Speedlite blown out question

Bluepoole

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Interesting effect I've noticed when shooting portrait shots with my Canon 430EX II Speedlite - when I zoom out completely, (at widest angle on my 17-85mm lens) white (or very light) clothes get blown out, but when I zoom in a bit, say at 35mm focal length, I get much better results.

Why the difference in results?

This happens in Program mode at ISO 100.

The purpose of this topic is not to get advice on how to get better shots, I use Manual mode and EV compensation of the flash to get great results, and the Recovery function of my Photoshop RAW plugin has also saved my butt a couple of times.

So all I want to know, why does the above (as explained) happen?
 
E-TTL flash metering is done with a metering pattern, just like exposure. I'm not sure, but I think that it defaults to centre weighted, even if that is not what your exposure metering on the camera is set to. It might just be evaluative metering though.

So when you zoom in or zoom out, you are most likely changing the areas that the camera is reading for the E-TTL flash metering...thus, it gives you different exposures.

If you think about this while shooting, you should be able to predict the differences in exposure...but it's still pretty easy to just keep an eye on the histogram and adjust the FEC as needed.
 

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