Canon EOS 5D MARK IV Instruction Manual page 110

S Selecting the AF Area and AF PointN
AF Point Display Indications
Pressing the <S> button lights up the AF points that are cross-type AF
points for high-precision autofocusing. The blinking AF points are
horizontal-line or vertical-line sensitive. For details, see pages 113-121.
Registering the AF point
You can register a frequently-used AF point to the camera.
When you use the button set with the [83: Custom Controls] (p.495)
menu's detailed settings screens for [Metering and AF start], [Switch
to registered AF point], [Direct AF point selection], [Selected AFpt
z Cent/Reg AFpt], or [Register/recall shooting func], you can
instantly switch from the current AF point to the registered AF point.
For details on registering the AF point, see page 500.
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