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The difference is that the ST-E2 is ETTL only; no manual control.
While you can vary the power by using FEC, it is still measuring the subject reflectance first, assigning an output level, and then modifying that level by the + or - FEC amount. If a person steps into the frame wearing a black shirt, the flash output can be different than if he or she is wearing a white shirt.
If you're using a 580EX as Master, you can set an actual power level -- from Full power to 1/128th power (1/64th on the 430EX) -- and it will be...Show more →
I'm a little confused. Are you saying you can control a 430EX in manual from your 580EX on camera (power settings)? You could change the 430EX from half power to full using the 580EX on the camera?