This function uses a slow shutter speed with flash. The end result is a photo that combines a foreground lit by flash with a background lit using the scene’s existing light, with both properly exposed.
Note: Use of a tripod is recommended to prevent background motion blur.
Slow sync can be used to express motion and has two options, depending on the scene and type of movement. Rear curtain sync fires the flash at the end of the exposure, allowing you to track the trajectory of a moving subject. Front curtain sync fires the flash at the beginning of the exposure, making it ideal for capturing the moment an action begins, like the beginning of a smile.
Front curtain sync
The flash is fired at the beginning of the exposure. If the subject moves after the flash fires, subject motion looks unnatural.
Rear curtain sync
The flash is fired at the end of the exposure (after capturing the subject's trajectory). Subject motion looks natural.
This makes the camera “memorize” the optimal amount of flash needed to properly expose a subject positioned in the middle of the viewfinder. Even if scene brightness changes (depending on composition), the flash delivers the optimal amount of light.
Note: EOS-1D Series allows you to set FE Lock on subjects positioned off-center.
FE Lock on
Although light from the flash is bounced, the amount of light delivered by the flash remains the same, resulting in an optimally exposed subject.
FE Lock off
Light from the flash is reduced due to the white background, resulting in an underexposed subject.