Canon EOS 5Ds Instruction Manual page 272

Using AF to Focus (AF Method)
The size of the AF point varies depending on the setting of [z4: Crop/
aspect ratio].
During movie shooting, if [Movie Servo AF] is set to [Enable], the AF
point will be displayed in a larger size.
Notes for AF
AF Operation
Even when focus is achieved, pressing the shutter button halfway
will focus again.
The image brightness may change during and after the AF
operation.
If the light source changes while the Live View image is displayed,
the screen may flicker and focusing may be difficult. If this happens,
exit Live View shooting and execute AF under the actual light
source.
If you cannot achieve focus with AF, set the lens's focus mode switch to
<MF> and focus manually (p.275).
If you shoot the subject at the periphery and it is slightly out of focus, aim
the center AF point over the subject you want to focus on, focus again,
and then take the picture.
The external Speedlite will not emit the AF-assist beam. However, if an
EX-series Speedlite (sold separately) equipped with an LED light is
used, the LED light will turn on for AF-assist when necessary.
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