Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 266)

3 Setting the Flash Function
266
Wireless functions / Flash ratio control
Wireless (multiple) flash shooting is
possible with radio or optical
transmission.
For details on wireless flash, refer to the
Instruction Manual of a Speedlite
compatible with wireless flash shooting.
With a macro flash (MR-14EX II, etc.)
compatible with flash function settings,
you can set the flash ratio between flash
tubes or flash heads A and B, or use
wireless flash with additional slave units.
For details on flash ratio control, refer to
the macro flash’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.