Canon MARK IV User Manual (en)

Using AF to Focus
Shooting conditions which can make focusing difficult:
Low-contrast subjects such as the blue sky and solid-color, flat 
Subjects in low light.
Stripes and other patterns where there is contrast only in the 
horizontal direction.
Under a light source whose brightness, color, or pattern keeps 
Night scenes or points of light.
Under fluorescent lighting or when the image flickers.
Extremely small subjects.
Subjects at the edge of the picture.
Subjects strongly reflecting light.
The AF point covers both a near and faraway subject (such as an 
animal in a cage).
Subjects which keep moving within the AF point and cannot keep 
still due to camera shake or subject blur.
A subject approaching or moving away from the camera.
Autofocusing while the subject is way out of focus.
Soft focus effect is applied with a soft focus lens.
A special effects filter is used.