Is it possible to trigger a camera from a flash, which is triggered by another flash?

Question

I have a wifi ad-hoc connection to a Canon 6D, that I trigger from my laptop. The Canon is physically connected via sync cable to an Impact brand monolight. The monolight triggers another monolight via a built-in optical slave.

Is it possible for a SECOND camera to be triggered by:
a sync cable between the 2nd monolight and the camera (via hot-shoe sync terminal)
some other device to attach to the hot-shoe,
a built-on optical slave function (camera model?)
or
other

Currently the 2nd camera is a Sony NEX5N, but can be anything, if I need to purchase..

Answer 1

I do not know the specific hardware mentioned, but I will attempt a general answer. The short version is "no".

Optical triggers of most studio flash gear are specifically for the flash itself, and not control and command information. As a counter example, Nikon CLS (Creative Lighting System) uses pre-flash pulses of light to communicate settings ahead of the primary flash. For most "dumb" flash triggers, the actual flash itself, during the camera's exposure, is what triggers the slave. The slave flash can fire quickly enough that the camera's shutter is still open.

A Camera, at least a DSLR with all the mechanics it requires, is not fast enough to fire if triggered by the actual flash output; it would miss the light of the triggering flash. For example, my Nikon D5, which is pretty fast, has a pre-focused shutter lag of about 41ms, which is about 1/24th of a second. That sounds fast, but the flash duration of fairly slow studio strobes is still in the hundredths of a second, so it is well extinguished before the shutter could open.

Firing a synchronized camera requires a common signal that will fire the camera slightly ahead of the flash, giving the camera adequate "advanced notice" that it can get all the mechanics moving and fire. Generally the trigger fires the camera; the camera fires the flash(s). Flashes work together because even if the flashes are not perfectly synchronized (as in remotes fired by a master flash output), they all still manage to fire during the relatively slow shutter open period. Using some form of radio or wired poppers on each device, fired together, is the straightforward answer, as then the camera lags and flash timing are coordinated. At the very least you need the cameras triggered from the same source. Even then you may have issues with mixed equipment in both cameras being able to see all flashes during their (perhaps not quite precisely synchronized) shutter open time. Increasing the length of shutter open can help with near-miss issues.

Incidentally the CLS mentioned is just for flash; to my knowledge it also will not fire a camera, but was mentioned as an example of pre-exposure control signals, as opposed to during-exposure light output.

Answer 2

The new Nissin MG10 can trigger a camera that has an Air10s installed on it...these two pieces are usually sold together. I would imagine with that setup, you can then trigger all your supporting (compatible) flash units. It's a bit of a different workflow that works well if you do not have a lighting assistant. Your camera would need to be on a tripod, and you would be directing your model's pose from the flash, not from the camera. But the neat thing is you can move up closer to your model and trigger (while your camera stays back) to achieve more dramatic light dropoff. Now all you need is something like the Camera Ranger to show you the results so you don't have to run back to your camera.