Wednesday, November 10, 2010

ETTL in the Studio

Strobist info:
* Key: 580EX II and 430EX II in a Westcott 28" Softbox above, camera right
* 430EX II in Morris 15x18" Softbox, camera left

* Savage 107" Seamless Charcoal Paper

In my past studio work, I'd use an Alien Bee B800 and several old Nikon speedlights triggered optically and by Pocket Wizards - all in full manual mode. This gives total control, but requires a lot more setup and tinkering. If the subject will stay still, you're set, but if you meter for the subject to be three feet from the key light and they move back a foot, they've just underexposed themselves a little bit. If they move closer by a foot, the difference is even more obvious, and you might clip some highlights.

This is where ETTL (Canon's fully automatic flash metering) really helps out. Here, I used a 580EX II on-camera to trigger three remote flashes - a 580EX and two 430EX II's. The remote 580ex and one of the 430ex's were in my softbox, camera right in group "A" and the other 430EX was in group B.

With ETTL, I can set the ratio of the two groups (A:B) in-camera - their power isn't determined until a split second before the shot. As I press the shutter, the on-camera 580EX quickly sends out coded pulses of light that tell each of the remote flash groups to send out a pulse of light at a known power. The camera quickly meters those pulses, then figures out how bright each flash needs to be. It then sends out another message to the flashes telling them how much power to use, and to fire at the same time. This all happens immediately before the shot - just about too fast to notice.

The drawback of ETTL is that two back-to-back shots might have slightly different exposures, but in the case of shooting children, the benefits outweigh it. If the kid moves forward, the flashes will fire with less power. If the kid moves back, the flashes fire with more. If the kid moves toward one light or the other, each light compensates accordingly.

In this shot, I used A:B mode on the flash with a ratio of 4:1 - the fill flash was 2 stops weaker (1/4 as bright) as key, just to fill in the shadows a little bit. Total flash exposure compensation was +2/3eV, which, sure enough, meant darkening it 2/3 stops in post-processing (but that's okay, because it actually reduces ISO noise to darken a photo). I was impressed at how well ETTL did. The on-camera flash was told not to contribute to the shot - just to master the remote slave flashes.

You may be wondering why I used two flashes in the key softbox - I've never done that before, and I didn't plan on it this time. It was a quick hack at the last second, because the 580EX II key light wasn't recharging as fast as I wanted, so I ball-bungeed a 430EX II next to it. With two flashes, each didn't have to work as hard, so recharging was faster.

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