Telephoto-Type Zoom Lens System

1. Field of the Invention

The present invention relates to a telephoto-type zoom lens system and, more particularly, to a telephoto-type zoom lens system free from aggravation of aberrations for the whole range of photographing distances from the infinite distance to short distance.

2. Description of the Prior Art

Some of known zoom lens systems in general are arranged to be focused by the first lens group. That is, the first lens group is advanced for focusing according to the photographing distance. This kind of focusing method, however, has a disadvantage that aberrations are aggravated considerably when the photographing distance changes as in the case of the front-lens group advancing method adopted for low-cost cameras. If, however, the lens system as a whole is advanced for focusing, in case of zoom lens systems, it is impossible to keep the focusing position constant in the zooming range. Therefore, the focusing method to advance the lens system as a whole cannot be adopted for zoom lens systems, and the focusing method to advance the front lens group is generally adopted.

Besides, the shortest possible photographing distance for zoom lens systems is usually decided at a constant distance for the whole zooming range. Therefore, even when the magnification for photographing on the short focal length side is small, the magnification for photographing on the long focal length side becomes large and, consequently, aggravation of aberrations on the long focal length side becomes necessarily large. Especially in telephoto-type zoom lens systems, aggravation of spherical aberration is conspicuous and, as a result, decrease of quality of the image and change of the focusing position are caused in the zooming range.

As a method to solve the above-mentioned problem in focusing of zoom lens systems, it may be considered to satisfactorily correct aberrations within the first lens group. In that case, however, it is necessary to adopt a first lens group of complicated arrangement and, as a result, the overall length of the lens system necessarily becomes long, weight of the lens system increases and, moreover, the lens system becomes extremely expensive. Therefore, this method is not practical for zoom lens systems for still-photographic cameras.

It may be also considered to solve the above-mentioned problem by dividing the first lens group into two lens components and to thereby correct aberrations in cases of close-up photographing. This method, however, is not preferable in practice to zoom lens systems for still-photographic cameras due to reasons same as the case of the above-mentioned method.

The most preferable method for solving the above-mentioned problem may be to arrange the first lens group by two or three lenses and to thereby reduce aggravation of aberrations in close-up photographing. Zoom lens systems of this kind of arrangement has excellent performance for photographing an object at the infinite distance. In close-up photographing, however, quality of the image decreases and, in addition, change of the focusing position in the zooming range becomes large. These disadvantages are caused by the fact that it is substantially impossible to favourably correct aberrations of the first lens group when a lens group of simple arrangement is adopted as the first lens group.

It is, therefore, a primary object of the present invention to provide a telephoto-type zoom lens system comprising a first lens group arranged to move along the optical axis at the time of focusing and, at the same time, arranged so that aberrations of said first lens group will not be varied even when said first lens group is moved for focusing.

The telephoto-type zoom lens system according to the present invention comprises an afocal lens system comprising three lens groups, i.e., a first lens group having function as a convex lens, a second lens group having function as a concave lens, and a third lens group having function as a convex lens, and a relay lens system comprising a fourth lens group, magnification of said zoom lens system being varied by moving said second lens group along the optical axis of the lens system, the position of the imaging plane of said zoom lens system being kept constant by moving said third lens group, said first lens group to be moved for focusing being arranged by two lens components of three lenses, i.e., a first lens component which is a biconvex lens and a second lens component which is a cemented doublet lens consisting of a biconvex lens and biconcave lens cemented to each other. Moreover, the telephoto-type zoom lens system according to the present invention satisfies the following conditions when reference symbols n.sub.1, n.sub.2 and n.sub.3 respectively represent refractive indexes of respective lenses constituting said first lens group, reference symbols r.sub.1 through r.sub.5 respectively represent radii of curvature of respective surfaces of said lenses in the order from the object side toward the image side, and reference symbol f.sub.1 represents the total focal length of said first lens group. ##EQU1##

Out of the above conditions defining the first lens group of the zoom lens system arranged as described in the above, conditions (1) and (2) are to define the first lens component. Out of them, the condition (1) defines the refractive power of the first surface r.sub.1 of the lens group. The condition (2) relates to the radius of curvature of the second surface r.sub.2 of the lens group when the refractive power of the first surface r.sub.1 is defined by the condition (1).

If it becomes [(n.sub.1 - 1)/r.sub.1 ]f.sub. 1 < 0.3 in said condition (1), aggravation of spherical aberration caused by change of the photographing distance at the time of focusing will become small. As, however, the absolute value of astigmatism becomes large in that case, it becomes impossible to favourably correct astigmatism in case of the lens group comprising three lenses and, therefore, it becomes very difficult to favourably correct astigmatism of the lens system as a whole unless the other lens groups are made complicated. If it becomes [(n.sub.1 - 1)/r.sub.1 ]f.sub.1 > 0.58 on the contrary, aggravation of spherical aberration at the time of focusing becomes large and, especially, it becomes extremely large at the time when the focal length is maximum.

If it becomes - r.sub.1 /r.sub.2 < 0.35 in the condition (2), aggravation of spherical aberration caused by change of the photographing distance becomes large even when the condition (1) is satisfied. In case of - r.sub.1 /r.sub.2 > 0.65, astigmatism is aggravated through aggravation of spherical aberration becomes small.

Conditions (3) and (4) relate to refractive powers of the front surface r.sub.3 and rear surface r.sub.5 of the second lens component arranged by leaving a small airspace from the first lens component. If it becomes [(n.sub.2 - 1)/r.sub.3 ]f.sub.1 < 0.57 in the condition (3) related to the refractive power of the surface r.sub.3, aggravation of spherical aberration caused by change of the photographing distance becomes large. In case of [(n.sub.2 - 1)/r.sub.3 ]f.sub.1 > 0.75, astigmatism is aggravated though aggravation of spherical aberration is small.

As for the condition (4), aggravation of spherical aberration will become small when the refractive power is made small as [(n.sub.3 - 1)/r.sub.5 ]f.sub.1 < 0.1. In that case, however, astigmatism cannot be corrected favourably. If the refractive power is made large as [(n.sub.3 - 1)/r.sub.5 ]f.sub.1 > 0.24, aggravation of spherical aberration caused by change of the photographing distance becomes large.

When, as described in the above, the first lens group does not satisfy the afore-mentioned respective conditions, aggravation of spherical aberration at the time of focusing becomes large, or considerably large aberrations will be caused by the first lens group and it will become very difficult to correct them by the other lens groups. In the above, the second lens component is described as a cemented doublet lens. This lens component, however, may be also arranged as a biconvex lens and biconcave lens spaced from each other by a small airspace.

FIG. 1 through FIG. 3 respectively show sectional views of respective embodiments of the telephoto-type zoom lens system according to the present invention;

FIGS. 4A, 4B, 4C, 5A, 5B and 5C respectively show graphs of aberration curves of the Embodiment 1 of the present invention;

FIGS. 6A, 6B, 6C, 7A, 7B and 7C respectively show graphs of aberration curves of the Embodiment 2 of the present invention; and

FIGS. 8A, 8B, 8C, 9A, 9B and 9C respectively show graphs of aberration curves of the Embodiment 3 of the present invention.

Preferred embodiments of the telephoto-type zoom lens system according to the present invention are as described below. These embodiments are of course arranged so that their first lens groups respectively satisfy the afore-mentioned conditions. Moreover, in order to make the lens system as a whole extremely compact so that its telephoto ratio (the value obtained by dividing the distance from the first lens surface of the lens system to the imaging plate by the focal length of the lens system as a whole) becomes smaller than 1, the varifocal afocal lens system of every embodiment is made small and the fixed relay lens system is also arranged compactly. Besides, for every embodiment, respective lens groups other than the first lens group are also arranged so that the quality of the image becomes favourable over the whole varifocal range and whole focusing range. That is, for every embodiment, the second lens group, i.e., the variator is arranged to have an extremely short focal length and is arranged by four lenses, i.e., a concave lens, a cemented doublet lens consisting of a convex lens and concave lens, and a concave lens in order to prevent aberrations. The third lens group, i.e., the compensator is arranged by using a cemented doublet lens consisting of a convex lens and concave lens. The fourth lens group, i.e., the fixed relay lens system is arranged by using a first lens component of a convex lens, a second lens component arranged as a cemented doublet lens consisting of a convex lens and concave lens or as a combined lens component consisting of a convex lens and concave lens spaced from each other by a small airspace, a third lens component of a convex lens positioned at some distance from said second lens component, a fourth lens component of a concave lens, and a fifth lens component of a convex lens.

f.sub.1 (Focal length of first lens group) = 126.05

f.sub.2 (Focal length of second lens group) = -40.00

f.sub.3 (Focal length of third lens group) = 115.05

f.sub.4 (Focal length of fourth lens group) = 138.20

where reference symbols r.sub.1 through r.sub.27 represent radii of curvature of successive lenses, reference symbols d.sub.1 through d.sub.26 represent thickness of successive lenses and distances between lenses on the axis, reference symbols n.sub.1 through n.sub.15 represent refractive indexes of successive lenses, .nu..sub.1 through .nu..sub.15 represent Abbe's number of successive lenses.

where reference symbols r.sub.1 through r.sub.26 represent radii of curvature of successive lenses, reference symbols d.sub.1 through d.sub.25 represent thickness of successive lenses and distance between lenses on the axis, reference symbols n.sub.1 through n.sub.15 represent refractive indexes of successive lenses, .nu..sub.1 through .nu..sub.15 represent Abbe's number of successive lenses.

where reference symbols r.sub.1 through r.sub.27 represent radii of curvature of successive lenses, reference symbols d.sub.1 through d.sub.26 represent thickness of successive lenses and distances between lenses on the axis, reference symbols n.sub.1 through n.sub.15 represent refractive indexes of successive lenses, .nu..sub.1 through

.nu..sub.15 represent Abbe's number of successive lenses.

Furthermore, for the fourth lens group, i.e., the fixed relay lens system of each of the above-mentioned respective embodiments, the overall length (the distance from the first lens surface of the fourth lens group to the imaging plane), focal length f.sub.4 and Petzval's sum are as follows.

As described in the above, lens systems according to respective embodiments all satisfy the afore-mentioned respective conditions and, at the same time, are arranged as follows. That is, for every embodiment, a lens group having an extremely small focal length and having functions as a concave lens is used as the second lens group. Besides, the fourth lens group, i.e., the relay lens system is arranged by six lenses positioned adequately and has Petzval's sum of large positive value which is required to offset Petzval's sum of negative value caused in the second lens group. Therefore, as the lens system as a whole, it is also possible to provide an extremely compact telephoto-type zoom lens system for which quality of the image is very high as shown by graphs of aberration curves and for which the telephoto ratio is smaller than 1.

The above fact also shows that the lens system which has the first lens group satisfying the afore-mentioned respective conditions, is capable of constituting an extremely compact zoom lens system for which quality of the image is very high over the whole zooming range and whole focusing range.

Out of graphs of aberration curves, FIG. 4A shows spherical aberration, astigmatism and distortion of the lens system according to the Embodiment 1 when the photographing distance is infinite and the focal length of the lens system is 86.5. FIG. 4B shows said aberrations of the Embodiment 1 when the photographing distance is also infinite, but when the focal length is 151.4. FIG. 4C shows said aberrations of the Embodiment 1 when the photographing distance is also infinite, but when the focal length is 250.0. FIG. 5A shows said aberrations of the Embodiment 1 when the photographing distance is 3 m and the focal length is 86.5. FIG. 5B shows said aberrations of the Embodiment 1 when the photographing distance is also 3 m, but when the focal length is 151.4. FIG. 5C shows said aberrations of the Embodiment 1 when the photographing distance is also 3 m, but when the focal length is 250.0. FIG. 6A shows said aberrations of the lens system according to the Embodiment 2 when the photographing distance is infinite and the focal length is 86.5. FIG. 6B shows said aberrations of the Embodiment 2 when the photographing distance is also infinite, but when the focal length is 151.4. FIG. 6C shows said aberrations of the Embodiment 2 when the photographing distance is also infinite, but when the focal length is 250.0. FIG. 7A shows said aberrations of the Embodiment 2 when the photographing distance is 3 m and the focal length is 86.5. FIG. 7B shows said aberrations of the Embodiment 2 when the photographing distance is also 3 m, but when the focal length is 151.4. FIG. 7C shows said aberrations of the Embodiment 2 when the photographing distance is also 3 m, but when the focal length is 250.0. FIG. 8A shows said aberrations of the lens system according to the Embodiment 3 when the photographing distance is infinite and the focal length is 86.5. FIG. 8B shows said aberrations of the Embodiment 3 when the photographing distance is also infinite, but when the focal length is 151.4. FIG. 8C shows said aberrations of the Embodiment 3 when the photographing distance is also infinite, but when the focal length is 250.0. FIG. 9A shows said aberrations of the Embodiment 3 when the photographing distance is 3 m and the focal length is 86.5. FIG. 9B shows said aberrations of the Embodiment 3 when the photographing distance is also 3 m, but when the focal length is 151.4. FIG. 9C shows said aberrations of the Embodiment 3 when the photographing distance is also 3 m, but when the focal length is 250.0.