| 摘要 |
An optical element (LG1, LG2) is guided along an optical axis by turning a first ring (18) about a parallel or coincident rotational axis (Z0). An outer ring (22) centered on Z0 holds the first ring (18). This has a circumferential groove (22d) on its external circumferential surface which faces the first ring (18). Between the outer circumferential surface of the first ring (18) and the inner circumferential surface of the outer ring (22), a mechanism for extension and retraction (18a, 22a), moves the first ring between front and rear end positions along the optical axis, when the first ring is turned. On the outer circumference of the first ring, there is a projection (18b) guiding rotation, which engages the groove (22d), so that the ring can be rotated in an axially-fixed position without movement along the optical axis, when moved by the extension-retraction mechanism into one of the end positions. A stop (26, 26b), can be inserted into or removed from the circumferential groove (22d) between its opposite end positions. This stop limits the range of rotation of the first ring about Z0 relative to the outer ring (22), by resting against the rotation guidance projection (18b), when positioned in the circumferential groove. The restriction of the range of rotation is removed when the stop is positioned outside the circumferential groove.
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