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1. Adjustable focal length eyewear comprising two lenses that are mounted to a frame for supporting the lenses in front of a user's eyes, thereby defining a viewing direction through each lens; at least one of the lenses being a variable focal length lens of the kind comprising two superposed lens elements having cooperating optical surfaces that are shaped such that the focal length of the variable focal length lens is variable according to the relative lateral disposition of the lens elements; wherein one of the lens elements is fixedly mounted to the frame and the other lens element is movable manually relative to the fixed lens element for varying the focal length of the variable focal length lens; wherein guiding parts are provided on the movable lens element which cooperate with corresponding guiding parts on the fixed lens element or the frame to define and constrain movement of the movable lens element relative to the fixed lens element to a transverse path that extends across the viewing direction; and wherein one or more releasable fasteners are provided for releasably fastening the movable lens element to the fixed lens element or frame whilst permitting movement of the movable lens element relative to the fixed lens element along the transverse path, the one or more releasable fasteners comprising mutually cooperating parts on the movable lens element and the fixed lens element or the frame.
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