Auto ISO capped at 400 when Flash Attached on 5D III

Started May 11, 2012 | Discussions thread
Enzyme01
Forum MemberPosts: 83
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Re: what????
In reply to Rumi Siddiqui, May 25, 2012

Don't worry. You don't need auto ISO for what you're doing.

When shooting without flash your camera has three exposure variables: Aperture, shutter speed and ISO. But when you add a flash in ETTL mode you add a fourth variable - the strength of the flash output from your speedlite.

Assuming that you are using ETTL mode then just set your ISO to 100, if that is what you want. Your camera will do some work the figure out how strong the flash should be and make the adjustments automatically for you. That is what ETTL is for.

If you lock your ISO at 100 and rely on the camera varying the flash output then the flash will be putting out more power than it would if the ISO was 400. You will go through the batteries a bit quicker, you'll have a longer recycle time between flashes, and your ability to freeze motion will not be as good (if there is not much ambient light). You will also need to be aware that you will hit the limit of your flash's power sooner if you are on ISO 100 instead of 400, so you may still need to increase the ISO in some circumstances.

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