| 摘要 |
PROBLEM TO BE SOLVED: To prevent the full-open diameter of a diaphragm from varying in a focus section by providing a cam mechanism with an invariable section wherein a restriction member is not displaced as the restriction member and an engagement member move relatively. SOLUTION: On a shutter unit (stop mechanism part) 8, the restriction member 41 projects which restricts the full-open position of a shutter blade (stop) 40. The engagement member 4b engages the restriction member 41 by the relative movement of a 2nd moving frame 4 to a 1st moving frame 3. The engagement member 4b has a stepped cam, which has a slanting part and a flat part (invariable section). The projection part of the restriction member 41 engages the cam of the engagement member 4b and rotates the restriction member 41 through the slanting part to increase the full-open diameter of the stop. When 1st and 2nd lens groups L1 and L2 move in the focus section, the projection part does not rotate since it move on the flat part, so that the full-open diameter of the diaphragm will not rotate during focusing operation.
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