Le 1D MIIn a une synchro X à 1/250
Mais le 550ex permet de dépasser cette limite quand on active le "highspeed sync" (cf, page 32 du manuel) :
When you set the synchronization mode to high-speed sync (FP flash),

. When highspeed sync has been set, is displayed in the viewfinder.
Dans le manuel du Multimax :
D: Fast Mode - RECEIVER – ~/MENU A D
The MultiMAX is designed to sync cameras and flash units at shutters speeds up to 1/250th for most focal plane shutters (35mm) and 1/500th for most leaf shutters.
Some camera and flash combinations are capable of fast sync speeds up to 1/1000th. The MultiMAX (set for RECEIVE mode) is capable of operation at these speeds in Fast Mode.

.
A MultiMAX (set for RECEIVE mode) in fast mode will show FAST MODE on the display. While in this mode Quad-Triggering and Relay Mode are not available and the A B C D L keys perform no function on a RECEIVE unit. A MultiMAX (set for TRANSMIT mode) will trigger RECEIVE units in FAST MODE with any remote zone selected, but will perform confirm ation only on zone A.
Donc, si je comprends bien, il est possible d'aller jusqu'à 1/1000 avec la combinaison 1DMIIn + 550ex + Multimax ?
Quelqu'un a un peu d'expérience ?