Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-03-28
2004-10-19
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000
Reexamination Certificate
active
06807015
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a focusing method for a zoom lens system.
2. Description of the Related Art
In conventional zoom lens systems, a front-focusing zoom lens system, in which the frontmost lens group out of a plurality of lens groups functions as a focusing lens group, has been widely used. In such a front-focusing zoom lens system, there is an advantage, i.e., the traveling distances of the focusing lens group are substantially the same regardless of the focal lengths. However, the diameter of the frontmost lens group is generally large, so that the frontmost lens group inevitably becomes heavy. Therefore the front-focusing zoom lens system is unsuitable for an AF camera in which the focusing lens group is driven through an electric motor. In the case of a mechanism for automatic focusing (AF mechanism) contained in the front-focusing zoom lens system, a driving mechanism for interconnecting the focusing lens group and a driving motor provided in the camera body becomes complicated (becoming large in size).
In an AF camera, if an attempt is made to miniaturize an electric drive mechanism and to increase the focusing speed, a lens group, which (i) is positioned close to the camera body, and (ii) generally has a small diameter, can advantageously be utilized as the focusing lens group. Accordingly, an inner-focusing lens system or a rear-focusing lens system have been often employed in AF cameras.
However, in conventional zoom lens systems, only a predetermined lens group is used, over the entire focal length constituted by a plurality of discrete focal length ranges, as the focusing lens group, no matter which focusing method is employed, i.e., any one of the front-focusing method, the inner-focusing method and the rear-focusing method. In other words, the above conventional zoom lens systems are required to move zoom lens groups along predetermined lens-group-moving paths at the time of zooming, and also required, at any discrete focal length ranges, to secure a space for the traveling distance of a lens group functioning as the focusing lens group from infinity to the shortest photographing distance. These requirements have to be satisfied even for discrete focal length ranges where distances between lens groups are short, i.e., where there is low freedom of movement of the lens groups. Consequently, these requirements have been obstacles when an attempt is made to miniaturize the zoom lens system, or to design a miniaturized zoom lens system with a high zoom ratio.
Furthermore, the zoom lens systems are classified into the two types, i.e., the telephoto zoom lens system including the positive lens group and the negative lens group in this order from the object, and the retrofocus zoom lens system including the negative lens group and the positive lens group in this order from the object. In the retrofocus zoom lens system, among the lens group from the second one or therebehind, there is a lens group, which has the transverse magnification, varying from a low magnification less than life-size magnification (m=−1) to a high magnification greater than life-size magnification (i.e., in a range including life-size magnification), with respect to an object at infinity in accordance with the change of the focal length of the entire zoom lens system. More concretely, the above lens group (hereinafter, the lens group with the life-size transverse magnification) can vary the transverse magnification of the lens group itself from a low magnification of less than the life-size magnification, passing through the life-size magnification (m=−1), to a high magnification of greater than the life-size magnification. A lens group with the life-size transverse magnification cannot be used as the focusing lens group. Accordingly, in the conventional retrofocus zoom lens system, since only a predetermined lens group is arranged to be the focusing lens group over the entire focal length, a lens group other than the lens group with the life-size magnification has to be used as the focusing lens group. This requirement has also been an obstacle when an attempt is made to miniaturize the zoom lens system, or to design a miniaturized zoom lens system with a high zoom ratio.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a focusing method for a zoom lens system, by which a miniaturized zoom lens system with a high zoom ratio can be obtained.
The present invention is based on a conception that a traveling distance for focusing, which is required to the focusing lens group, can easily be secured without causing interference between the focusing lens group and other lens groups, if one lens group to be the focusing lens group, which has conventionally been limited to a predetermined lens group only, is changed to another lens group, in accordance with the change of the discrete focal length ranges.
Furthermore, in regard to the retrofocus zoom lens system, the present invention is based on a conception that the lens group with the life-size transverse magnification, which has not been able to be used as the focusing lens group, can be used as the focusing lens group, if the lens group with the life-size transverse magnification is, not used as the focusing lens group at a focal length where the transverse magnification m becomes −1.
In order to achieve the above-mentioned object, there is provided a focusing method for a zoom lens system including at least two lens groups.
According to the focusing method, a portion of the zoom lens system is arranged to function as a focusing lens group over the entire focal length constituted by a plurality of discrete focal length ranges, and the portion functioning as the focusing lens group is changed to another portion, in accordance with to the discrete focal length ranges.
More concretely, in the case of the two-lens-group zoom lens system including a first lens group and a second lens group in this order from the object, in a predetermined discrete focal length range, the first lens group functions as the focusing lens group, whereas, in another predetermined discrete focal length range, the second lens group functions as the focusing lens group.
Furthermore, in the case of the three-lens-group zoom lens system including a first lens group, a second lens group and a third lens group in this order from the object, in a predetermined discrete focal length range, the second lens group functions as the focusing lens group, whereas, in another predetermined discrete focal length range, the third lens group functions as the focusing lens group. In this case, the first lens group can be a stationary lens group which is made immovable upon both zooming and focusing, or can be a zoom lens group which is made movable upon zooming.
Still further, in the case of the four-lens-group zoom lens system including a first lens group, a second lens group, a third lens group and a fourth lens group in this order from the object, in a predetermined discrete focal length range, the second lens group functions as the focusing lens group, whereas, in another predetermined discrete focal length range, the second and fourth lens groups are made integrally movable so that these two lens groups function as the focusing lens group. As an alternative, in a predetermined discrete focal length range, the second, third and fourth lens groups are made integrally movable so that these three lens groups function as the focusing lens group, whereas, in another predetermined discrete focal length range, the third and fourth lens groups are made integrally movable so that these two lens groups function as the focusing lens group.
In any of the above cases, according to the present invention, (i) the lens group with the life-size transverse magnification can be a part of the focusing lens group, and (ii) in that case, the lens group with the life-size transverse magnification can function as the focusing lens group in a predetermined discrete focal length range other than
Eguchi Masaru
Ito Takayuki
Greenblum & Bernstein P.L.C.
Sugarman Scott J.
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