Read the “Second Curtain” blog before this one to make sure you understand the basics of the camera shutter mechanism.
The Shutter Sync speed is the minimum shutter speed that allows for the flash to lighten up the entire shutter in one go.
Lets take a step back to understand the above statement.
When you use your default camera flash, it would need to be fired such that the entire sensor gets light because of the flash correct?
Now, imagine a scenario where you have set the shutter speed as 1/1000. Now, when you click the shutter release button, the first curtain moves out and before the first curtain can move out completely the second curtain starts its journey thus not giving any chance for the flash to get the entire sensor open. Before the first curtain moves out completely, the second curtain starts its journey in-order to maintain the basic requirement of exposing each pixel for the same duration of time which in this case is 1/1000.
Generally, for all the DSLRs this speed that allows the first curtain to move out completely before the second curtain moves in is called the shutter sync speed and is around 1/250 or 1/200 for most cameras.
Any speed slower than this is ok for the flash but any speed faster than this and the flash will not be able to do its job well.
So what happens if you set your shutter to 1/1000 and pop open the flash?
The camera will reset the shutter speed back to the ‘shutter sync speed’ automatically.
Does it mean you can never click faster than this with a flash?
Not with the hot-shoe flash. There are however, better external flash guns available which have a High-Speed Sync mode that allows you to maintain the shutter speed yet evenly light all the pixels.
More on the High Speed sync later.