Canon EOS 5DS Instruction Manual page 119

AI Servo 2nd image priority
You can set the AF operation characteristics and shutter-release timing
during continuous shooting after the first shot with AI Servo AF.
: Shooting speed priority
Priority is given to the continuous shooting speed instead of
achieving focus.
: Focus priority
Priority is given to achieving focus instead of the continuous
shooting speed. The picture will not be taken until focus is achieved.
Under shooting conditions which activate anti-flicker shooting (p.191), even
if [Shooting speed priority] is set, the continuous shooting speed may
become slightly slower or the shooting interval may become irregular.
3 Customizing AF FunctionsN
: Equal priority
Equal priority is given to focusing and
continuous shooting speed. In low
light or with low-contrast subjects,
shooting speed may slow down.
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