How to trigger multiple canon cameras at the same time?
I've done this to combine the video onboard an aircraft with the video shot from the ground, and if both of my cameras were good quality, it worked pretty well. I tried to merge the videos using time codes, but that doesn't work even though they both had GPS- things with identical time-codes just weren't happening at the same time.

So, here's what I do now... (and this is only valid for video for obvious reasons)


Start all the cameras
Before the scene starts, use a clapperboard or your hands to record a sharp, short sound.

a. It is important to be equidistant from the cameras for this to work

b. If all the cameras can also "see" the clapperboard, that's better
After the scene ends, clap it again! This is very important! You may also want to create syncing sounds during the scene if it's a long one.


The last part is to deal with the fact that not all video plays at the correct speed. For example, when I mixed clips shot at 29.97 with clips shot at 30, they didn't line up after a few minutes.

The over-arching point here is, you will need to use something physically "in the video" to sync with, and in my experience syncing time codes or trying to start the cameras simultaneously just doesn't work.