Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-11-18
2001-08-07
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S714000
Reexamination Certificate
active
06271973
ABSTRACT:
This application claims the benefit of Japanese Patent Applications Nos. 10-329500, 11-112753, and 11-116897 which are hereby incorporated by references.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable focal length lens system and, in particular, relates to a variable focal length lens system having a compactness and a large zoom ratio suitable for an objective lens used for a lens shutter type camera.
2. Related Background Art
It has recently become common to use a zoom lens in a picture-taking optical system for a camera. In particular, since portability is regarded as important for a lens shutter type camera, In order to reduce size and weight of a camera itself, a small-sized lightweight zoom lens used for an objective lens is proposed.
As a zoom lens has become popular for an objective lens, various proposals have been made regarding a zoom lens having a long focal length in a telephoto end state and a large zoom ratio (a quotient of a focal length in the telephoto end state divided by a focal length in the wide-angle end state). A long focal length in the telephoto end state makes it possible to take picture closer to an object and a large zoom ratio makes it possible to take picture with high degree of freedom.
In these zoom lenses, because of the long focal lengths in the telephoto end state, the total lens lengths in the telephoto end state become long, so that the portability is spoiled. Therefore, the portability while being carried not for taking picture is improved by means of storing lens groups in the camera body in a state where the distances between adjacent lens groups are made minimum. By using such structure for the lens barrel that plurality of lens barrel elements are nested, it is possible to accommodate each lens barrel element as if the lens barrel element is folded when the camera is carried, so the portability while being carried is improved.
However, as the focal length in the telephoto end state becomes long, the barrel length also becomes long, so that each lens barrel element consisting of the lens barrel also becomes long. Then, the portability is spoiled in consequence. Accordingly, it is conceivable to reduce the thickness of the camera body while being accommodated by means of increasing the number of lens barrel element and, then, reducing the thickness of each lens barrel element. On the contrary in this case, however, the height and width of the camera become larger because the diameter of each barrel becomes larger, so the portability is also spoiled.
According to an optical design, it can be conceivable to avoid inconvenience of portability by means of shortening the total length of the lens in the telephoto end state. In a conventional zoom lens, in order to shorten the total length of the lens system in the telephoto end state, it is necessary to increase refractive power of each lens group or to increase the number of movable lens group. Because of the increase of the number of the lens element in each case, this is the result is contrary to objections of compactness and lightweight design and, therefore, contrary to the objective of good portability.
SUMMARY OF THE INVENTION
The present invention is made in view of the aforementioned problems and has an object to provide a compact variable focal length lens system suitable for a large zoom ratio with small number of lens elements.
According to one aspect of the present invention, a variable focal length lens system includes a first lens group having a positive refractive power, a second lens group having a positive refractive power disposed to an image side of the first lens group, and a third lens group having a negative refractive power disposed to the image side of the second lens group. The first lens group, the second lens group, and the third lens group are moved toward the object in such a manner that a space(air gap) between the first lens group and the second lens group is increased, and a space(air gap) between the second lens group and the third lens group is decreased when the system goes from a wide-angle end state to a telephoto end state. The following condition is satisfied:
0.28<f1/ft<0.48;
where f1 denotes a focal length of the first lens group, and ft denotes a focal length of the lens system in the telephoto end state.
In one preferred embodiment of the present invention, the second lens group is comprised of an aperture diaphragm, a negative sub lens group having a negative refractive power arranged to the object side of the aperture diaphragm, and a positive sub lens group having a positive refractive power arranged to the image side of the aperture diaphragm. The following condition is satisfied:
0.4<f2/(fw·ft)
½
<0.8;
where f2 denotes a focal length of the second lens group, fw denotes a focal length of the lens system in the wide-angle end state, and ft denotes the focal length of the lens system in the telephoto end state.
In one preferred embodiment of the present invention, the following condition is satisfied:
0.15<&Dgr;1/f1<0.3;
where &Dgr;1 denotes a variation of a space(air gap) along the optical axis between the first lens group and the second lens group when the system goes from the wide-angle end state to the telephoto end state, and f1 denotes the focal length of the first lens group.
According to another aspect of the present invention, the following condition is satisfied:
1.10<f12T/f12W<1.50;
where f12W denotes a combined focal length of the first lens group and the second lens group in the wide-angle end state, and f12T denotes a combined focal length of the first lens group and the second lens group in the telephoto end state.
According to another aspect of the present invention, the following condition is satisfied:
0.3<fw·&Dgr;1/(f1·f2)<0.45;
where fw denotes a focal length of the variable focal length optical system in the wide-angle end state, &Dgr;1 denotes a variation of the space(air gap) between the first lens group and the second lens group when the system goes from the wide-angle end state to the telephoto end state, f1 denotes a focal length of the first lens group, and f2 denotes a focal length of the second lens group.
In a three-group type zoom lens according to prior art comprising, from the object side, a positive lens group, a positive lens group and a negative lens group, variation of a lateral magnification of the third lens group is very large when a state of the focal length changes from a wide-angle end state to a telephoto end state. In this case, the following inconveniences (I) and (II) come out.
(I) Since the lateral magnification of the third lens group in the telephoto end state is large, the total length of the lens system becomes larger.
(II) Since the lateral magnification of the third lens group in the wide-angle end state is small, a sufficient back focus cannot be secured, so that the diameter of the lens system becomes larger.
Since the zooming ratio according to prior art has been just over three, the focal length in the telephoto end state has been rather short, so that the magnitude of the total length of the lens system has not affected much. If the zooming ratio becomes larger, then the magnitude of the total length of the lens system may cause big problem. Moreover, because the oversized lens diameter makes the width and height of the camera body larger, it does not meet the demand of the market for portability.
Therefore, in the present invention, controlling the variation of the lateral magnification of the third lens group from the wide-angle end state to the telephoto end state, the compactness of the total length of the lens system and the compactness of the diameter of the lens system are intended. To give an actual example in the present invention, a variable focal length optical system, in order from the object side, comprise:
a first lens group having a positive refractive power,
a second lens group having a positive refractive power, and
a third lens group having a negative refractive power, wherein said ea
Hayakawa Satoshi
Ohtake Motoyuki
Miles & Stockbridge PC
Nikon Corporation
Sugarman Scott J.
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