Canon Speedlite 600EX II-RT Manual (en)

e: Setting the Flash CoverageN
When you use the flash’s built-in wide panel together, you can perform 
flash photography covering the angle of view of an ultra-wide angle lens 
with focal length as wide as 14 mm.
Pull out the wide panel.
Pull out the protruding area located in 
the center of the wide panel.
The white catchlight panel is pulled 
out at the same time.
Push back the catchlight panel.
Push back the catchlight panel only, 
keeping the wide panel down.
Wide Panel
Since underexposure may occur, the <N> warning is displayed on 
the LCD panel when using the wide panel with bounce flash.
Do not pull out the wide panel with excessive force. Doing so may detach 
the wide panel from the Speedlite.
Angle of view of EF15mm f/2.8 Fisheye or EF8-15mm f/4L Fisheye USM 
is not supported.
The flash coverage is set automatically when using the wide panel. You 
cannot change the setting. (When the <[> button is pressed, <WIDE 
> is displayed.)