It's not so much that flash photography is actually complex, but more that the available information tends to skip the fundamentals. In short, I love Strobist very much (and I'm not alone), but I felt left in the dark (pun intended) about a couple of small items after reading a recent post and I felt I should share the answers I came up with.
Land of Confusion
So there I was, reading this Balance Light: Twilight post on Strobist, which is admittedly part of the second and more advanced series on lighting (Lighting 102), but nevertheless I became overwhelmed with total misunderstanding. I don't want to be too quick to discredit David because, after all, it was Lighting 102, so maybe I missed something that was discussed earlier, but all of the examples of aperture and shutter settings raised a boat load of questions in my mind.
Questions that burned within me!
Time Is of the Essence
My first question, and the simplest one, was how flash lighting actually interacts with aperture and shutter settings. What I didn't realize is that the exposure of an object lit entirely by your flash is not affected in any way by your shutter speed. The easiest way to explain why this is true is to discuss maximum sync speeds. All mid- to high-end Canon cameras have a maximum sync speed of 1/250th of a second, which means that you can set your shutter to anything up to 1/250th while using a synchronized flash (which is to say, any flash that is fired by the camera, either using a sync cable, wireless system, or master/slave setup). With the high-end Nikons, I believe you can get 1/500th. The maximum sync speed represents the shortest length of time during which your camera can guarantee that the flash will turn on and turn off.
By definition, if your shutter speed is set to the maximum sync speed or slower, your camera is guaranteeing that its shutter will be open for at least the entire duration of the flash's discharge, exposing your image to all of its generated light (by “all” I mean the brightness as well as duration components).
I had never really thought about it that way before.
What About TTL?
My other big question was how this house of cards you've meticulously built up by manually adjusting all of your settings could possibly be thrown asunder by the TTL (or A-TTL or E-TTL) metering that the camera does. If you are using a TTL-capable flash unit on your Canon camera, the camera will be able to adjust the duration of flash discharge to get the “proper” exposure.
Any recent Canon camera and flash system is able to use what they call E-TTL, which means “evaluative through-the-lens” metering. E-TTL meters the flash by firing a “pre-flash” milliseconds before the actual flash and measuring its effect on the scene using the same through-the-lens metering system that meters non-flash photographs. Because the pre-flash is a set intensity, all of the variables are known to the camera and the result of the actual flash exposure can be calculated. It seems almost impossible, but it works.
So the short answer to my question is that the metering system will, theoretically, change the results of whatever settings you have selected. However, the E-TTL system should be “smart” enough to properly expose the scene for flash, meaning that whatever adjustments you make to the ambient light's exposure should be virtually unaffected. E-TTL simply ensures that your flash-lit subject isn't over-exposed by a flash burst that lasts too long.
The real point of talking about all of this is to figure out how to balance ambient light with the light from your flash. The moral of the story is that the shutter speed has an effect on the ambient light only, while the aperture has an effect on both. It's not a walk in the park to get precisely the results you want without plenty of in-the-field experience, but knowing that simple rule is a pretty good start!
A very straightforward approach would be to set your shutter speed to the slowest you feel comfortable with (if you're shooting hand-held, that is your main limitation) and then adjust your aperture to where your subject is getting the light from the flash that you desire (by chimping on your LCD using the histogram display of course… Right?). Then you can start to ramp up the shutter speed to see how it affects the ambient lighting (because it won't affect the exposure of your subject if it is lit solely with the flash).
In the cases where ambient and flash light mixes on your subject, you're going to need a lot more time to chimp the histogram.
This article on photonotes.org.org/articles/eos-flash/ was invaluable to me in learning about how flash works in the Canon EOS system. If you shoot Canon, you should read this from top to bottom!