How to use Fill Flash

Fill flash is a method of using flash - usually in backlit situations - so that it doesn't look like flash at all, but a per-fectly exposed, ambient-light picture.

Using fill flash correctly means understanding a single basic principle: Every flash shot is really two exposures. Because these two exposures occur simultaneously, and are largely independent of each other, sometimes people have trouble grasping the concept. But consider:

One exposure is the ambient exposure. Lightness or dark-ness is determined by your combination of f-stops and shutter speeds.

The other exposure is the flash exposure. Lightness or darkness is determined by flash power and, if your flash is an auto unit, flash duration. You also can vary exposure by changing the distance of flash to subject, or your lens' f-stop. But shutter speed has no effect whatsoever on an electronic-flash exposure, except for the limitation your camera's shut-ter imposes on maximum flash-sync speed.

Keeping that principle in mind, here's how to get the fill-flash exposures you want if you're using a TTL flash/ camera system with flash-exposure compensation:

1) Set the ambient exposure first; this is the general expo-sure for the background - the sunset, the skyline, the garden trees, etc. Whatever camera mode you use, whether manu-al or autoexposure, make sure the exposure selected is at highest shutter-sync speed or slower.

2) Determine how much fill you want. Usually you don't want a full flash exposure (that will look like a typical bright, harsh flash shot), but one half, one third, or one quarter of full exposure. For half intensity, dial in -1 EV of flash compensation; for one-third fill, -1 1/3 or -1 1/2 EV; for one quarter, -2 EV. (Unfortunately, some autoflash units don't have variable set-tings; you just have to go with what they give you.)

3) Shoot!

For generic (non-TTL) auto-flash units, the method is a lit-tle trickier:

1) Set the camera to a general exposure for the background, as above. Note the aperture set-ting for the exposure.

2) Set the aperture selection on your autoflash as follows:

For one-half flash power, set it one stop open from the camera setting (e.g., if the camera is set tof/S.settheflashtof/5.6). For one-third flash power, set the flash a stop and a half open. For one-quarter flash power, set the flash two stops open. Leave the camera set-tings as they were.

3) Shoot.

Sometimes you may find you don't have an autoflash aperture available - you need f/4 set on the flash, say, but it only offers f/5.6. If so, change your shutter speed to shift the ambient exposure to an equiv-alent setting (e.g., from 1/250 at f/5.6 to 1/125 at f/8) and recalculate the flash setting.

Finally, for manual flash units, the best method is to use an electronic flash meter and vary the flash power to bal-ance ambient/flash readings.

Note on flash-sync speeds: SLRs use focal-plane shutters which can synchronize with electronic flash only up to a certain speed. Older cameras could sync only to 1/60 sec; most newer cameras can sync to at least 1/125 sec; high-end AF cameras often sync to 1/250. Check your manual for your camera's top sync speed. If you exceed maximum sync speed, you'll get just a band of flash or no flash at all in the picture.