Canon EOS 90D Advanced User's Manual page 80

Fully Automatic Shooting (Scene Intelligent Auto)
Recomposing the Shot
Depending on the scene, positioning the subject toward the left or right
to include a balanced background will result in a picture with better
perspective.
Pressing the shutter button halfway to focus on a still subject will lock the
focus on that subject. Recompose the shot while keeping the shutter button
pressed halfway, and then press the shutter button completely to take the
picture. This is called "focus lock".
In Live View shooting, the camera continues to focus on any faces initially
o
detected and focused on, even if you recompose the shot.
Shooting a Moving Subject
Pressing the shutter button halfway tracks moving subjects to keep them in
focus.
Keep the subject in the Area AF frame (in viewfi nder shooting) or on the
screen (in Live View shooting) as you hold down the shutter button halfway,
and at the decisive moment, press the shutter button completely.
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