Canon Speedlite 600EX II-RT Manual (en)

E-TTL II/E-TTL Autoflash by Shooting Mode
EOS DIGITAL cameras have three sizes of image sensors, and the effective 
shooting angle of view of the attached lens varies depending on the size of image 
sensor. 600EX II-RT automatically recognizes the image sensor size of the EOS 
DIGITAL camera and automatically sets the flash coverage that is ideal for the 
effective shooting angle of view of a lens for the focal length range of 20-200mm.
This function optimizes the white balance during flash photography by 
transmitting the color temperature information to the EOS DIGITAL camera 
when the flash fires. When you set the camera’s white balance to <A> or 
<Q>, the function is enabled automatically. Refer to the specifications in your 
camera’s Instruction Manual to find out if it is compatible with this function.
When it is difficult to autofocus on the 
subject in low-light or when contrast is low 
during viewfinder shooting, the infrared 
AF-assist beam built into the flash 
automatically flashes to help autofocus.
The AF-assist beam supports all EOS cameras’ AF points. The AF-
assist beam supports 28 mm and longer focal lengths of the lens and its 
effective range (at 28mm focal length) is approx. 0.6 - 10 m/2.0 - 32.8 ft. 
at the center in the viewfinder and approx. 0.6 - 5 m/2.0 - 16.4 ft. at the 
periphery (AF points other than the center AF point).
Auto Zoom Adjustment to Image Sensor Size
Color Temperature Information Transmission
AF-Assist Beam
If a peripheral AF point or a wide-angle or telephoto lens is used, achieving 
focus may be difficult with an EOS-dedicated, external Speedlite’s AF-assist 
beam. In such a case, use the center AF point or an AF point close to the center.
During Live View shooting, the AF-assist beam is emitted even when the 
AF method is set to [Quick mode].
AF-assist beam firing can be disabled (C.Fn-08/p.115).