Originally posted by derekkite Actually no. The Metz AF50-1 at 1/1 power is 1/250th of a second, at 1/2 it is 1/900. 1/4 to 1/16 it gets interesting as a way to freeze movement.
The sync speed it the fastest speed that the sensor is entirely exposed to light during the exposure. Faster speeds the trailing curtain follows the leading curtain across the sensor, and if a flash is used, gives a band of exposed area, the rest darker.
The limit is the shutter design and construction.
Ok, 1/250s instead of 1/1,000s is the maximum shutter speed to fire this cobra flash at full power (I had studio flashes in mind, the flash duration of which is shorter at full power).
As long as the synch speed is lower than 1/250s (in your example), you will get the same illumination power from the flash whatever the synch speed is and whatever actual shutter speed, lower than the synch speed, you are using.
At shutter speeds higher than the synch speed, the trailing curtain follows the leading curtain across the sensor, exposing a progressive band, as you wrote. In this case you need HSS (High Synch Speed): the flash fires several times during the exposure to successively illuminate several portions of the sensor. The flip side of the coin is that each flash then corresponds to a limited power : 1/4, 1/8, 1/16 or 1/32 of max power, depending on the number of successive flashes. There you 'loose' a lot of the flash's illumination power.