Make sure you are using the right camera sync speed with your flash.
Camera sync speed refers to the fastest shutter speed to which you can set your camera in order for it to sync with the camera's flash. This shutter speed varies greatly depending on the type of shutter you have and age of your camera. You can use lower shutter speeds than the sync speed, but using faster shutter speeds can result in big problems for your images.
Focal Plane Shutters
Exposure problems with a flash are directly related to how a focal plane shutter works. This is the most common type of shutter found in 35mm SLR (single-lens reflex) cameras. Focal plane shutters consist of two overlapping metal curtains that travel across the film plane or sensor. As the first curtain moves, it creates a window through which light enters the camera. The second curtain follows after the designated shutter speed duration, closing the window and ending the exposure.
Sync Speed Problem
At high shutter speeds, the second curtain begins to close before the first one has completely opened the window, creating a traveling slit of exposure. The problem with this slit is that only a portion of the image receives the very brief blast of light from the camera's flash. If you choose a shutter speed that is higher than your camera's sync speed, the image may only register on the lower portion of the composition, or only on one side, depending on the direction that your focal plane shutter travels. Using the proper sync speed ensures that the flash fires only as the entire image receives exposure rather than just a portion of it.
Leaf Shutters
A leaf shutter is a different type of shutter mechanism that is built into a camera's lens rather than its body. It consists of multiple, overlapping leaves that retract from the center, open fully, then close again. Because leaf shutters always open completely no matter what you set the shutter speed to, they do not suffer from the same issues with sync speed as a focal plane shutter. These cameras have an "X-sync" setting that fires the flash only once the shutter is fully open.
Different Sync Speeds
Older cameras usually have a particular shutter speed listed on the dial in a different color to indicate its sync speed. In the 1970s and 1980s, sync speeds faster than 1/60 of a second were not common. Even today some camera models, such as Leica rangefinders, may have sync speeds as slow as 1/30. Contemporary SLRs (both amateur and professional models) as well as digital point-and-shoots have sync speeds that range from 1/125 to 1/500. Consult your camera's manual to determine its sync speed if it is not indicated on the shutter-speed display.
Tags: sync speed, shutter speed, your camera, focal plane shutter, plane shutter, shutter speeds