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| p.1 #10 · Can I modify my flash to act as a wireless transmitter? |
...I understand my 7D will slave other [flashes] but it NEEDS to discharge too (right? sounds like poor engineering to me).
Not poor engineering, but poor understanding on your part of how the system works and what are the full capabilities.
Because the Canon (and other camera company's systems) Master/Slave function is controlled by light pulses (other than the new 600EX-RT, which is radio), the built-in flash or attached Speedlite needs to fire metering and control preflashes.
However, an attached Speedlite or the pop-up can be set not to fire a flash for exposure, and only fire the command-and-control pulses. No need to dial down the power of the exposure flash, because there won't be one when set that way.
As mentioned, if there is a highly reflective surface in the frame, or if the Master flash is very close to the subject, the final "Fire now!" pulse may be visible because it obviously has to fire after the shutter opens in order to sync the Slave flashes. That can be mitigated in two ways: by aiming the Master away from the subject, or by using a visible-light filter that still lets enough of the infrared spectrum through to trigger the Slaves.
I use a four-light setup sometimes, and a three-light setup regularly, and I use the pop-up on my 7D as the Master with excellent results in typical home-sized rooms. (With my 20D I used an Off-camera ETTL cable to let the Speedlite in my soft box act as the Master.)
Using a Speedlite as the Master is an obvious way to aim the flash away from the subject, but here's a way to do it using the pop-up flash that will work in rooms with a light-colored ceiling and walls:
For an IR filter, the Nikon model works on Canons if you don't push it all the way into the shoe (Canons have a pop-up disable switch in the hot shoe), or if you make a cutout in the filter's foot.