1. Field of the Invention
The present invention relates to a zoom lens barrel (zoom lens apparatus) in which a focusing lens is moved in association with the movement of a variator lens.
2. Description of Related Art
In zoom lens barrels in recent years, a type in which focusing is performed by moving a lens unit other than a front lens unit, which is called an inner focus type or a rear focus type (referred to as an inner focus type below), has been a mainstream in order to achieve the miniaturization of the lens barrel by reducing the diameter of the front lens unit on the object side, to speed up an automatic focusing operation (referred to as an AF below) by making the focusing lens lighter, and to improve the operability of a filter and the hood effect by arranging the front lens unit to have a non-rotary structure.
However, in a zoom lens barrel of such a type, in general, the amount of movement of the focusing lens must be varied in accordance with the variation of the focal length due to the zooming operation. As a method for this purpose, a structure for automatically correcting the amount of movement of the focusing lens according to the variation of the focal length is disclosed in Japanese Patent No. 2561350, for example.
However, according to an increase of the zoom ratio, which is another tendency in recent years, the following problems have been actualized.
In an ordinary zoom lens barrel of the inner focus type, the amount of movement of the focusing lens with respect to the same object distance increases gradually as the zooming proceeds from the wide-angle side toward the telephoto side. In particular, in a zoom lens barrel having a high zoom ratio (generally, a zoom lens barrel having a zoom ratio of 5xc3x97 or more), the difference in the amount of movement of the focusing lens between the wide-angle side and the telephoto side is large, and the amount of movement of the focusing lens rapidly increases as the zooming approaches the telephoto side.
If the above-mentioned conventional method is applied to the high-zoom-ratio zoom lens barrel having such characteristics, the amount of shift of zoom parameters caused by the optimization of a focus cam, that is, the operation of forming one smooth cam by joining together loci of the amounts of movement of the focusing lens in the respective focal lengths is increased, and, therefore, the inclination of the locus of movement of the variator lens becomes greatly different between the wide-angle side and the telephoto side.
Then, as a result, problems have arisen, such as an operability problem that operational torque is increased and changes rapidly during the zooming operation, and a problem that changes in focus have to be permitted during the zooming operation across the permissible range for solving the former problem.
It is an object of the present invention to provide a zoom lens apparatus capable of restraining changes in focus during the zooming operation to have a sufficiently small value without having a bad influence upon the operability during the zooming operation. In particular, it is another object of the present invention to provide a zoom lens apparatus having small changes in the operational torque during the zooming operation.
In accordance with an aspect of the present invention, there is provided a zoom lens apparatus having an optical axis, comprising a zoom cam tube arranged to rotate around the optical axis and having a cam groove for moving a variator lens along the optical axis, and a focus cam tube having a cam groove for moving a focusing lens along the optical axis, wherein the focus cam tube rotates around the optical axis and moves along the optical axis in association with the rotation of the zoom cam tube during zooming, and wherein a rotational speed of the zoom cam tube and a rotational speed of the focus cam tube during zooming are different from each other.
In particular, the rotational speed of the focus cam tube during zooming becomes lower on a wide-angle side and higher on a telephoto side as compared with the rotational speed of the zoom cam tube.
In particular, the zoom lens apparatus further comprises a zoom operation ring for driving the variator lens, wherein the zoom cam tube rotates around the optical axis according to an operation of the zoom operation ring.
In particular, the zoom lens apparatus further comprises a fixed tube having a straight groove extending along the optical axis, wherein a cam follower provided on the variator lens engages the straight groove of the fixed tube and the cam groove of the zoom cam tube.
In particular, the fixed tube further has a curved groove formed thereon, the zoom cam tube further has a curved groove formed thereon, and the focus cam tube has a cam follower engaging the curved groove of the fixed tube and the curved groove of the zoom cam tube.
In particular, the zoom lens apparatus further comprises an operation ring for driving the focusing lens, wherein the focusing lens moves along the cam groove of the focus cam tube while rotating around the optical axis according to an operation of the operation ring.
In particular, the focusing lens has a cam follower engaging the cam groove of the focus cam tube, and a long groove part extending along the optical axis, which the cam follower of the focusing lens engages, is connected to the operation ring.
In accordance with another aspect of the present invention, there is provided a zoom lens apparatus having an optical axis, comprising a zoom cam tube arranged to rotate around the optical axis and having a cam groove for moving a variator lens along the optical axis, and a focus cam tube having a cam groove for moving a focusing lens along the optical axis, wherein the focus cam tube rotates around the optical axis and moves along the optical axis in association with the rotation of the zoom cam tube during zooming, and wherein, while the focusing lens rotates around the optical axis during zooming, a rotational speed of the zoom cam tube and a rotational speed of the focusing lens are different from each other.
These and further objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.