because of the engagement of the key 72 and the key groove 113 also since the focus ring 5 is not applying a rotational force to the helicoid ring 8 screwed together with the moving ring 7 the helicoid ring 8 is caused to move along the optical axis direction interlocked with the moving ring 7 this motion further causes the lens frame 21 of the front lens group 2 bayonet engaged with the helicoid ring 8 to move along the optical axis direction thus causing the front lens group 2 to move along the optical axis direction in this way both the front lens group 2 and the rear lens group 3 are moved to a desired position along the optical axis direction at a time thus to complete the zooming operation during such operation since the lens frame 21 of the front lens group 2 is restricted from rotating by the key engagement with the inner tube 11 accessories such as a filter and a hood not shown attached to the lens frame 21 of the front lens group 2 are kept from being rotated in the focusing operation rotating the focus ring 5 causes the helicoid ring 8 to interlockedly rotate fig5 illustrates the identical portion of the lens body tube to that shown in fig1 for indicating the moving direction of the respective components with the same marks as those used in fig4 when the helicoid ring 8 rotates the moving ring 7 is kept from rotating by the engagement of the.