Canon EOS 5D Mark III Instruction Manual (en)

An EX-series Speedlite (sold separately) makes flash photography as 
easy as normal shooting.
For detailed instructions, refer to the EX-series Speedlite’s 
instruction manual. 
This camera is a Type-A camera that can use all 
the features of EX-series Speedlites.
To set the flash functions and flash Custom Functions with the camera’s 
menu, see pages 193-198.
FE lock
This enables you to attain a proper flash exposure for a specific part 
of the subject. Aim the viewfinder center over the subject, then press 
the camera’s <B> button and take the picture.
Flash exposure compensation
In the same way as normal exposure compensation, flash exposure 
compensation can be set. You can set flash exposure compensation 
up to ±3 stops in 1/3-stop increments.
Press the camera’s <m> button, then turn the <5> dial while 
looking in the viewfinder or at the LCD panel.
 Flash Photography
EOS-dedicated, EX-series Speedlites
Shoe-mount Speedlites
Macro Lites
If [z2: Auto Lighting Optimizer] (p.144) is not set to [Disable], the image 
may still look bright even if a darker flash exposure compensation amount 
has been set.
When it is difficult to achieve focus with autofocus, the EOS-dedicated, 
external Speedlite may automatically emit the AF-assist beam.