Canon MARK IV User Manual (en)

An EX-series Speedlite (sold separately) makes flash photography as 
easy as shooting without flash.
For detailed instructions, see the EX-series Speedlite’s instruction 
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 121-124.
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 <I> button and take the picture.
Flash exposure compensation
In the same way as normal exposure compensation, you can set 
exposure compensation for flash. You can set flash exposure 
compensation up to ±3 stops in 1/3-stop increments.
Press the camera’s <Q> button, then turn the <5> dial while 
looking at the top LCD panel or viewfinder.
 Flash Photography
EOS-dedicated, EX-series Speedlites
Shoe-mount Speedlites
Macro Lites
If [8C.Fn II -4: Auto Lighting Optimizer] (p.81) is set to a setting other 
than [3: Disable], the image may still look bright even if a darker flash 
exposure has been set.
If the camera has difficulty autofocusing, the EOS-dedicated, external 
Speedlite may automatically emit the AF-assist beam.