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Not only can your camera control an attached flash unit, it can control others via radio signals.
Flash modes tend to be less well-defined because the relationship between a pulsing, external light source and cameras goes way back to the days when you had to load your external flash with a special pyrotechnic powder and light it off at just the right time. How cool would that be? Hit the shutter and SHA-PHOOM! If you were using flash powder today you’d probably be interrogated by the authorities as a terrorism suspect and shooting a portrait would involve setting off the smoke alarm.
Flash sync is a complicated dance because the external flash and the camera may be manufactured by different companies. That means they have to trade specs on metering modes and how to communicate with the camera’s computer. If you’ve ever been involved with a project like that between two big companies, then you know the process is fraught with potential pitfalls. Still, overall, they do a pretty good job of getting it right most of the time.
By adjusting the relationship between the flash and camera you can achieve some really interesting effects. Toward that end it would be useful to cover different flash modes that are available in modern cameras.
Auto Flash Sync
In automatic modes if your camera has a built-in flash the computer will tell the flash to fire. If you have an external flash that communicates with the camera, the computer will take over the exposure settings.
Flash On or Fill Flash
Turn your camera’s flash on outdoors, even when it thinks there’s enough light.
The built-in flash on your camera may be the worst photography lighting source ever developed, but the situations where it can turn average pictures into amazing shots are outdoors in open shade or direct sun. Those are the very times your camera is sure it has enough light without a flash. The ironic nature of built-in flash photography leads to a simple rule: Turn the camera flash off indoors and turn it on outside. Just the opposite of what you might expect.
Managing The Curtains
The shutter in your camera works a little like a stage with two sets of curtains; one that comes down from the top and one that sinks down into the stage. As the bottom curtain drops, the top curtain comes down to control how long the stage is exposed to the light. The length of time the stage is open to the light corresponds to the shutter speed. The default most cameras use out of the box is called First Curtain. That means the flash fires as the first curtain drops. There’s another option which most high-end cameras support and that’s called Second Curtain. Instead of firing right away, the flash waits until nearer to the end of the shutter cycle and fires just before the top curtain comes down to shut off the light to the stage.
While it doesn’t sound like a huge difference between First and Second Curtain, it can make quite a lot of difference in how the shots turn out, especially if the subjects are moving.
Slow Flash Sync
Remember our mythical stage with the curtains? Now imagine the curtains are going to stay open for a bit longer on a night shot and, in the middle of that long open time, the electronic flash fires and freezes the subject. It takes longer to describe than it does to happen, with all that going on in 1/30th to 1/15th of a second. That’s a long exposure for a hand-held shot and, if you time it right, you can have swirly lights and subjects frozen by the flash. It’s an interesting effect, definitely worth trying.
1/250th of a second is plenty fast enough to freeze a shot like this.
High Speed Sync
Just like the name says, this mode is at the other end of the scale from Slow Sync. Typically a flash unit syncs at 1/250th or 1/125th of a second. But when the flash and camera support high speed sync, the flash sync speed can be pushed up as high as 1/5000th of a second. It’s not very often you need a flash sync that high but it does come in handy if you’re trying to freeze something very fast, like a hummingbird.
Understanding your camera’s flash adjustments gives you the power to experiment with different effects. Since how these flash modes are implemented sometimes varies between camera manufacturers, I would encourage you to spend a few minutes with your camera manual, and then experiment with different flash settings. The results will surprise you.
