noisejammer said:
A lot depends on what you're trying to photograph, how much background light it present and how bright your strobe is.
If you can set the shot up so the cameras are in the dark, the obvious way is to open both apertures and then fire the strobe. If you do this, your images can easily be synchronised to less than a millisecond. Don't control the strobe with either camera - you can use the strobe's light measurement to get the exposure about right
If this isn't practical then you need to define how closely you want the exposure to be fired because the 5D2 and 7D have different delays when you trip the shutter. It certainly helps if you can set the cameras to the same exposure (trade off ISO against f-stop) and run everything in manual mode. You can then use a wired connection and put them in parallel (I think this will work if they are wired together but - some electrical buffering would probably be better.)
Another random thought - if you have an ultra-fast strobe (say a Q-switched laser), don't forget that light travels at a finite speed - this can make a difference. You need to get the distance to the subject the same for all the cameras.
Oh thank you, you make so much sense! I don't have ultrafast strobes, the fastest my speedlights do is 1/20000s. It's fast enough for almost anything but not for freezing bullets of course. I am not good with electronics, I wouldn't know how to wire the parallel without them exploding! There are some millisecond timers on the market. Do you know if it's somehow possible to sync two wired timers? If I set a different delay to each timer (different by the amount of ms that the shutter delay of the two cameras differ) and could sync the time release triggers then it might work. Ambient light won't be a problem, I can get it down to absolute zero (minus light leak from the cameras and the strobes after I put black gaffer tape on all the lights and lcds) if I need to. The problem is that the strobe duration is so much shorter than the shutter speed that the shutter has to be open when the strobe fires, not the other way around. The shutter durations can overlap as much as they want, but can both be open at the same time? My master speedlight has HSS and can sync all the way up to 1/8000 when on the camera (I haven't tried it with a hotshoe extension cable but I doubt it would work). It can only sync up to 1/180 when off the camera. Wait a second...
I just had an idea! Tell me if this would work: I set all the strobes to fire at the command of one camera, the one with
shorter shutter lag. I set the same shutter speed for both cameras. I set all strobes to rear curtain sync. When the 2nd curtain of the faster camera is about to come down, the flash fires.
Shutter lags:
5d2: 206ms
7d: 131ms
Shutter speed: 1/4s (250ms)
5d2 shutter open: from 206ms to 456ms
7d shutter open: from 131ms to 381ms
7d 2nd curtain coming down: at about 375ms, strobes fire
at 375ms, 5d2 shutter still open: check!
I think that would work!
