If we then only increase the ISO by three stops we get ISO 3200. You can see the resulting photograph has now more details from the overall scene (the ambient light was more recorded by the camera) and our subject is now over-exposed:
If we then decrease the ISO value down to 100, change nothing else and take an exposure the subject will then become un-exposed and the ambient light not recorded:
I hope you now understand why I compared the ISO as a master switch for your Flash Photography exposure. By changing its value, you affect the entire exposure: the part of the scene lit by your flash as well as the rest of the scene lit by the ambient light.
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