Canon MARK IV User Manual (en)

: Selecting the AF Mode
Suited for still subjects. When you 
press the shutter button halfway, the 
camera will focus only once.
When focus is achieved, the AF point 
which achieved focus will flash in red, 
and the focus confirmation light <o> 
in the viewfinder will also light.
With evaluative metering, the 
exposure setting will be set at the 
same time focus is achieved.
While you hold down the shutter 
button halfway, the focus will be 
locked. You can then recompose the 
shot if desired.
AF is also possible by pressing the 
<p> button.
One-Shot AF for Still Subjects
Focus confirmation light
AF point
If focus cannot be achieved, the focus confirmation light <o> in the 
viewfinder will blink. If this occurs, a picture cannot be taken even if the 
shutter button is pressed completely. Recompose the picture and try to 
focus again. Or see “When Autofocus Fails” (p.100).
If the [2 Beep] menu is set to [Off], the beeper will not sound when 
focus is achieved (p.52).
Focus Lock
After achieving focus with One-Shot AF, you can lock the focus on a subject 
and recompose the shot. This is called “focus lock.” This is convenient when 
you want to focus a subject not covered by the Area AF frame.