If you get confused every time someone tries to explain balancing flash with ambient light in manual flash mode, don’t worry there is an easy alternative. You can get brilliant fill-flash results using automatic flash and still have control!
What is Fill Flash?
Fill flash is usually used to light your subject in situations where the background is too bright and your main subject will be too dark without flash. It can eliminate hard dark shadows when the sun is very strong, it will often put a lovely sparkle in someone’s eye.
It is really easy to use fill flash pictures outdoors in daylight, even in bright sunlight using an external flash or your cameras built in flash. With most DSLR cameras using Program mode (P), shutter speed priority Tv, or aperture priority Av, if you pop up your flash, or turn on your speedlight your camera knows you are outdoors and will do a really good job of filling your subject with just enough light. If you’ve set a shutter speed in TV mode or an aperture in AV mode that your camera cannot use to expose correctly, the problem value will blink on and off in your viewfinder. You can then adjust your speed or aperture until the value stops blinking. Sometimes you can change the ISO (making your sensor more or less sensitive to light) if you really want to keep the shutter speed or aperture the same.
Your camera will estimate how much fill-flash you need so that the flash appears subtle and not too artificial. Sometimes, if the sun is in front of your subject you will be asking your camera to estimate how much flash is needed to fill in harsh shadows. Sometimes, if your subject is backlit, you are asking your camera to estimate the amount of flash needed to be the primary source of light on your subject.
Using Flash Exposure Compensation for more control
What if leaving your camera to decide doesn’t give you the effect you want? Using Flash Exposure Compensation is a really easy way to add or subtract a little flash while keeping the correct exposure for the ambient light in your background.
Using Exposure Compensation for more control
You can use Exposure Compensation to lighten or darken your background, the flash output will remain unchanged, so your subject will be exposed correctly. A great technique for getting a really moody sky and making your subject appear to pop out of the image. It’s also really useful for balancing flash with low light, indoors or at night outside.
If you are shooting against the sun, your flash may not have the power to balance such a powerful light source.
A good rule of thumb is that if you are more than 6 to 8 feet away from your subject your on camera flash will struggle. Speedlites may struggle if the subject is more than 15 or 20 feet (4-6m) away.
Night-time and indoor flash
Usually if you use automatic flash settings indoors you get a really brightly lit subject and a dark background. To get a more balanced background exposure you will need to expose the background for longer, or increase the sensitivity of your sensor. To achieve this you can increase the ISO (making your sensor more sensitive to light), use a wider aperture (smaller f numbers) or choose a slower shutter speed.
In aperture or shutter speed modes the camera will automatically try and expose your background properly, if you choose a shutter speed or aperture that won’t allow this the opposite value will flash in your viewfinder.
Unlike using outdoor, daylight fill flash, P mode is not the best option indoors or in low light because your camera limits it’s shutter speed and won’t allow speeds slower than 1/60th of a second with flash. You may want the background to be exposed for longer than this.
Your flash will be limited by its sync speed, most flashes will flash for 1/250 sec, and the flash will keep your subject reasonably sharp even when the exposure is longer than the flash duration.
Sometimes simply choosing an ISO setting of 400 or 800 whilst in aperture or shutter speed mode is all you may need to do.
Choosing a slower shutter speed (allowing more background light to hit your sensor) is another option, but remember to hold your camera as still as possible, and ask your subject to keep as still as they can.
You can still experiment with flash and exposure compensation to get the effect you need. If the background is still too dark use exposure compensation and the flash output will remain the same but the background will be brighter (with a + setting). Alternatively if your subject is too bright use flash compensation with a minus value and your background will remain the same but your subject will be less brightly lit.