So today my 400D and I did our first engagement photo gig--my wife's sister's. (I've had the camera for 3 years, but I've done mostly landscape and astrophotography; this is the first "big event" where others are counting on me.) I took several photos in Av mode with a wide aperture to separate the couple from the background. Then I popped up the flash to fill in some shadows. Mistake.

I found when I downloaded my pictures that the ones with fill flash were like 3 stops overexposed. Highlights irretrievably blown even though I shot RAW. "Crazy," I thought. "The flash can't be THAT bright."

Then I noticed the non-flash shots--which were exposed properly--had exposure times of 1/1000 - 1/1600 s, and the flash ones were all 1/200.

Aha. So that's this min flash sync thing I vaguely remember reading in the specs a very long time ago.

Just a note of warning to those who might not be aware of this little quirk. Av + sunny day + fill flash = bad news.

The affected photos are not a total loss--at least they work as high-key B&W shots...