Fill-Flash use with EOS cameras and Speedlites

The intent is to give you fill-flash pictures with foreground subjects that dont appear to obviously have been blasted with frontal flash. 

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.

Balanced-fill flash can also easily be done when you're indoors, or outdoors at night. In a typical flash snapshot in these conditions, we're used to seeing a brightly-lit subject, with a black or nearly-black background. This happens because the ambient light in the room or in the scene was underexposed, but the flash illumination on the subject was correct. To get a more natural-looking, balanced-fill result in these scenes, we need to extend the exposure so that more ambient light is recorded in the scene.

This means one of three things, or a combination of them: raise the camera's ISO setting; and/or shoot at wider lens apertures (a lower f/number); and/or shoot at slower shutter speeds. Again, EOS digital SLRs can automatically perform this task for you. The simplest way is by changing your exposure mode to the Av mode, or the Tv mode (in the latter case, you must also select a fairly slow shutter speed). In either mode, the camera will always try to properly expose the ambient light in your background, no matter how dark it is – if your chosen settings won't allow this, the speed or aperture (whichever you're not setting) will blink on and off in your viewfinder. And keep in mind that none of this is possible in P (Program) mode, or the full-auto "green zone" setting. In those modes, to avoid possible blurs, the camera never allows shutter speeds slower than 1/60th of a second when you use a dedicated flash.

Raising your ISO to something like 400, 800 or higher will give the camera more light-gathering power when you're in low light, and will minimize the use of extremely slow shutter speeds in Av or Tv mode with flash.

There's another even simpler option – on cameras like the Digital Rebel series, or mid-range models like the EOS 60D, there's a fully automatic mode called "night portrait" on the camera's mode dial. Set it there, and the camera will allow slow shutter speeds with flash to blend in ambient light.

Any time you're working at slow shutter speeds to blend-in natural light in a flash picture, remember to be careful to hold the camera very steadily, and be sure your subjects are stationary as well. Any movement will usually result in blurs or ghost images. In other words, if you're looking for sharp action pictures with flash at a high school basketball game, it's not the time to use the balanced-fill technique.

Again, the camera will usually tend to reduce the flash output when you properly expose your backgrounds, so that the flash isn't obvious in the lighting of the final picture. Often, this gives a very natural looking result. But if you want more or less flash output, you can easily achieve this with your Flash Exposure Compensation. And, you can also subdue the ambient light in your background by darkening it with standard Exposure Compensation, or alternatively brighten it by adjusting Exposure Compensation in the "plus" direction. Once again, this is entirely separate from adjusting flash power up or down with the Flash Exposure Compensation. Experimenting with the two will give you a tremendous amount of control over the final look of your fill-in flash pictures.

Don't be afraid to use your flash in daylight – it's actually one of the most effective times to use it, as long as you're not too far from your subjects. And in low light, don't feel locked-in to 1/60th of a second shutter speeds, and resulting unnatural dark backgrounds. Experiment a little with the balanced fill-flash capabilities of an EOS digital SLR, and you'll probably be quickly convinced of its usefulness.