Canon Speedlite 600EX II-RT Manual (en)

Flash coverage (the range covered by the flash light) can be set 
automatically or manually. With the <L> (automatic) setting, the flash 
coverage is adjusted automatically according to the focal length 
(shooting angle of view) of the lens in use and the image sensor size 
(p.26). With the <d> (manual) setting, you can manually set flash 
coverage in the range of 20 to 200 mm.
Press the <[> button.
Press function button 1 <[>.
The flash coverage value is 
Set the flash coverage.
When you want to set the flash 
coverage automatically, select <L>. 
When using the manual setting for 
the flash coverage, select <d> and 
the value (indicating the focal length 
in mm).
Turn <9> to set the flash coverage, 
then press <8>.
e: Setting the Flash CoverageN
When you set the flash coverage manually, set the same or a wider 
coverage than the angle of view for shooting to avoid darkening the 
periphery of the picture.
When a lens with a focal length less than 20 mm is attached, the 
<O> warning is displayed on the LCD panel. When using a camera 
with the image sensor size smaller than full-frame, the <O> 
warning is displayed when the actual shooting angle of view is wider than 
the angle of view of a 20 mm lens.
When shooting with the camera and Speedlite’s PC terminal connected 
by a commercially-available sync cord, set the flash coverage manually.