I am still working out the various settings on my new Canon Speedlite 430EXII which is on my Canon 500D. Mostly I just leave it on auto and it does a great job, but I like playing around with the settings to see what they all do. Yes I do have the manual and have read it, but there's nothing like hands on.
There are a few different ways to adjust the flash output.
1 ..... In manual mode I can set it from -3 to +3 via the flash.
2 ..... In ETTL mode I can adjust it via the "SEL/SET" button on the flash, and
3...... In the camera under Flash control / Ext flash func setting / Flash exp comp and then adjust from -2 to +2.
To me they all do the same job except that in manual mode it only affects the output when bouncing the light.
Am I right in my assumptions ?