Slave flash with no visible master flash

This thing is firing by itself? Just like that? No cables, but also no light coming from the camera? What’s the trick?

Yesterday, I was asked to do some kind of fashion shoot in Thailand’s new airport which has yet to open to the public. With backlights being so in fashion (no less than 3 shoots were backlit in the latest issue of Wad), I suggested we try that. Backlight also goes well with the cold, artificial and metallic quality of the airport. Since I’ve never done a fashion ahoot, you'd think I’d try something simpler, or get decent equipment—but no. We’re going to do this with 10 dollar slave flashes and burnt out negs taped to the main flash.

In case you hadn’t guessed, backlight is light coming from the back. It creates sharp burnt out highlights on the sides of the subject, leaving the background and center of the subject in darkness, sometimes in total darkness. Something like this:



I don’t think I have to draw a diagram of this one. You can see where the slave flashes are and the master flash is right on top of the camera, which I’m holding. What should surprise you, if you paid attention to “slave flashes: part I”, is that my face is so dark, meaning there’s no light coming from the master flash. I linked to that trick already but I’ll spell out in full right here because I found out a few things about putting the theory into practice.

The theory is that developed film that has been exposed to light will become dark, blocking off visible light, but will continue to let infrared light pass through it. Since your slave flashes’ sensors are sensitive to infrared light, you can tape an exposed neg to your main flash to stop visible light while still triggering the sensors. Actually a single layer of negative is not enough, letting way too much red light through. Your subject will be backlit in white and lit from the front in red—not exactly the effect I’m after. Doubling the layers of exposed negatives worked well at cutting off nearly all the visible light emitted by the main flash while allowing enough infrareds through to continue to trigger the slaves.

After I had all that figured out, I decided I also wanted to be able to have some light from the master flash so that I could continue to control the exposure of the central, non-backlit, zone of the subject (in this case, myself). In the end, this is how the camera was rigged:



90% of the master flash is covered by the dual-layer of exposed negative but notice the little white rectangle at the top. That’s the part I can choose to open or close—by moving the duct tape around—to control how much visible light is coming from the main flash. The problem is that the shape of the light becomes a small rectangle too! But remember we’re talking 61 dollar setup here. This is the same picture as the one above but with a little “window” opened in the main flash:



I don't exactly feel ready for the shoot though. I'd need to test this stuff in a setting bigger than my room and with a model.