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Old Oct 10, 2011, 12:29 PM   #1
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Default ISO Confusion

I just don't understand something here. Two shots - first is with auto-ISO and the camera selected 500, the second is fixed at 100. Sony a580, 16-80Z, F58AM flash.

Both are 0ev, 1/80th, f/4.5. Why isn't the ISO100 one under-exposed and why would the camera select ISO500 when the flash is on and ISO100 works fine?



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Old Oct 10, 2011, 2:45 PM   #2
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That is Sony's "intelligence" automated,to make sure the there is no failed shot.They would rather compromise the higher ISO poor IQ than a failed shot.
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Old Oct 10, 2011, 3:09 PM   #3
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But what really confuses me is that two shots with the same exposure compensation, shutter speed and aperture but different ISO look completely the same. I'd expect if you held everything constant and quadrupled the ISO it would be *much* brighter
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Old Oct 10, 2011, 3:25 PM   #4
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What happens is the camera tells the flash to output more power for the ISO 100 shot and less power for the ISO 400 shot. Because the subject and background are so close, in both shots you have both the subject & background equally affected by the combination of flash/ambient light. So, if the TTL logic in the camera is doing it's job correctly, the shots SHOULD look the same.
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Old Oct 11, 2011, 2:54 AM   #5
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That makes sense - and it's worked because they do look paractically identical but I'd prefer it if the camera used the lowest ISO possible.
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Old Oct 11, 2011, 9:18 AM   #6
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The second shot ISO 100 is a lot brighter than the first.
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