steve, a workaround to solve the FP issue with the Sigma is to shoot TV at your desired shutter speed (presuming to be faster than the your camera's max sync speed) and check the histogram for exposure.
Canon's 420 EX has a "full-time" high speed sync model by flip of a switch. I don't under why the 500 DG Super, being a newer and more than comparable model, doesn't have such feature.



