Bracketing for HDR by Changing Flash Intensity vs. Exposure

[William Steinmetz]

Who can help me solve this riddle? I recently shot an HDR table-top sequence by stepping my flash intensity through 5 increments from -2 to +2. The illuminated foreground changed as expected, BUT so did the distant sun-lit background even though the exposure was constant through the sequence at 1/125 at f 2.4. Why would the background exposure change too?

 

[KmH]

What camera make/model?
What focal length lens?
What flash unit, what focal length was the flash unit set to?

 

[john.margetts]

If shutter and aperture were constant, the only way the background exposure could change is if you had ISO set to Auto.

 

[480sparky]

The camera adjusted the exposure based on the changes made to the output of the speedlight. Doing so caused the unchanging natural-light background to be rendered different according to the changing exposures.

Same reason you can easily be blinded by a match in a dark room, but you may not even see it in direct sunlight. The match isn't any darker.... your eyes just adjusted from the dark room to the bright sunlit area.

 

[William Steinmetz]

I was fooled; the background IS the same in the attached comparison shots from the ends of the HDR range. I was visually tricked by not isolating the background in my comparison, but viewing the whole image.

Camera: Olympus C8080WZ, 1/125 @ F2.4, ISO 50, internal flash, 31 mm focus