1. Field of the Invention
The present invention relates to a zoom lens device whose lens barrel elements rotate relative to each other for changing the focal length, i.e. for zooming. The present invention relates more particularly to a zoom lens device having a zooming position detector mounted in a compact fashion.
2. Background Arts
A zoom lens device has at least front and rear lens groups whose positions are changed relative to a film surface and relative to each other in the direction of the optical axis of the zoom lens device. Since there is a clearance between barrels that hold the front and rear lens groups respectively, extraneous light would enter inside the zoom lens device through the clearance without any light-shielding device. JPY 5-31634 discloses a light-shielding device for a zoom lens device, which is constituted of a light-shielding ring having a resilient lip around its inner rim. The light-shielding ring is placed behind a face flange surrounding an opening of a front wall of a camera body such that the resilient lip stays in contact with the outer periphery of a lens barrel or lens cover frame that moves with the front lens group in the axial direction through the opening of the front wall.
The light-shielding ring disclosed in JPY 5-31634 is useful for a clearance around a lens barrel that moves in the axial direction relative to a fixed barrel. However, the light-shielding ring is not preferable for a lens device where a front lens frame is rotatably held in a barrel, and a front cover member having an opening for exposing the front lens is attached to a front end of the barrel, and a clearance is provided between the front lens frame and the front cover member for allowing the front lens frame to rotate relative to the front cover member. If the light-shielding ring is used for the clearance between the front cover member and the front lens frame, the outer periphery of the front lens frame would rub against the lip of the light-shielding ring as the front lens frame rotates relative to the front cover member. The friction between the front lens frame and the front cover member would raise the necessary driving force for the lens device. The lip of the light-shielding ring would sooner be worn out or heat-deformed by the frequent friction. Besides, the light-shielding ring increases the requisite number of parts necessary for the lens device.
On the other hand, it is known in the art to provide a zooming position detector in a zoom lens device. For example, JPA 50-36118 discloses a zooming position detector consisting of an encoder plate and a brush device. The encoder plate is coupled to a lens barrel of a zoom lens device through gears and a lead screw, such that the encoder plate moves in the axial direction of the zoom lens device as the lens barrel rotates for zooming. As the encoder plate moves in the axial direction, the brush device brushes conductor patterns on the encoder plate detecting signals corresponding to the rotational angle of the lens barrel. Japanese Utility Model Registration No. 2521469 discloses a zooming position detector wherein a code plate having conductor patterns are tightly provided on an outer periphery of a cam ring that rotates for zooming, whereas a brush device brushing the conductor patterns is secured to a stationary frame outside the cam ring.
Either of the prior zooming position detectors needs a mounting space outside the movable lens barrel, which inevitably enlarges the whole size of the camera.