20d aperture priority problem

I recently shot a wedding including some outdoor shots. I have the 17-85 cannon

lens with the 580ex in high speed sync. iso at 200, f5.6. For the outdoor shots

I had a series of pics were the s/s was 250 and all shots were overexposed then

the s/s kicked up to 1250 for properly exposed shots. I didn't make any

adjustments during those shots so I'm not sure what happened. Anyone else out

there ever heard of this problem?

 

thanks, bill

could it be the iso 200 AND the f5.6? try f 11 or so? is ti still doing it?

The "sunny 16" rule suggests a non-flash exposure of 1/1600 at f5.6 at ISO 200, assuming the sun was out. So 1/250 would certainly result in overexposure. I don't understand why your camera didn't set a higher speed i.e. the 1/1250 which it subsequently correctly selected.

Having had similar experiences with the same camera, I usually use manual exposure for flash, sometimes shutter priority but rarely aperture priority. Check the viewfinder meter to be sure the shot is not overexposed.

Sounds like for the first series, you weren't really in high speed sync, since 1/250th is the fastest sync speed possible and the camera will default to that if you aren't in high speed sync. Switch problem, maybe? I'd suggest glancing at the histograms of the first few images in a series, and then checking again occasionally. Even though the LCD is hard to see outside, you can see completely overexposed images as well as the histogram.