Canon EOS 5DS Instruction Manual page 122

3 Customizing AF FunctionsN
One-Shot AF release priority
You can set the AF operation characteristics and shutter-release timing
for One-Shot AF.
122
: Focus priority
The picture will not be taken until
focus is achieved. Effective when you
want to achieve focus before
capturing the shot.
: Release priority
Priority is given to the timing of
shutter release instead of achieving
focus. This gives priority to capturing
the image rather than achieving
correct focus.
Note that the picture will be taken
even if focus has not been
achieved.