Those info are needed to better explain what's going on....
I notice all your shots were taken at ISO 200 and f2.8 with shutter speed 1/90 or faster. I think at these settings indoor, there should not be enough ambient light to light up any of your photos. They would all had to be lighted by the flash.
Typical fan speed is 1200 RPM which means 20 revolutions per second or 1 revolutions per 1/20 second or 360 degrees per 1/20 second. To freeze the motion of the fan, you probably don't want the fan blade to move more than 1 degree during exposure. And the fan will move 1 degree in 1/(20X360) or 1/7200 sec.
Your flash at minimum power will flash for a duration of 1/20000 sec and maximum power for 1/1200 sec.
For your three photos at or below the sync speed, the flash happened to flash at close to minimum power. Thus the fan blades were all frozen by the flash firing at around 1/20000 sec to 1/10000 sec. Note the shutter speed would not have affected the flash exposure. It would only affect the ambient exposure which in this case would be very dark and have no effect on your photos.
In high speed sync mode, the flash fires continuously, many thousands of times per second. All these pulses of light essentially merge together into one long "pulse" that stays on the entire time the shutter is open. So for HSS, the light from the flash kind of becomes the "new ambient light". This resulted in capturing the fan movement. With HSS, the fan movement will have to be freezed by the shutter speed. You can actually see less fan blur as the shutter went up to 500. You should get less blur at 1/4000. But still not be as good as the ones you got at or below sync speed with the flash.
(I think I said in another post that HSS would fire the flash a discreet small number of times. I think I was wrong. It actually flash thousand of times in a second.)
Last edited by ma318; 01-28-2010 at 09:57 PM.