First of all, until PocketWizard’s ControlTL and RadioPoppers came along, off camera flash for the most part was a 100% manual affair. This may seem intimidating to beginners, but really, with a few simple rules, setting the exposure isn’t that difficult.

Let’s get the technical details out of the way: these shots were taken on a Canon EOS 5D Mk ii, with either the EF24-70mm f/2.8L or the EF70-200mm f/2.8L IS. Lighting was provided by a single, film era Nikon SB-25 ($60.00 used). The flash was gelled CTO and the camera’s white balance set to daylight. Balanced against daylight, the flash’s output takes on an orange tinge – making it appear that the subjects are lit by fire light.

As an engineer, I approach setting the correct exposure as an algebra problem. For any off camera shot there only exists 4 user-controlled variables:

* The output intensity of the flash (as seen by the subject)
* Shutter speed
* Aperture
* ISO

Just like solving an algebra equation, we try to nail down the unknowns. The first one I tackle is the flash’s output. I know that to have any chance of getting good shots, I need to shoot *a lot*, given the unpredictable nature of the dance subjects’ movements. So, having to sit around and wait for the flash to recharge (2-3 seconds) is probably not acceptable. My choices are really M1/4 and M1/8 – quarter and eighth power. I know that I’m going to be trying to drag my shutter speed (more below) out to possibly 1 sec exposure, so M1/4, and being mindful not doing a “spray and pray”, would be my good starting point.

Once I set my flash output, I arrive at preliminary numbers for 2 more variables: ISO and Aperture. By now, the sun had set and the sky’s rapidly darkening, so for a first order approximation I can afford to ignore the ambient light. From experience, I know that the combination of a gell and a diffuser is about 1.5 stops of light being taken out, so I started at f/2.8 and ISO200. the exposure is decent and so I run with it.

Now, my last variable: shutter speed. this really depends on the dancer i’m photographing. I want to capture as much of the fire trails as possible, so i experimented with various speeds, from 1/6 to 1 second.

Occasionally, the amount of fire being spun creates a large enough “light source” that it contributes to the ambient exposure and blurs my final images. when that happens, I up the flash output to 1/2 power or full power, and then stop down my aperture the corresponding 1-2 stops to compensate. this way, the ambient light contributes less to the overall exposure compared to the flash pulse – and thus a sharper, more well defined image.  See here – notice how there’s a lot less ghosting of the face, etc.  The flash’s much stronger pulse is what’s recording the face, but the aperture, stopped down to f/5.6, means that the ambient light sources isn’t recording much onto the sensor for the rest of the exposure.

Sometimes, having the flash misfire isn’t a bad thing.  The flash failed to go off for the following image, but I really liked the results.  It also gives you a good idea of what the ambient only contribution to the image is: