Friday, April 23, 2010

Optical slaves, a cautionary tale.

I spent about an hour last night thinking my new camera was defective. After repositioning a few of my strobes, trying for whiter backgrounds with less spill onto my subject, I took a few test shots, and noticed that my background wasn't blowing out. This was not completely unexpected, I had moved the lights after all... I watched carefully to make sure that all the strobes were firing for every shot, and they were.

So, I bumped the power a little on the strobes, from 1/2 or so to 3/4 (AB-400's). No luck. Wondering how far off I was, I opened up a lot, from f/8 to f/4. Nope. Dropped from 1/250 to 1/125. That started to help, now some of the background was blowing out. Bumped the AB's to full power, and now I had it, but this seemed like way too much light.

Spot metering with the light meter I almost never use, the background read f/22. So why no blowout? Worse still, as I kept shooting, the background would blow out sometimes, but not always. Had I really moved the lights that much? I didn't think so, but I put them back where they were, and the problems remained.

Before pitching a fit, I sat down and went through all the camera settings, hoping a clue would hit me. And that's when I noticed it. The on-camera flash was set to manual, as usual. But A and B were set to TTL! Ugh. So, my TTL pre-communications were firing all the strobes. After they fired, or at the end of their output, my shutter opened and an image was recorded. Obviously, all of this was way too fast to see by eye. It looked like everything was working fine, until I checked the exposure, that is.

Switching off the A and B units in the commander mode menu fixed everything, almost. Now, at 1/125 f/4 and all strobes at full, EVERYTHING was blown out. Way blown out. Back to about 3/8 power on the strobes, f/8 and 1/250. All better after a few more deep breaths.

Perhaps I should try a sync cord, now that I have a camera with a port... But more importantly, I need to remember the lesson: When it seems like my gear isn't working properly, assume user error, and re-check the setup.

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