I've been following the action over at Strobist for a few weeks now. The author of the blog, David Hobby, has a great writing style, and is a fantastic teacher. If you're interested in flash photography, I highly recommend you check out his site. There's something of a cult following on Flickr as well, and they've produced some very impressive results!
I've had more than a couple "A HA!" moments when reading his blog, and I thought I'd share a couple of things that finally clicked in my brain in the hopes that it will help somebody else to "click"...or maybe everybody else just gets these things and I'm really slow.
In any case, my first epiphany happened after reading several of his posts and thinking, what's with this guy's unhealthy obsession with aperture? I mean, sure, aperture is important if you want to control depth of field. But if you're want to change exposure you can tweak both the shutter speed and the aperture, right?
Well, at least not in the same way as when you're just using available light.
The thing is, your flash fires pretty fast. Way faster than your shutter speed in most cases. It's like 1/10,000th of a second. So it doesn't matter if your shutter is open for 1/500th of a second, or 1/30th of a second, the same amount of light from your flash is going to hit your sensor either way.
But your aperture does affect how much light is able to reach your sensor from that brief burst of light. A larger aperture (smaller f/stop) lets more light in.
This isn't to say that you can ignore your shutter speed, because it does affect how the ambient light contributes to the exposure. You can use this to your advantage to control the flash / ambient ratio.
Understanding all of this led to my second "A HA!" moment. David often says to set your camera's shutter to its fastest flash sync speed to make things easier for your flash...which is a bit counter-intuitive at first. If you want things to be easier on your flash, you should have a longer shutter speed to let more light in!
After thinking about it a bit more, I realized that choosing a faster shutter speed means that your aperture is opening up to maintain a proper exposure of the ambient light. And since the aperture is open wider, the flash doesn't have to put out as much light to light your subject properly.
Which is great when you're competing with the sun as the primary light source :)