I am not a control freak when it comes to most forms of photography. I prefer to silently lurk in the background, observing the actions and interactions of my subjects, and make a photograph when something occurs that is emotional, sentimental, humorous, or visually interesting. This candid moment was a grab shot, as you can see by my brother-in-law in the background. This is the real me, "running and gunning", unfettered by the need to have every photograph a contender for next year's Pulitzer prize. I kid you.
One might assume that this would be a typical available light photo, since we're standing outside in bright sunlight. The problem comes from the exposure extremes your camera's metering system must consider when reconciling both highlights and shadows in the same photograph. Normally, the camera would simultaneously meter them both and select a compromise exposure, one that will inevitably overexpose the highlights and underexpose the shadows.
In this discussion, "highlights" are areas in the photograph that receive their illumination from the sun, and "shadows" are the regions that don't receive any direct sunlight.
For this shot, I manually set the exposure as follows: 1/250 of a second shutter speed (the shortest exposure that supports iTTL speed lights), F 16, and ISO 400. If you're old enough to remember the "Sunny 16 Rule", you would conclude that the image would be overexposed by 2/3 stop in a front-lit situation. Now this would explain the brightness of the highlights, but what about the shadows? I my case, I had a bare (no BFT or diffuser) SB-900, mounted in the hot shoe and aimed straight ahead, but dialed down to 1/2 power. (9/13/12: Ooops. I should have said it was dialed to -1/2 EV. This means that the exposure from the speedlight would give 1/2 of the light required for a full exposure. Sorry about that). Based on the Sunny 16 rule, I am already overexposing the highlights by 2/3 stop. Now here's the catch. My speed light, functioning as an on-axis fill light, is dropping an additional 1/2 stop of exposure onto the shadows and on top of the already "overly exposed" highlights. For you math over-achievers, the highlights receive an over exposure of 2/3 of a stop, plus the addition of 1/2 stop from the fill flash giving a total overexposure of slightly more than one stop, 1 1/6 stop to be exact.
Now examine the shadow areas in this portion of the lead photograph. The shadows received 1/2 of the proper exposure from the speed light. If you look at Suzi's face, you can see that the tip of her nose receives light from the sun and the speed light, while the shadows are illuminated by the fill flash. If you look under her chin, you can see that without the flash, there would be very little shadow detail.
If I had the presence of mind to refine my exposure, I would have reduced the ISO to 125 which would have given me a "sunlight" aperture of F 11 and kept the speed light output at 1/2 power. I could then drop the aperture to F 16 if the sunlit highlights were too bright. And I'm not above reducing the speed light output even further, depending on what I am trying to achieve.
In an earlier post, I explained that you can use your speed light/s to expose your subject any way that you want if you can place your subject in a shadow of a building or tree. This shot combines a large key light source and a small, on-camera flash for fill.
For those who argue that "flash is dead" are missing an important point. Sometimes you need some sunlight in your pocket to bring the details in the shadows back to where they belong. In the end, it's all a matter of altering the lighting conditions to better match what we thought we saw at the time. And your built-in flash just won't have the power to be any any use when wrestling with the sun.