Keep ISO low for flash
11Generally speaking, when shooting with flash it’s often best to shoot with the ISO set low, ideally between 100 and 400 to ensure the best image quality. Flashguns adjust power output when set to TTL mode, so when shooting portraits the subject will most often be perfectly lit. If you’re shooting in trickier situations where you would like the background to expose correctly, you can force the flash to emit a lower light level that’s balanced with ambient light by shooting at a higher ISO such as 800 or 1600.
Balance flash with ISO