canon 580EXII speedlite out of control
Trying to figure out why my flash gives out a large stroboscopic flash while i'm shooting. ... Am I pressing something on the 5Dmrk3 that trigers this? It used to happen on my 1dsmrk2 as well.
You are probably pressing the DoF button (maybe accidentally), which by default produces a lengthy burst of "modelling flash". You can switch this behaviour off by a CF on the flash.