Using a Flash Outdoors
When I first got in to nature photography, I couldn't grasp the need for a flash. I wasn't taking pictures in the dark, so what good would a flash do?
And then over the years, I read books and commentary from professional photographers many of which advocated the use of flash outdoors. They used the term “fill-flash” to describe the benefits of augmenting ambient light with that of a flash. I also found myself in situations where a flash would come in handy.
The problem was that my camera's built-in flash was pretty weak. The drop-off in light was fairly quick as the distance between me and my subject increased. And so I eventually picked up Canon's 580ex flash; a fairly bulky piece of equipment, but one that has become part of outdoor camera gear.
And like the pros described, I use the flash to increase overall light levels. Most of the time there is enough light to get a focus lock, but not quite enough to keep the shutter speed high enough to eliminate blurring from camera shake. The flash gives me a little extra wiggle room to crank up the shutter speed which is quite handy when using a zoom lens.
The flash I have also allows me to set a high shutter speed and, in effect, tell the flash that I don't care if the background is underexposed (by default flash units aim to expose everything in a scene adequately). This has a desirable side-effect of making the subject bright, but leaving the background dark so that the eye is automatically drawn to the subject.