Canon EOS Rebel T5i 18-55mm IS STM Kit Instruction Manual page 165

Shooting conditions that make focusing difficult
Low-contrast subjects such as the blue sky, solid-color flat surfaces
or when highlights or shadow details are lost.
Subjects in low light.
Stripes and other patterns where there is contrast only in the
horizontal direction.
Subjects with repetitive patterns (skyscraper windows, computer
keyboards, etc.).
Fine lines and subject outlines.
Under a light source whose brightness, color, or pattern keeps changing.
Night scenes or points of light.
When the image flickers under fluorescent or LED light sources.
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 that 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 very far out of focus.
Soft focus effect is applied with a soft focus lens.
A special effect filter is used.
Noise (spots, banding, etc.) appears on the screen during AF.
If focus is not achieved with the above shooting conditions, set the lens
focus mode switch to <MF> and focus manually.
If you use AF with any of the following lenses, using [Quick mode] is
recommended. If you use the [u+Tracking], [FlexiZone - Multi] or
[FlexiZone - Single] for AF, it may take a longer time to achieve focus or
the camera may not be able to achieve correct focus.
EF50mm f/1.4 USM, EF50mm f/1.8 II, EF50mm f/2.5 Compact Macro,
EF75-300mm f/4-5.6 III, EF75-300mm f/4-5.6 III USM
For information on discontinued lenses, refer to your local Canon Web
site.
Changing the Autofocus Method (AF Method)
165