wherein one or both of the lenses are adjustable, comprising two superposed lens elements and a selectively operable optical power adjuster, the lens elements being slidable relative to each other in a direction transverse the optical axis of the lens under the control of the optical power adjuster and having respective lens surfaces that are shaped to act together to form a corrective lens, the focal power of which varies according to the relative disposition of the two lens elements; wherein upon increasing the focal power of the adjustable lens, the centration distance between the two lenses progressively decreases to match substantially the interpupillary distance of the user's eyes.