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Terry Murphy's picture

High ISO with Flash

I have just bought a Canon 77D and it's first use was at an daytime, indoors children's birthday party.
The camera was initially set on Auto as I hadn't had an opportunity to become familiar with it. In general the photos looked a little overexposed and I was surprised to find the camera had chosen ISO of around 3200 even though flash was used.
After this I turned flash off and the camera selected an ISO of around 600 - 800. The shots were of similar subject matter to the first set but were of much better appearance.
My question is this - why would the camera select a much higher ISO when using flash and a lower ISO when not using flash under the same photographic conditions?

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Pat McEntee's picture


To explain a little deeper, your shutter has two curtains. The first covers the sensor while the second is to the left side. When you release the shutter, the first curtain moves to the right. allowing light to hit the sensor. When the proper time has elapsed, the second curtain moves to the right to cover the sensor. With faster shutter speeds, the second curtain will start to move even before the first is completely across. With very high shutter speeds, this can end up as just a sliver of light moving across the sensor.

For a flash, the camera synchronization wants the shutter completely open so that microsecond flash hits the entire sensor equally. If the flash fires before the first curtain is completely across or after the second curtain has started, you end up with shadows of no flash on a side.

When the camera is setting up the next shot with a flash, it takes into consideration how much light it needs with the shutter wide open (usually around 1/200 sec). Even with a wide open aperture, it needs a lot of light so opens the only thing it can, the ISO.

Set your ISO to 400 and pick a comfortable aperture, such as f8.0, and shoot in Av priority. Your flash will automatically take care of the rest if it is Canon compatible or with the built in flash. The camera will adjust the flash output to match the settings.

If you want to try a different ISO or aperture, feel free to experiment. Full auto tries too hard to be everything. Trying to retain some control does make a difference.

Terry Murphy's picture

Thanks Pat. I'm sorry to take so long to acknowledge your reply. Yes, my habit now is not to use auto ISO when using flash as you suggest so this is no longer an issue for me. It just seems an awkward and unsatisfactory strategy for the camera to adopt when using auto ISO with flash and I don't recollect this being an issue with my previous Canon cameras. Since my post I've read articles from others, much more knowledgeable than me, who have expressed surprise at this for both the 80D and 77D. I can imagine beginners, quite reasonably, setting ISO to auto and being very disappointed with the results and with no understanding of what caused them nor how to work around the issue.
Yesterday I revisited this (maybe I did something weird when first encountering the problem) and again tried some indoor shots with Auto ISO and flash and, sure enough, the camera selected ISO 3200 for every shot. Needless to say the images were unsatisfactory.
I should add that, in every other way, I'm very happy with the 77D.