Balancing flash with ambient light is quickly becoming one of the short list of skills that most define me in my own mind. This article I’ll discuss more the creative decisions than the technical.
The method I usually use, unless time makes this impossible is to manually set an ambient exposure that gives the look I want. At this time I need to make the main creative choice; do I want the flashed subject to be brighter than the overall image, or use the flash as fill. If I want the flashed subject to be brighter, than I set the overall exposure to be a little dark, possibly as much as a stop. If I’m using the flash as fill, then I set a normal looking overall exposure.
Then you set the flash. If I’m using on camera speedlights, I use the E-ttl method. If I’m using off-camera then I set it manually. I am considering upgrading to Profoto Off Camera Flash system, if I do that I’ll use the method where you set the initial flash power with ttl and then tweak it to give the effect you want.
Finally, one has to consider the flash modifier used. I usually use something that softens the flash, softbox or beauty dish, but you need to look at the ambient and consider how you want it blend.