I have just upgraded to a 20D and spent a few hours yesterday getting used to all the new functions, I also tried my 580 flash with it, and by accident i pressed the depth of field preview button and it fired the flash in a strobe like burst. I have been unable to find reference to function in the 20D or the 580 instruction books, has any body else experienced this before with this camera and flash combination?
Quadphoto.

