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I've observed this behaviour on my Canon 550D, and I was wondering why it didn't use a lower or even variable ISO. I'm not sure if this is a camera/brand specific setting.

Is there any particular reason for this behaviour? Are there any scenarios where a different ISO is chosen, as I have not encountered it so far?

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Are you using "green square" mode? –  Nzbuu Dec 13 '11 at 21:12
    
@mattdm Your answer addresses most of the points. If you could add some information regarding the auto-ISO logic, then I will accept the answer. If it always defaults to ISO 400 (negligible noise), then things are fine. However, if it switches to a higher value under certain circumstances, it would be good to know. –  ab.aditya Jan 12 '12 at 4:35

1 Answer 1

up vote 10 down vote accepted

While the specific value chosen will be brand-specific, you're right that this is common behavior. My Fujifilm point & shoot favored ISO 800.

Increasing the ISO from 100 to 400 doubles the effective range of your flash, which is important with a relatively-anemic built-in flash. It also means that half of the flash power can be used for a subject within the normal range, saving battery life, decreasing the time it takes to be ready to flash again, and shortening the duration of the flash pulse to better freeze motion.

Another approach would be to increase aperture (more open; smaller numbers), which would have a similar effect, but would reduce depth of field. That means focus has to be more dead-on, and is generally not what auto-exposure modes are programmed to go for unless they have no other option.

I don't know the specifics of the auto-ISO logic in your camera model, but I wouldn't be surprised if it favors keeping ISO at around 400 and only increasing it only when there's not enough flash power.

When I'm using off-camera flash, I usually set ISO manually, but I generally default to around 400, just like your auto mode. This gives me a lot of flexibility in aperture, and even though my camera isn't the latest sensor generation, that ISO is clean enough that I don't really worry about noise.

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I do not have a direct reference, but I believe I read somewhere that the actual duration of all flash is something around 1/1000 to 1/3000 of a second. Increasing the ISO should not affect the duration of the flash to better freeze subject as it is super fast as it is. If I am not mistaken, in a scene which is lit solely by a flash, shutter speed of 1/10 and 1/200 should not make any difference. (if it goes above the max flash sync speed thats a different story) –  Gapton Dec 14 '11 at 4:19
    
@Gapton, at full power, the flash duration can be significantly longer. On my Metz flash, the T.1 duration is 1/125ths of a second — slower than my camera's sync speed! Reducing power quickly gets into the much faster range you mention. (Down to around a tenth of that, even.) –  mattdm Dec 14 '11 at 4:24
    
It all depends on the type (studio, speedlights, etc.) and even specific model of flash. Some control power by altering the duration (most speedlights), others do it by primarily altering the amount of peak power (Many, but far from all, studio types). –  Michael Clark Aug 23 at 20:41

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