ETTL & flash exposure compensation

When you put your speedlite into manual mode, you can adjust the flash output on the speedlite itself in 1/3 stop increments (where each full stop is shown as a halving of the previous output - i.e. 1/2, 1/4, 1/8 etc. of full output).

When the speedlite is in ETTL mode, then you can adjust the flash output using FEC on your camera.

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Cheers,
Damien