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?

  • \$\begingroup\$ Are you using "green square" mode? \$\endgroup\$
    – Nzbuu
    Commented Dec 13, 2011 at 21:12
  • \$\begingroup\$ @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. \$\endgroup\$
    – ab.aditya
    Commented Jan 12, 2012 at 4:35

2 Answers 2


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.

  • \$\begingroup\$ 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) \$\endgroup\$
    – Gapton
    Commented Dec 14, 2011 at 4:19
  • \$\begingroup\$ @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.) \$\endgroup\$
    – mattdm
    Commented Dec 14, 2011 at 4:24
  • \$\begingroup\$ 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). \$\endgroup\$
    – Michael C
    Commented Aug 23, 2014 at 20:41

This solution or "hack" from Branko Mitic (link below) seems to achieve the desired effect. "Auto ISO" while using flash by way of setting super high manual ISO and utilizing Safety shift to push it down to what is needed. Not perfect but beautiful how it fakes the camera into using higher than 400 ISO with flash.

I have a 1Dx -I imagine most others are similar in being able to achieve same settings. You can go from bright sun to total dark and get predictable exposures.

Speedlite: Set Flash to TTL Disable HSS Set Flash sync speed in Av mode to "Auto" (important)

Camera Set mode to AV (mandatory) Set manual ISO to very high value 512000 etc. Set ISO Min. shutter speed to 1/250 (or slower) Enable Safety shift ISO (option: set upper and lower shutter speed "Range" to a narrow setting to taste for more/less ambient- mine is set to Highest speed 1/60 Lowest speed 1/30 )

Watch your "auto ISO" change to the available light via "Safety shift" with flash !

Adjust Exposure Compensation and Flash Exposure Compensation to taste. If the ambient light is Tungsten use a CTO filter on your flash Tungsten WB will be in the ball park.



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