Extension spring installation structure of a lens barrel

1. An extension spring installation structure of a lens barrel including a pair of relatively movable members which are movable relative to each other in an optical axis direction and at least one of which supports an optical element, said extension spring installation structure comprising:

at least one extension coil spring which biases said pair of relatively movable members in directions to approach each other, said extension coil spring including a coil spring portion and a pair of engaging portions provided at opposite ends of said coil spring portion, respectively; and

at least one spring-hook portion and at least one tilt-restriction portion which are formed on at least one of said pair of relatively movable members,

wherein one of said pair of engaging portions is engaged with said spring-hook portion by a movement of said extension coil spring toward the spring-hook portion in a direction along an axis of said coil spring portion,

wherein said spring-hook portion is shaped so as to be prevented from being disengaged from said one engaging portion of said extension coil spring even in the case where said extension coil spring is moved along said axis of said coil spring portion in a state where said one engaging portion of said extension coil spring is engaged with said spring-hook portion, and

wherein said tilt-restriction portion prevents said coil spring portion from moving in a radial direction of said coil spring portion in said state where said one engaging portion of said extension coil spring is engaged with said spring-hook portion.

2. The extension spring installation structure according to claim 1 , wherein said spring-hook portion comprises a deformation guide surface,

wherein, when said extension coil spring is moved in an insertion direction along said axis of said coil spring portion so as to be engaged with said spring-hook portion, said deformation guide surface firstly comes into contact with said one engaging portion of said extension coil spring and subsequently resiliently deforms said one engaging portion of said extension coil spring in a predetermined deforming direction so that said one engaging portion of said extension coil spring rides over said spring-hook portion to be seated over said spring-hook portion.

3. The extension spring installation structure according to claim 2 , wherein said spring-hook portion further comprises a retaining projection which prevents said one engaging portion of said extension coil spring from moving in a direction so as to be disengaged from said spring-hook portion after said one engaging portion of said extension coil spring rides over said spring-hook portion and is seated over said spring-hook portion.

4. The extension spring installation structure according to claim 2 , wherein said tilt-restriction portion restricts movement of said coil spring portion in at least said predetermined deforming direction.

5. The extension spring installation structure according to claim 1 , wherein said tilt-restriction portion comprises a cylindrical surface which surrounds said spring-hook portion.

6. The extension spring installation structure according to claim 5 , wherein said spring-hook portion is shaped as a projection which projects from said cylindrical surface in a radially outward direction of said one of said pair of relatively movable members.

7. The extension spring installation structure according to claim 6 , wherein said spring-hook portion comprises a first projection and a second projection which project from a radially outer end portion of said spring-hook portion substantially in opposite directions away from each other along said optical axis direction,

wherein said deformation guide surface is formed on said first projection, and

wherein a radially outer end surface of said spring˜hook portion is provided between said first projection and said second projection and lies in a plane substantially parallel to said optical axis.

8. The extension spring installation structure according to claim 2 , wherein said deformation guide surface lies in a plane which is inclined with respect to said optical axis direction.

9. The extension spring installation structure according to claim 1 , wherein said one of said pair of relatively movable members, which includes said tilt-restriction portion, comprises an annular member, and

wherein said tilt-restriction portion is formed along an inner peripheral surface of said annular member.

10. The extension spring installation structure according to claim 9 , wherein said one of said pair of relatively movable members comprises an inward flange which projects radially inwards from said inner peripheral surface of said annular member, and

wherein a portion of said tilt-restriction portion is formed in said inward flange.

11. The extension spring installation structure according to claim 1 , wherein said one engaging portion of said extension coil spring is shaped as a loop, and

wherein said spring-hook portion is inscribed in said loop at four corners of said spring-hook portion in said state where said one engaging portion of said extension coil spring is engaged with said spring-hook portion.

12. The extension spring installation structure according to claim 1 , wherein said extension coil spring, said spring-hook portion and said tilt-restriction portion comprise at least two extension coil springs, at least two spring-hook portions and at least two tilt-restriction portions, respectively, which are arranged at different circumferential positions about said optical axis.

13. The extension spring installation structure according to claim 12 , wherein said two extension coil springs, said two spring-hook portions and said two tilt-restriction portions are symmetrically arranged with respect to said optical axis.

14. The extension spring installation structure according to claim 1 , wherein each of said pair of relatively movable members supports a lens group of a zoom lens optical system.

15. The extension spring installation structure according to claim 1 , wherein said pair of relatively movable members are moved relative to each other in said optical axis direction by a rotation of a cam ring.

16. A lens barrel comprising:

a front annular movable member and a rear annular movable member, which are arranged on an axis common with an optical axis of an optical element, to be movable relative to each other in said optical axis direction, at least one of said front and rear annular movable members supporting said optical element;

at least one extension coil spring which is extended to be installed between said front and rear annular movable members to bias said front and rear annular movable members in directions to approach each other, said extension coil spring including a coil spring portion and a pair of looped ends provided at opposite ends of said coil spring portion, respectively; and

at least one spring-hook portion and at least one tilt-restriction portion which are formed on at least one of said front and rear annular movable members,

wherein one of said pair of looped ends is engaged with said spring-hook portion by a movement of said extension coil spring toward the spring-hook portion in a direction along an axis of said coil spring portion,

wherein said spring-hook portion is shaped so that said one looped end of said extension coil spring is prevented from being disengaged therefrom even if said extension coil spring is moved along said axis of said coil spring portion in a state where said one looped end of said extension coil spring is engaged with said spring-hook portion, and

wherein said tilt-restriction portion restricts movement of said coil spring portion in a radial direction of said coil spring portion in said state where said one looped end of said extension coil spring is engaged with said spring-hook portion.