Help -- Overexposing Flash shots w/ diffuser

[NeoteriX]

For some reason, when I try using bounce shots with my Sigma EF500 Super I just got in plus the Omnibounce diffuser, the shots seem to be really overexposing. When I take a shot with the setup, you can hear the capacitor fully discharge and the whine of it recycling. When I take a bounce shot or otherwise without the diffuser, it's a typical shot and doesn't overexpose. I have to turn the on flash FEC to like -1 1/3 for it to resemble the shot w/out the diffuser.

Also, an aside question -- is the FEC adjustment on camera and the one on flash one and the same or are they adjusting different things?

pics below. 1st shot is a bounced flash w/out diffuser. 2nd shot is a bounced flash w/ Omnibounce diffuser. 3rd shot is of the flash with the diffuser on it.

[Collins]

At the time of me reading this, I can't see your pics, but typically, when using a diffuser that slides over the front of the flash, the main problem is it partially covers the flash's sensor that it uses to meter the scene.

With that said, it is getting less light than is available and therefore ends up firing at max output or close to it.

Use a TTL flash metering mode or the manual mode on your flash to work around this.

[mattchow]

fec is controlled by the flash on the sigma, as is 2nd curtain sync. make sure your flash dosent have the "17mm" cover out either. since the omnibounce dosent cover any sensors just check what mode you are in for shooting, i believe the consensus is to start at a 1/200 shutter speed with a aperature of f/8 and adjust from there.