When I'm asked about my favorite piece of photography kit, people are often surprised that I say 'a flash and an umbrella' instead of quoting a fancy high-end body or an expensive fast telephoto. In fact, if money were a concern, I'd prefer a cheaper camera with a flash than an expensive one without.
Being a travel photographer with a bias for the Far East, the second question I get is why I'd take a flash with me to countries that have a much sunnier climate than dreary ol'Belgium where I live.
Using flash on location is not about adding more light, it's about being able to control the light. Especially in tropical countries, where the ambient sunlight is harsh and therefore photographically unpleasing during most of the time, the ability to take down the ambient light and bring a subject up to exposure using the soft, pleasing light of an umbrella'd flash can make all the difference.
So, using flash on location is more about changing the quality of light than the quantity. And a big part of getting that quality is taking the flash of the camera. You've probably noticed that on-camera flash (especially when you turn your camera in portrait orientation) is not the most flattering light.
Your options improve greatly when you take your flash off the camera. This not only allows you to work at different angles, which can help to sculpt the light on your subject's face, but it also lets you work with modifiers that soften the light output. As the softness of a light source is determined by the relative size of that light source to the subject, it pays to use a softening like an umbrella or a softbox in combination with that flash. I use a Westcott Collapsible Reversible umbrella because it folds down to mere 19 inches and I can easily attach it to the steer of my bike (or my camera bag), ready to be whipped out at a moment's notice.
Taking your flash off camera means you have to use a way to trigger that flash. Luckily, there are now countless ways, ranging from the built-in proprietary systems of Canon, Nikon and Sony over cheap, 30GBP radio triggers to the industry standard but more expensive PocketWizards.
Adding flash to an ambient-only setup doubles your light sources, giving you two lights for the price of one.
In the picture below, I used the setting sun as a rim-light, defining the outline of this harvesting woman. A splash of flash provided the necessary fill light.
It doesn't have to be human to throw a dash of flash at it! I took this image at noon. Underexposing the harsh ambient light and supplementing it by raking, orange gelled flash light parallel to the sculpture brings out texture, much like 'golden' evening light would. A hand is used to block unwanted spill in the top left part of the frame.
A flash with was shot through a Westcott Collapsible umbrella. The setup was held by an assistant. The flash had an orange gel over it to help it match the warm ambient light of the bulbs in the background. Holding the flash at a 45° angle gives depth to the face and makes for a pleasing portrait.
If you want to know more about what off camera flash can mean for your photography in general and your travel photography in particular, check out Making Light 1 & 2, available at www.craftandvision.com.