Canon EOS-1D C EOS 1D X Instruction Manual - Page 200

3 Setting the Flash Wireless functions Wireless (multiple) flash shooting is possible with radio or optical transmission. For details on wireless flash, refer to the Speedlite's instruction manual. Flash zoom (Flash coverage) With Speedlites having a zooming flash head, you can set the flash coverage. Normally, set this to [AUTO] so that the camera will automatically set the flash coverage to match the lens focal length. Shutter synchronization Normally, set this to [First-curtain synchronization] so that the flash fires immediately after the exposure starts. If [Second-curtain synchronization] is set, the flash will fire right before the shutter closes. When this is combined with a slow shutter speed, you can create a trail of light such as from car headlights at night. With Second-curtain synchronization, two flashes will be fired: Once when you press the shutter button completely, and once immediately before the exposure ends. If [High-speed synchronization] is set, the flash can be used at all shutter speeds. This is especially effective for portraits using fill flash when you want to give priority to the aperture setting. 200