While shooting outdoors with high speed sync I've encountered a strange behaviour on my of camera flash (D300); the exposure went down a 1/3 stop with every shutter release and stayed at the maximum flash compensation of -3EV. Everything went fine before I changed to manual mode. The SB 900 and a SB 800 showed the same behaviour.
What happend? Does high speed sync has some restrictions while shooting in manual mode? Did I switched the camera to some unknown flash bracketing settings?
BTW: What is the advantage of 250 vs. 320 high speed sync?