Canon EOS 7D Instruction Manual page 144

Using AF to Focus
You cannot take a picture during autofocusing. Take the picture only while
the Live View image is displayed.
144
Focus the subject.
3
Aim the AF points over the subjects
and press the shutter button halfway.
The Live View image will turn off, the
reflex mirror will go back down, and
AF will be executed.
When focus is achieved, the beeper
will sound and the Live View image
will reappear.
The AF point used to focus will be
displayed in red.
Take the picture.
4
Check the focus and exposure, then
press the shutter button completely to
take the picture (p.132).