Canon EOS 5D Mark III Instruction Manual (en)

 Setting the FlashN
Wireless functions
Wireless (multiple) flash shooting is 
possible with radio or optical 
transmission. For details on wireless 
flash, refer to the Speedlite’s instruction 
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 
] 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.