A77 flash overexposure problems: hypothesis

Started Jan 16, 2013 | Discussions thread
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Chris Malcolm
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A77 flash overexposure problems: hypothesis
Jan 16, 2013

I have an A77 and a Sony HVL F43am flash. I've taken a few dozen flash photos so far, mostly TTL-metered ceiling bounce, and so far haven't encountered any of the intermittent overexposure problems others have mentioned.

I have noticed that when the camera is in flash mode with the HVL F43 on the hot shoe and ISO is on AUTO, that a half press of the shutter button doesn't tell me what ISO the camera has chosen. Presumably because it won't choose until it's done the pre-flash to test the flash lighting. When it does choose the ISO it usually chooses a much higher ISO than I would be happy with, i.e. one which will degrade image quality with noise compared to an ISO 100 shot, even if the gun is capable of producing enough power for an ISO 100 shot.

For example on my last flash shot on auto ISO I let it choose any ISO. It chose ISO 1600, and exposed correctly. Yet there was enough power for an ISO 100 shot, because when I selected ISO it exposed correctly again, identical histograms in both the reviewed shots. I personally regard that as a fault in the camera's flash exposure settings algorithm -- for best image quality it should choose the lowest ISO within the power reach of the flashgun. On the other hand for people who want to take lots of flash pics as rapidly as possible there is the immediate problem of flash recharging delay, and the possible later problem of overheating the flash (or having the flash turn off because it's scared of becoming too hot). So it's arguable that these high ISO flash exposure choices aren't a fault, just a compromise which happens not to best fit my own photographic habits.

I note that some people who have complained of flash overexposure have discovered that the problem was that they were shooting with a wide aperture and a high ISO which required their flash to be turned down to a lower power than it was capable of. So I wonder if these A77 flash overexposure shots are due to the camera's flash exposure choice algorithm not knowing how low the flash gun power can be turned down, and sometimes expecting lower power than it can manage? That is a well known problem in macro photography when the gun is very close to the subject -- special macro flashes can be turned down very low to avoid that problem. Flashes vary in how much they will permit their pwoer to be turned down, perhaps because there's a compromise involved between flash duration and colour temperature drift.

If so, these flash overexposure problems should be avoidable if ISO is manually set low (and possibly as well in some cases aperture is not too wide). Which might explain why so far I haven't found my A77 doing any of these flash overexposures -- because when using flash I usually set the ISO as low as it will go.

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Chris Malcolm

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