Balanced fill-flash is a wonderful technique that can add life to otherwise ordinary pictures, or even rescue images that previously might have not been useable. What's really nice is that fill-flash can be done with fully automatic exposure control! No special manual settings are needed (unless you want to put your own personal touch on your pictures). You can even use the built-in flash unit on some EOS cameras. Photographers use different terms to describe this technique. Fill-flash, fill-in flash, balanced-fill flash, and so on all refer to the same thing.
By definition, a fill-flash picture is one in which the ambient light in the background – sunlight outdoors, room lighting in an indoor shot, and so on – is properly exposed and therefore normally visible in the picture. But along with this, flash has been used, and subjects in the foreground of the picture are illuminated by this extra burst of flash. With a properly exposed background, and also a flash-illuminated subject that's got proper flash exposure, the two light sources (flash and ambient light in the scene) are balanced, and neither will appear overly dark or light. This usually gives us a very natural-looking picture. It can also brighten otherwise subdued faces, lighten shadows from harsh sunlight, or sometimes just add a little sparkle in a subject's eyes.
First off, there's no special trick to taking balanced-fill flash pictures outdoors in daylight, even in bright sunlight. With a Canon EOS SLR in any automatic exposure mode such as Program (P), Tv, or Av, just pop-up the built-in flash by pressing the flash-on button, or alternatively mount and turn on an EX series Canon Speedlite flash unit. The camera "knows" the flash has been activated, and all you need to do is focus and shoot the picture. In the Tv and Av modes, if you've set a shutter speed (in Tv) or aperture (in Av) that simply won't enable a proper exposure of the ambient light in the scene, the opposite value blinks on and off in your viewfinder; just turn the camera's Main Dial and switch to a different shutter speed or aperture, until the display stops blinking on and off. You're ready to take the picture.
There's no need to take the flash out of its automatic "E-TTL" setting (unless for some reason you want to use manual flash with an accessory EOS Speedlite). The beauty of fill-in flash with a modern SLR is that the camera adjusts background exposure and flash exposure automatically, with little input required from the user.
To preserve a natural look in fill-flash pictures, the camera deliberately tends to reduce the intensity of your flash output when it detects that you've properly exposed the ambient light in your background. The intent is to give you fill-flash pictures with foreground subjects that don't appear to obviously have been blasted with frontal flash. (This occurs automatically, with no input required from the photographer.) This way, shadows are gently filled-in, without looking like your subject was lit-up by a searchlight.
When you have a subject that is primarily lit by sunlight, and you're just trying to lighten a few shadows on his or her face, this automatic flash reduction tendency works very well. But sometimes, you may be asking the flash to do more than just fill-in a few shadows. If your subject is heavily back-lit, and entirely in shadow, you may want the flash to become the primary source of light. An example might be a shot of a person against the setting sun. With no flash, you'd likely get a silhouette. But if you simply turn on your flash and take a picture at normal settings, you might end up with an effect that still looks like a silhouette. The flash may not have lit your subject up much at all. What when wrong?
The amount of flash illumination in a fill-in flash picture is a starting point. And while it often works beautifully, in instances like the sunset shot, all the bright sunlight in the scene can fool the camera into thinking that not much flash is needed. But as we said, in a totally back-lit scene, you're basically asking the flash to be the main source of light hitting your subject. How to get more flash on the scene? Easy. Use your camera's Flash Exposure Compensation. All EOS Digital SLR cameras, with the exception of the original (6MP version) of the EOS Digital Rebel, have this feature. And it's also available on most of Canon's accessory Speedlites, like the 320EX, 430EX II, or 600EX-RT. By deliberately adding "plus" up to two stops of flash illumination, you're intentionally brightening the flash output. Don't be alarmed or surprised if you find that you need to use Flash Exposure Compensation from time to time in fill-in flash pictures. And remember if you think the flash output in your fill-in shots is too much, you can also use Flash Exposure Compensation's "minus" settings to reduce it. One last thing: Flash Exposure Compensation affects only the flash output. The ambient light in your background will remain unchanged as you adjust Flash Compensation.
All EOS digital SLRs also have "standard" Exposure Compensation as well as Flash Exposure Compensation. Exposure Compensation lets you deliberately lighten or darken the ambient lighting in the background. It's entirely separate from Flash Exposure Compensation, and has absolutely no impact on how much flash output strikes your subjects. It solely impacts the ambient light in the scene. It's another very useful control which gives you very complete command over the look of your fill-in flash pictures. It's particularly useful when balancing flash with low light, indoors or outside at night.
Remember that if you're outdoors in daylight, your flash is competing against a mighty strong opponent - the sun. Don't expect fill-in flash to be effective when you're too far from your primary subject(s). In sunlight, the built-in flash really loses most effectiveness in fill-in situations when you're much more than 6 or 8 feet (2-2.5m) from your subject. Accessory Speedlites like the Canon 270EX II or 600EX-RT will reach farther, but may be against their limits if you're trying to light up a back-lit group picture from more than 15 or 20 feet (4-6m) away. Even if you boost flash output with the Flash Exposure Compensation, any flash unit has a maximum amount of power it can deliver, after which it simply can't produce any more flash power.