Speedliter's Handbook: Learning to Craft Light with Canon Speedlites
Understanding Flash Photography: How to Shoot Great Photographs Using Electronic Flash
1/200 or 1/250 is the flash x-sync limit, because that is the fastest shutter speed that has both shutter curtains fully open during an exposure.
Canon calls the 2 shutter curtains the first and second curtains, while Nikon calls them front and rear.
By default, most cameras are set to sync the flash to fire when the first/front shutter curtain is fully open - front/first curtain sync.
When first curtain sync is set, if anything in the scene moves during the exposure a 'ghost' trail will
precede the direction of motion, because the movement happens
after the flash has fired.
Images like that don't look very natural, because the ghosting does not support the notion of motion.
Setting the camera to rear/second curtain sync the flash fires just before the rear/second curtain starts to close and end the exposure. - second/rear curtain sync.
When rear curtain sync is set, if anything in the scene moves during the exposure a 'ghost' trail will
trail the direction of motion, because the movement happens
before the flash has fired.
Ghosting that trails looks more natural, because the ghosting does support the notion of motion.
At shutter speeds faster than the cameras x-sync speed, one or both shutter curtains block a portion of the image sensor during an exposure, even if flash isn't used.
In other words the 2 shutter curtains form a slit. The faster the shutter speed, the narrower the sit becomes.
Flash can be done at those higher shutter speeds, but the flash unit has to fire multiple times during the exposure.
Canon calls this flash mode High Speed Sync, or HSS. Nikon calls it Auto-FP sync.
Not all DSLRs or flash units can do high speed sync, particularly entry-level grade cameras and flash units.
Flash units take time to re-cycle the main flash capacitor from a full power flash - 2 seconds and longer depending on the flash unit and how it is powered.
Since high speed sync requires multiple flashes per exposure and the shutter speed is faster than 1/200 or so, there isn't time to re-cycle the flash during an exposure time shorter than the camera's x-sync speed.
So high speed sync has to be done using well less than full power for each of the multiple flashes.
If 2 flashes are needed during the exposure each flash has to be made at 1/2 of full power.
If 4 flashes are needed during the exposure each flash has to be made at 1/4 of full power.
Because of the inverse square law, if the power is reduced by 1/2, only 1/4 as much light reaches the subject with each of the 2 flashes. At 1/4 power, only 1/16 as much light reaches the subject with each of the 4 flashes.
Flash units need to be much more powerful to be used on high speed sync mode so they can deliver enough light when they flash.
The camera and flash unit electronics also need to be more sophisticated to make high speed sync happen, which is why most entry-level cameras and flash units are not high speed sync capable.
