In the past I have mentioned that the Morse-code "FIRE!" command used in the Wireless Flash protocol happens after the shutter has already opened, and because the intensity of the Morse-code commands is relatively small, it will probably never be noticeable in your shot. However, there are situations (like when you're shooting up close and wide open) when it will show up, and in the past (like last month) I have always recommended putting an IR filter in front of the controller flash to eliminate this possibility.
While presenting the Nashville seminar, one of the attendees, Rick Werp, also reminded me of a property of High-Speed Synch flash which also can completely eliminate the "FIRE!" command from showing up in your image. And he even created some test shots (which were designed specifically to make the "FIRE!" command look as bright and distracting as possible) showing how well the technique works!
Here's what he did. He had a nice glass sculpture in a display case, with a mirror behind and a piece of glass in front. (I can't think of a better worst-case scenario!) He then took two wireless flashes, and placed one on the glass shelf above (pointing down) and the other on the shelf below (pointing up), to illuminate the sculpture from above and below. The first shot shows the result when shooting at 1/200th of a second (with HSS OFF); as you can see the "FIRE!" command not only can be seen off the rear mirror, but its reflection off the front glass can be seen as well.
The next shot was taken at 1/250th of a second, at which speed the HSS feature automatically turns on:
Why does it work? In HSS mode, the "FIRE!" command actually occurs BEFORE the shutter starts traveling, whereas in normal, non-HSS mode, it waits until the shutter is open completely before issuing the "FIRE!" command. (This cuts down on shutter lag a little bit.) Knowing this fact has enabled this difficult shooting situation to be executed beautifully without the need for infrared filters. Thanks, Rick!
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