I have a Canon EOS 40D camera. We are having problems with using an external flash. The pictures always come out extremely dark with the external flash. The flash seems to work fine, it flashes brightly. The pictures look fine using the built in flash. The flash is a Promaster but we tried a Canon flash and it did the same thing. The camera store we bought the camera and the flash from says it looks like a synchronization issue and that it is a hardware problem with the boot. Their suggestion is to send it off to Canon to get it fixed. Before I did that, I wanted to see if there was an option.

I don't know if this will help and this post is two months old. The camera store may be right that the synchronization is the issue which tends me to point a finger at the external flash. There are at least three different flash units that will flash when the picture is made but only one type will sync with the camera. A digital flash unit is different from a film flash so a Canon EX type is required to sync with Canon TTL or TTL II digital cameras. An EZ will flash but it's off a bit, so I can only assume that your Promaster and Canon flash units are for film cameras. Okay and another thing that could be throwing things off a bit is the camera may have been programed in Custom Functions to use "Second Curtain" or "Rear Curtain". Just seams to me if the built in flash works then it's the external units at fault.