7.6 Fill Flash Versus Flash as Main Light

If you’ve spent any time reading popular bird photography articles, you’ve very likely encountered the term “fill flash”.  Indeed, I’m often asked by novices exactly what setting on the flash unit corresponds to “fill” flash versus “normal” flash.  In truth, there is no special setting on the flash unit that corresponds to “fill flash”.  The idea is, of course, to “fill in” the shadows left by “natural light” (typically direct sunlight, though sometimes directional ambient light).  The figure below illustrates this very nicely: although the sunlight is shining on the bird from the right, the left and underside of the bird are lit almost as well as the right, due to the use of flash (as well as some postprocessing—see Chapter 11).


    The real phenomenon of interest here is the manipulation of the flash ratio—the ratio of flash to ambient (including both direct and indirect sunlight).  When the ratio approaches 1:0 (i.e., mostly flash and little or no ambient), we say that flash is being used as the main light source; otherwise, it’s fill flash.  The figure below shows an example in which flash is being used as the main light source.  In this case, the flash illuminated the bird very well, without adding any illumination to the background whatsoever (due to the angle of the water relative to the camera), while the ambient light provided only negligible amounts of illumination to the background, overall.



    In the case of the above image, I used flash as the main light because I was forced to do so: the sun had just set, and no practical combination of aperture / shutter speed / ISO would have resulted in a bright-enough exposure to give a low-noise and blur-free image of the bird.  Photographing birds after dark is, of course, something that most people rarely do, and so the use of flash as main light tends to be fairly rare in bird photography.  Hence, most uses of flash in bird photography constitute “fill flash”, and there’s little need to worry over whether your use of flash is technically “fill” flash or not.  In other words: just worry about illuminating the bird.  If you can do that and still have access to enough ambient light to also illuminate the background, then utilize whatever flash ratio you can that will give you the desired effect.
    As a caveat to the foregoing discussion, it must be noted that there are several very notable applications of the use of flash as main light.  The first is the use of flash to freeze extremely fast motion, such as the motion of hummingbird wings; we will deal with this in detail in the next section.  The other application is in producing what I call “low key” images, in which only the bird is illuminated and the rest of the frame (or most of it) appears black.  The image below is one such example.  In this case I intentially used an extreme flash ratio to render the bird fully illuminated upon a field of (mostly) black.




    This type of image is very specialized, and considered by some (perhaps many?) to be cliché; certainly, you don’t want to make all of your bird photos look this way.  A more judicious use of flash ratio (thereby staying within the domain of “fill flash”) will give you images that, on average, have more colorful backgrounds and less startling foreground / background contrast.
    Note that using flash as main light doesn’t always give you a dark background.  If the backdrop is close enough to the bird to be illuminated by strong flash, but far enough back to be rendered reasonably out-of-focus (given the effective depth-of-field for your selected aperture and distance), flash as main light can work quite well.  In order to illuminate the backdrop, however, you typically need to be using fairly strong flash, and that more than likely means shooting at your camera’s sync speed (typically 1/200 or 1/250 sec for consumer-grade models, or 1/300 sec for pro models).  The down-side of shooting at these slow speeds is that motion blur can occur if the flash ratio is too low.  But when flash is used as the main light, by definition the flash ratio is generally high enough to keep motion blur and ghosting at a minimum.
    The best strategy, when you’re just starting out with flash, is to choose some reasonable exposure and flash settings (based on prior experience with your particular camera and flash unit) and to take a few shots at the start of each session in the field and see how your exposures are turning out.  Concentrate first on correctly exposing the bird, and after that on getting a nice background exposure.  Modifying the in-camera and flash exposure parameters in tandem will then allow you to explore the range of possibilities for the relative exposure of the bird versus its background.  At that point you can begin to form your artistic goals for the scene, and then fine-tune the camera/flash settings in pursuit of those goals.  And keep in mind that much of your artistic inspiration may come later, during post-processing on the computer.  As such, it’s often a good idea to experiment with different flash ratios in the field, so as to maximize your range of feasible options later during post-processing.