Canon EF 70-300mm f4-5.6 IS USM - Manual (Page 7)

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ENG-6
5. Fixing the Zoom Ring
The zoom ring can be fixed to keep the lens at the shortest point. This function is convenient for
carrying a camera on a strap because it prevents the lens from extending.
1 Turn the zoom ring to
the widest position
(70mm). 2 Slide the zoom ring
lock lever in the
direction indicated by
the arrow.
¡ To release the zoom ring,
slide the zoom ring lock
lever in the direction
opposite to the arrow.
The zoom ring can only be locked at maximum wide angle.