Patent ID: 8019213

Claim:
An exchangeable lens comprising: a cam member configured to have a cam driving part and be capable of rotating in a direction around an optical axis; a plurality of lenses configured to be disposed on the optical axis and include at least one movable lens that is capable of moving along an optical axis direction and allows focus adjustment or zoom adjustment through movement of the movable lens along the optical axis direction; a lens holding frame configured to have an engagement part slidably engaged with the cam driving part of the cam member and hold the movable lens, the lens holding frame being moved monolithically with the movable lens along the optical axis direction in linkage with rotation of the cam member in the direction around the optical axis; a drive motor configured to serve as a drive source for movement of the movable lens along the optical axis direction; a worm screw configured to be fixed to a motor shaft of the drive motor; a worm wheel configured to be meshed with the worm screw; a planetary roller configured to be supported on the cam member rotatably in a direction around an axis perpendicular to the optical axis and be revolved around the optical axis in linkage with rotation of the cam member; a first rotating member configured to be in contact with an outer circumferential surface of the planetary roller in a state of being biased in such a direction as to be pressed against the outer circumferential surface of the planetary roller, and be rotated in the direction around the optical axis through transmission of driving force of the drive motor to the first rotating member via the worm screw and the worm wheel; a first bearing seat configured to receive the first rotating member on an opposite side to the planetary roller; a second rotating member configured to be in contact with the outer circumferential surface of the planetary roller from an opposite side to the first rotating member in a state of being biased in such a direction as to be pressed against the outer circumferential surface of the planetary roller, and be rotated in the direction around the optical axis by manual operation; and a second bearing seat configured to receive the second rotating member on an opposite side to the planetary roller and restrict rotation of the second rotating member in the direction around the optical axis at the time of driving of the drive motor, wherein the worm screw and the worm wheel are so configured as to satisfy a condition represented by μ·cos α>sin α, where α is a lead angle of the worm screw and μ is a friction coefficient on a contact surface between the worm screw and the worm wheel.