Canon MARK IV User Manual (en)

Using AF to Focus
AF operation
Focusing will take slightly longer.
Even when focus has been achieved, pressing the shutter button 
halfway will focus again.
The image brightness may change during and after the AF 
If the light source changes while the Live View image is displayed, 
the screen might flicker and focusing can be difficult. If this happens, 
stop the Live View shooting and autofocus under the actual light 
source first.
If you press the <u> button in the Live mode, the AF point area will 
be magnified. If focusing is difficult in the magnified view, return to 
the normal view and autofocus. Note that the AF speed may differ 
between the normal and magnified views.
If you autofocus in the Live mode’s normal view and then magnify 
the image, the focus might be off.
If you shoot a peripheral subject and the target subject is slightly out 
of focus, aim the center AF point over the subject to focus, then take 
the picture.
In the u Live mode, pressing the <u> button will not magnify the 
The external Speedlite will not emit the AF-assist beam.
Live Mode and u (Face Detection) Live Mode Notes
When you press <
> straight down, it will switch to the Live mode 
(p.131). You can tilt <9> to move the AF point. If you press <9> 
straight down again, it will switch back to u (face detection) Live mode.
Since AF is not possible with a face detected near the edge of the 
picture, the <p> will be grayed out. Then if you press the shutter button 
halfway, the center AF point < > will be used to focus.