's ST-E2 flash controller, which uses IR LED's to communicate with the external flashes. The built-in flash emits the full spectrum of light which includes IR light that is received by the slave flashes. The ST-E2 pulses light in the same manner, you just can't see it.
6. Do I need line of sight to all my flashes?
Yes, but ...
The IR receiver of the flashes needs to be able to "see" the flash emitted by the 7D. But that does not necessarily mean that the camera needs to be aimed directly at the flash. In an outdoor setting, you have to point the camera directly at the flash. In an indoor studio or smaller room however, the pre-flash may be reflected by the walls and ceilings however and still reach the flash even without direct line of sight.
7. Can I use "dumb" optical slaves in combination with this wireless ETTL control feature?
No. Most regular optical slaves will trigger already when they see the pre-flash from the camera. Some optical slaves that are designed specifically to work with the Canon system and ignore the pre-flash may exist however and can be used.
8. Does the 7D support wireless high-speed sync?
No. Unlike the 580 EX II in master mode as well as the ST-E2, the built-in wireless flash control on the 7D cannot enable wireless high-speed sync or 2nd curtain flash.
9. How many flash groups/ratios does the 7D support?
The built-in wireless flash controls supports three flash groups - A, B and C.