Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 260)

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Using an EX-series Speedlite (sold separately) makes flash
photography easy.
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 on the camera’s
menu screen, see pages 263-269.
Flash exposure compensation
In the same way as normal exposure compensation, flash output
can be adjusted. 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 viewfinder or top LCD panel.
FE lock
This enables you to attain an appropriate flash exposure for a
specific part of the subject. Aim the viewfinder center over the
subject, press the camera’s <
B > button, then compose the shot
and take the picture.
D
Flash Photography
EOS-dedicated, EX-series Speedlites
Macro Lites Shoe-mount Speedlites
If [ z 2: Auto Lighting Optimizer ] (p.189) is set to any setting other than
[ Disable ], the image may still look bright even if a decreased exposure
compensation for a darker image is set.
If it is difficult to achieve focus with autofocus, the EOS-dedicated, external
Speedlite will automatically emit the AF-assist beam as necessary.