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Joewoo....
The way to tell what the flash is doing is to start from a baseline of no flash. Take an ambient test shot and adjust exposure until no highlight in the foreground you'll light with the flash is clipping. In Av mode that will typically require about - 2 EC and result in a shot that looks underexposed OVERALL.
But what makes the flash metering work correctly is keeping the ambient lit highlights below clipping. Then its just a matter of turning on the flash and letting the evaluative metering do its magic...
That shot above as at FEC =0 and matches perfectly. In the RAW file the sunlit parts are still below clipping and the flash lit parts lit by the flash are slightly darker, similar to how you'd perceive it in person. Actually giving the context of the darker than normal background, which is a result of the camera's limited dynamic range, the foreground may seem a bit over-flashed.
That shot was taken from about 12 ft at f/2.8. I put the second stand in the background to show the effect of the flash fall off. If you perform some similar tests you'll get a good understanding of the range you can expect. But there are two key concepts you need to understand to make flash outdoors work:
1) You can't overlap sun and flash: To make flash work outdoors the keep the sun off the front of the face entirely. Adjust ambient for the sunlit hair and clothing below clipping, then add flash to balance
2) The flash is the KEY LIGHT: By definition the key light is the one that creates the highlight pattern and that's what the flash does outdoors. The fill illuminating the spots the flash doesn't reach comes from the sky, and is 3 stops darker than the sunny side. So flash outdoors needs to be positioned like a key light above the head of the subject where is will create a natural looking mask pattern of highlights and no distracting nose shadow. A flash bracket is ideal for this.
3) The sky is the FILL LIGHT: WIth a subject in backlight front of the face is in God's own softbox the sky. There's plenty of soft light, its just a bit to dark for the camera to record the face as your adaptive eyesight perceives it in person. So in most portrait situations outdoors where sun is used as backlight its better to use two flashes: One off camera to create the highlight pattern, with a second on a camera bracket for fill. The only difference between indoors and out is the outdoors the flash doesn't need to work as hard because the light from the sky supplies most of the light and flash just needs to kick it up a notch.
3) The wider the aperture the less flash power is needed: High Speed mode cuts flash power so when using it try to shoot as wide open as your DOF requirements allow.
4) Its better to augment the ambient that fight it: Hot shoe flashes have limited power so the willing strategies are to work with the ambient rather than fighting it. That's limiting to some degree, but that's the nature of all lighting situations: you need to pick the strategy which works best within the limits of your equipment. In most situations the best strategy is to keep the sun off the face entirely in closer shots of people and then use flash in front to augment the light from sky.
More than anything what makes lighting unflattering is harsh dark shadows, so
For longer full body shots, such as for glamor or fashion work, where the emphasis is on the the clothing not the face flat lighting facing the sun can often be a better strategy than trying to fill with flash and not having enough power to do it adequately.
Chuck
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