One of the most creative ways to use speedlights is to apply what is known as Rear Sync Flash. Rear Sync is fundamentally quite simple. It is best used in low-light situations where it allows you to apply a slow shutter speed to pickup more of the ambient light, then freeze your subject with the flash which fires at the end of the shutter cycle. In other words, the shutter opens and remains open for an extended period of time before the flash fires just before the shutter closes. This also allows the camera to pickup some movement before freezing the subject. Used creatively, you can obtain some amazing results.
Trial and error, and applying visual compositional equations that work allowed me over time to build confidence in using off camera lighting. The Night Train Portrait shown here is one of my first portraits using speedlights where I stretched the imaginative realm beyond what I would ordinarily do. It was a moment when the use of artificial light became a larger part of the equation and was blended with a wonderful nostalgic setting along with appealing ambient light. It became a moment where the creative impact of off camera lighting was applied fully to create an exciting photographic moment.
Two lights were required for this shot. One Key Light and one Kicker Backlight. The backlight was placed to create a halo around the model and the trailing train of material. All lights were fired remotely using a camera mounted transmitter. The train consisted of 3 yards of thin cotton material in a Burgundy color because burgundy will work with almost any color...white, blue, black, red....My model was wearing a delicately styled patterned dress which added a measure of elegance to the composition.
The Key light, set initially to about 1/8th power was on a stand set to be slightly higher than and about 4 feet in front of my model, just out of the line of sight of the camera lens. The kicker light was placed on a stand behind the model. Its power setting was initially around 1/32 power, about 2 stops lower than the key light...just enough to provide a rim light effect. My camera, as always when using speedlights, was set to manual and the flash mode was set to Rear Sync. I used a wide angle lens and needed a bit of depth of field to keep the background somewhat in focus so I used an aperture of f/6.3. I also needed more light gathering ability so I bumped the ISO to 800 which allowed me to use a shutter speed of 1/6th of a second.
The beauty of using flash is that it freezes your subject so even with a slow shutter, my model was still sharp and clear, yet the ghost-like movement of the material was captured during the non-flash portion of the exposure. To keep the background sharp I set the camera on a tripod collapsed all the way down to its lowest level and made the shot from a low angle.
At that point it was simply a matter of choreographing an interesting look with the model which took some trial and error, but the results turned out better than I hoped for.