Focus Mode Switch; Focusing Ring; Zoom Ring - Canon EF24-105mm F4L IS USM Instruction

3. Setting the Focus Mode
To shoot in autofocus mode, set the focus mode
switch to AF.
To use only manual focusing, set the focus mode
switch to MF, and focus by turning the focusing
ring. The focusing ring always works, regardless
of the focus mode.
After autofocusing in ONE SHOT AF mode, focus
manually by pressing the shutter button halfway
and turning the focusing ring. (Full-time manual
focus)
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4. Zooming
To zoom, turn the lens' zoom ring.
Be sure to finish zooming before focusing.
Changing the zoom ring after focusing can affect
the focus.