nico1

No, i think you can't : if you put the sigma flash on slave position it will fire when the E-TTL flash of the built camera flash fires ! Your settings must be wrong because the sigma must fires. But in any case it won't contribute to the exposure of the shot.

About what have been said previously :

"The focus point selection makes no difference to exposure determination unless you are using flash."

 

No that's wrong. The exposure system of the 300d calculates the exposition with a "higher weight" on the area of the active focus point.

That's in my opinion one the most annoying aspect of the exposure system of this camera because the importance given to the active focus point area in the exposure calculation is too high.

 

(You can very easily test this point)

The sigma 500DG can act as an optical slave...

I've just bought one but i have got some problems with my Canon 300D. If the sigma flash is used as a slave (optical) and fired by the integrated 300D flash, it doesn't work.. I think it's what the user manual indicates effectively about E-TTL : "if your are using a canon brand flash unit with ETTL function [...] do not use ETTL Function as the monitor pre flash may cause the slave to fire prematurely".

Okay but we can't disable the ETTL on the 300D. So is there a solution ? Help welcome ! Regards