As described in “Flash Sync Problems” in Chapter 5 of Stunning Digital Photography, flash synchronization problems can lead to uneven lighting in a flash picture (shown next). Most name-brand flashes support high-speed sync with the manufacturer’s camera bodies, allowing you to use flash at any shutter speed. Thus, if you use a name-brand flash, you can usually shoot at any shutter speed with anybody (but check your flash’s manual to be sure). 
When using a flash that does not support high-speed sync, such as a generic flash, you cannot use shutter speeds faster than your camera’s X-sync speed. The X-sync speed is the fastest shutter speed at which the camera’s shutter fully exposes the entire sensor at once, giving the flash the opportunity to fire and illuminate the entire picture evenly. I don’t know that anyone has ever chosen one body over another based on the X-sync speed. It’s only important if you want to use a generic flash with faster shutter speeds, and even then, X-sync speed on camera bodies varies very little. For example, most of the Canon lineup has an X-sync speed of 1/200th, but the top-end camera, the Canon 1DX, has an X-sync speed of 1/250th. The Sony NEX-6 has an X-sync speed of 1/160th. The Nikon D40 has a remarkably fast X-sync speed of 1/500th, due to a rather special shutter mechanism, but the more expensive D4 has an X-sync speed of only 1/250th. While I don’t expect you to choose a camera body based on its X-sync speed, portrait photographers using studio lighting or generic flashes in bright sunlight should be familiar with their camera’s X-sync speed. In bright sunlight, you often need to use shutter speeds that might be faster than your camera’s X-sync speed. If that’s the case, and your flashes don’t support high-speed sync, you will need to watch your shutter speed closely and verify that your flash is evenly illuminating the frame. The PocketWizard wireless camera triggers have a clever feature called HyperSync that can actually increase a camera’s X-Sync speed. For example, the Canon 50D normally has an X-sync speed of 1/250th, but when using a PocketWizard FlexTT5 and the HyperSync feature, can achieve X-sync speeds of 1/400th. For more information about wireless flash triggers, refer to the Flash Buying Guide later in this book.
