Today's post comes from the book Flash and Ambient Lighting for Digital Wedding Photography: Creating Memorable Images in Challenging Environments by Mark Chen. It is available from Amazon.com and other fine retailers.Personally, I am not a big fan of using flash outdoors as fill light. Reflectors can generate a much more pleasant look, as illustrated in the image below. The advantages using a reflector are many: they provide directional light; they can be used very close to the subject (making the light softer, as noted in chapter 1); and they use the sun, so there is no difference in color temperature between the main light and the fill source.
At a wedding shoot, however, having an assistant running around with a reflector—blinding guests with the bounced light—is way too intrusive. I’m sure someone, somewhere has done this and infuriated the guests (wedding photographers are so diverse that one can assume all possibilities!), but this scenario does not happen with my studio.
Reflector fill creates a pleasing look. With and Without Fill Flash
The two images below are good examples. Clearly, the first image was created without fill flash, while the second image was shot with fill flash. When it comes to using flash under very bright ambient lighting, the camera’s maximum flash sync speed becomes a major roadblock on the way to successful shots. Here is what happened: the image without fill was shot at ISO 400 with an aperture of f/5 and shutter speed of 1/2,000 second. When the flash was turned on for the next image, the shutter speed was limited by the maximum flash sync speed, which is 1/250 second on y D2X. If the aperture was not adjusted (here, to f/11), this shot would have been seriously overexposed. (Nikon’s solution to keep this from happening is a “high” warning signal in the viewfinder.)
Without fill flash.
With fill flash. Battery Life and Flash Stress
What could have worried me was that when the shutter was pressed, the flash went off and started sending me the “stress” signal (this is a blinking red light on my SB-800; on my Canon 430EX, it’s a lack of the green light).
This means the flash TTL has asked the flash to dump all its power on the last shot. Therefore, the software concluded that the shot could be underexposed. Upon examining the preview of the shot, I came to the conclusion that the exposure was fine. In this case, the flash was merely filling in—so even if it did not fill enough, the subject would not look underexposed.
However, there is a legitimate concern that the battery could quickly be drained when creating fill in the aforementioned situation. The image below illustrates this scenario with our water analogy. The rain is pouring (symbolizing bright ambient light), so the hole on the bucket has to be kept small (at a low aperture). Because the opening is small, however, the faucet needs to be on for a while to fill the bucket. This depletes the tank. Refilling this deleted tank required lots of water. In the electronic world, this means the battery could be drained quickly.
When the tank is depleted, lots of water will be required to prepare it for another fill. 
You can see a moderate shift toward blue. Time to Turn the Flash Off
Not all outdoor lighting conditions need our human interference. In certain situations, turning the flash off and leaving it in the hands of Mother Nature is the best approach. These situations include when you are shooting in a shadowed area, an shown the image above. Another case is when you are shooting under an overcast sky. In both situations, the whole sky is transformed into an oversized softbox.
However, looking at the image above again you might find a moderate color shift toward blue. This is caused by the much higher (more blue) color temperature in the shaded area. Think about it: on a brilliant day, when the direct sunlight is blocked, what is illuminating the subject? The big blue sky! That is where the blue hue is coming from.
Technical Summary
• Watch for overexposure warnings (a “high” signal inside the viewfinder on Nikon cameras) and reduce your aperture until the warning is gone.
•Watch for an underexposure warning from your flash, but do not be overly concerned when it happens.
• Check the camera’s LCD preview to see if the shot was successful.
• If the flash underexposure warning goes on often, the recharge time will be long—hence, longer intervals will be needed between shots. Additionally, the batteries could become depleted, so be ready to replace them.
• When the subject is in shadow or when you are shooting under an overcast sky, turn the flash off.
All these troubles stem from the flash trying to challenge the sun—which is like arm-wrestling match between an ant and an elephant. However, when the ant manages to make a difference, it could yield a nice result, as seen below.
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