Canon MARK IV User Manual (en)

Autofocus can fail to achieve focus (the focus confirmation light <o> 
blinks) with certain subjects such as the following:
Subjects difficult to focus
Very low-contrast subjects
(Example: Blue sky, solid-color walls, etc.)
Subjects in very low light
Extremely backlit or reflective subjects
(Example: Car with a highly reflective body, etc.)
Near and far subjects covered by an AF point 
(Example: Animal in a cage, etc.)
Repetitive patterns
(Example: Skyscraper windows, computer keyboards, etc.)
In such cases, do one of the following:
(1) With One-Shot AF (p.95), focus an object at the same distance 
as the subject and lock the focus before recomposing.
(2) Set the lens focus mode switch to <MF> and focus manually.
Set the lens focus mode switch to 
Focus the subject.
Focus by turning the lens focusing 
ring until the subject looks sharp in 
the viewfinder.
When Autofocus Fails
MF: Manual Focusing
During automatic AF point selection (p.97), if you hold down the shutter 
button halfway and focus manually, the focus confirmation light <o> will 
light when the center AF point achieves focus.