Canon EOS 5DS Instruction Manual page 274

Using AF to Focus (AF Method)
Magnified View for FlexiZone - Single
The image will be magnified at the AF point.
If you press the shutter button halfway, AF will be performed in the
magnified view.
If focusing is difficult in the magnified view, return to the normal view
and perform AF.
Magnified view is not possible with [u +Tracking].
If AF is perfomed in the normal view and then magnify the view, accurate
focus may not be achieved.
AF speed differs between normal view and magnified view.
Continuous AF (p.264) or Movie Servo AF (p.313) does not work during
the magnified view.
The [x6] and [x16] views are always displayed in magnification of the
view with [Full-frame] setting, regardless of the [z4: Crop/aspect
ratio] setting.
During magnified view, achieving focus becomes more difficult due to
camera shake. Using a tripod is recommended.
If [z4: Crop/aspect ratio] is set to [1.3x (crop)] or [1.6x (crop)], [X1.3]
or [X1.6] will be displayed when starting the magnified view.
In magnified view, the exposure is locked. (Shutter speed and aperture
will be displayed in red.)
274
With [FlexiZone - Single], pressing the
<u> button will magnify the image to
check the focus in the following
sequence: approx. 6x, then approx. 16x.