A640 Flash overexposure

I was taking a shot with my new A640, using the self timer, of a group of 14 people outdoors. I used aperture priority set at f2.8 and ISO at 400. I took a total of 3 identical shots with flash enabled. The reason for the wide aperture and high ISO was to ensure that the subject would be adequately lit. There was evening daylight, with an overcast sky.
Two of the shots were grossly overexposed, and unusable. The third was also overexposed, but less so, and could be corrected in PP.
The EXIF information shows shutter speeds of 1/60, 1/100 and 1/400 respectively, for the 3 shots.
So I am wondering why the exposure varied when the conditions did not alter.
Also I thought, perhaps mistakingly, that the duration of the flash would be controlled to suit the ISO and aperture setting, to get correct exposure.
If my settings were wrong, I would have expected similar results with the 3 shots.
I would welcome any comments as to what may have gone wrong here.

Examples?

You don't say how far away from the camera that the subject was. It doesn't take much strobe to overpower the scene at f2.8 and ISO 400. Perhaps a scene mode would have been a better choice. Your other option would have been to look at the first shot, see the overexposure, and then manually reduce the power of the flash or increase the f stop to balance the natural and strobe lighting.

Examples

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Was the camera on a tripod or hand-held? What metering mode were you using? The two you posted were only 2/3 stop apart, slight differences in composition when the meter reading was taken may make a difference there.

I also suspect that you may have underestimated the light level; judging from the EXIF (photos taken at 8:25 PM, Aug. 1) you still had a good half hour before sunset. That makes for fairly soft lighting, but not necessarily as dim as your meter settings led to. Judging by the shadows (the dog's in particular, is a give-away, also the (white?) trousers on the man to the right), the flash didn't actually do very much at all in these shots. Overexposure came from the available light. Might you have had Exposure Compensation set? The full EXIF isn't available in these re-sized images, so I can't see either that or the ISO values.

Jon wrote in post #3655088
Was the camera on a tripod or hand-held? What metering mode were you using? The two you posted were only 2/3 stop apart, slight differences in composition when the meter reading was taken may make a difference there.

I also suspect that you may have underestimated the light level; judging from the EXIF (photos taken at 8:25 PM, Aug. 1) you still had a good half hour before sunset. That makes for fairly soft lighting, but not necessarily as dim as your meter settings led to. Judging by the shadows (the dog's in particular, is a give-away, also the (white?) trousers on the man to the right), the flash didn't actually do very much at all in these shots. Overexposure came from the available light. Might you have had Exposure Compensation set? The full EXIF isn't available in these re-sized images, so I can't see either that or the ISO values.

The camera rested on a wall; the selftimer was in operation; 3 shots were taken because I didn't get in to position in time for the first 2 shots!
Metering mode was pattern.
Exposure compensation value was 0, i.e. not set.
I am wondering why the exposure is different for each of the 3 shots.
Perhaps I should have left it on Auto.

I would have . I notice a slight difference in composition between the two shots you posted; if the camera wasn't sitting firmly on the wall, pressing the shutter release to fire the camera might have rocked the camera down a bit so it was metering more on the deck than on the people, which would (especially with a dark wood deck) lead to overexposed light colours.

And I deduce, then, that you're the gent in the white trousers I mentioned above.

Jon wrote in post #3655735
And I deduce, then, that you're the gent in the white trousers I mentioned above.

Actually I am in the blue trousers with wine pullover; what happened was that I failed to get in frame for the first shot, but this was the least overexposed, as you can see in the link below below. The man looking sideways in the blue shirt is looking at me trying to get around in time, but the camera went off BEFORE the 10 seconds elapsed.
So I took 2 more shots which I did get in frame (sorry I misled you on this) but the details are washed out. The gent in the white trousers got in frame too - all of which is unimportant of course. I like the results of the A640 in daylight without flash, but in future will remmeber to put it in Auto for flash. Thanks for your comments.

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