For some time now, I have this problem with my 300D + sigma 500DG flash. When in AV mode, the camera dosent seem to take the flash into consideration when it meters. As a result, the exposure time when the flash is attached and when I am not using the flash is the same.... Its like the camera ignore my flash when it is in AV mode. other mode such as P and A-DEP works fine with the flash.
Any clues as to why this happend? is it due to how the different metering modes used by the 300D in different modes?
Btw, Flash is set to E-TTL mode.