Title:
ZOOM LENS
Kind Code:
A1


Abstract:
An inner-focusing zoom lens that includes multi groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole. At least one group of positive refractivity among the trailing groups includes two or more negative lens pieces, at least one of which is connected on its surface to another lens piece to form a duplicated composite lens so that the junction between two of them functions to diverge incident beams. Assuming now that a sum of the refractivities of all the junctions of the composite lenses can be expressed as Σφ=Σ|(N1−N2)/R| where N1 and N2 are refractivities that substances before and after the junction of the composite lenses respectively have, R is a radius of curvature of the junction, ft is a focal length of the comprehensive lens optics of the zoom lens at the telephoto end.



Inventors:
Yamanaka, Hisayuki (Saitama-shi, JP)
Li, Dayong (Saitama-shi, JP)
Application Number:
12/893672
Publication Date:
01/27/2011
Filing Date:
09/29/2010
Assignee:
TAMRON CO., LTD
Primary Class:
Other Classes:
359/683
International Classes:
G02B15/14
View Patent Images:



Primary Examiner:
SCHWARTZ, JORDAN MARC
Attorney, Agent or Firm:
JACOBSON HOLMAN PLLC (400 Seventh Street N.W. Suite 700, Washington, DC, 20004-2218, US)
Claims:
What is claimed is:

1. A zoom lens comprising at least five groups of lens pieces, namely, a first lens group or the leading lens group of positive refractivity, a second lens group of negative refractivity, a third lens group of positive refractivity, a fourth lens group of positive refractivity, and a fifth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis, or comprising at least six groups of lens pieces, namely, a first lens group or the leading lens group of positive refractivity, a second lens group of negative refractivity, a third lens group of positive refractivity, a fourth lens group of positive refractivity, a fifth lens group of positive refractivity, and a sixth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis, wherein the first lens group at least includes one negative lens piece and two or more positive lens pieces, and the first lens group provides optical properties as expressed by the following formulae:
40<vd1<55 (2)
20<vd2<35 (3) where vd1 is an Abbe number of the negative lens piece(s) in the first lens group, and vd2 is the Abbe number of the positive lens piece(s) in any one of the lens groups of positive refractivity succeeding to the first lens group.

2. In an inner-focusing zoom lens that does not conduct the floating focusing and that comprises multi groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole, the second lens group includes a meniscus negative lens having its convex surface faced toward the photoshot subject, and a composite lens having negative and positive lens pieces connected to one another, and the second lens group provides optical properties as expressed by the following formulae:
1<R/f(wide)<8 (4)
2ω(wide)<40 (5) where R is a radius of curvature of an objective surface of the foremost lens piece in the second lens group, f(wide) is a focal length of the comprehensive lens optics at the wide-angle end, and ω(wide) is a half field angle of the comprehensive lens optics at the wide-angle end.

3. The zoom lens according to claim 2, comprising the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole, at least one group of positive refractivity among the trailing groups including two or more negative lens pieces, at least one of which is connected on its surface to another lens piece to form a duplicated composite lens so that the junction between two of them functions to diverge incident beams; assuming now that a sum of the refractivities of all the junctions of the composite lenses can be expressed as Σφ=Σ|(N1−N2)/R| where N1 and N2 are refractivities that substances before and after the junction of the composite lenses respectively have, R is a radius of curvature of the junction, f(tele) is a focal length of the comprehensive lens optics of the zoom lens at the telephoto end, and φ(tele)=1/f(tele) is the refractivity of the comprehensive lens optics at the telephoto end, the requirement as defined in the following formula are satisfied:
2<Σφ/φ(tele)<10 (6)

4. The zoom lens according to claim 2, wherein at least one group of positive refractivity among the third lens group and all the succeeding lens groups includes a triplicated composite lens of negative-positive-negative power configuration of three of the lens pieces.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No. 12/318,515, filed on Dec. 30, 2008, and hereby incorporated in its entirety by reference, which claimed priority to Japanese Patent Application No. 2008-004557, filed Jan. 11, 2008, and Japanese Patent Application No. 2008-004558, filed Jan. 11, 2008, under 35 U.S.C. §119, and which are hereby incorporated in their entireties by reference.

FIELD OF THE INVENTION

The present invention relates to inner-focusing or rear-focusing telephotography zoom lenses, and more particularly, to zoom lenses suitable for 35-mm cameras, video cameras, electronic still cameras, and so forth that are of approximately 4.3 to 5.7 in variable power ratio and that are capable of attaining enhanced optical performances throughout the entire ranges of both power ratio and objective distance.

The present invention is especially directed to the zoom lenses that have merely a single group of lens pieces among other groups utilized for the inner-focusing or the rear-focusing to well compensate for varied spherical aberration at the telephoto end during the focusing.

BACKGROUND ART

In the prior art, it is well known that some zoom lenses are adapted to simply displace the leading group of lens pieces closest to a photoshot subject for the focusing, which may be referred to as a ‘front lens focusing system’. With such a front lens focusing system, switching a focusing mode to the Automated forces the large heavy lens pieces to move instantaneously, resulting in an unsatisfactory rapidity.

Additional types of the focusing system, which are known as ‘inner-focusing’ and ‘rear-focusing’ have been developed in the art which enable rapid focusing by virtue of the reduced dimensions of the focusing lens groups.

With such inner-focusing and rear-focusing, however, their rapid focusing ability is a tradeoff of more largely varied aberration, and as a result of the focusing, it becomes hard to compensate adequately for the aberration varied so much.

In a specific type of the zoom lens, which is for telephoto shooting with a field angle 2ω of less than 40 degrees at the wide-angle end, a positive lens piece(s) in the first lens group has its convex surface faced toward the image plane while a negative lens piece(s) closest to the photoshot subject in the second lens group has its objective surface flattened or convexed in shape and faced toward the photoshot subject, so as to reduce variations in spherical aberration at the telephoto end, thereby facilitating the compensation, which is a tradeoff of considerable difficulties in adjusting and controlling the varied curvature of the image plane at the wide-angle end.

For instance, one typical inner-focusing zoom lens already disclosed consists of five groups of lens pieces, including their respective optical properties, i.e., of positive, negative, positive, positive, and negative refractivities, respectively (see Patent Document 1 listed below).

Another zoom lens coping with the aforementioned disadvantage in the art is of six groups of lens pieces where only the fifth succeeding to the first or the leading group of them is displaced for the focusing (see Patent Document 2 listed below).

Still another typical zoom lens in the prior art is of six groups of lens pieces, namely, the ones respectively having positive, negative, positive, negative, positive, and negative refractivities, and the sixth serving as the primary focusing lens along with the fourth behave as floating lens during the focusing for a shift from long distance zooming to short distance zooming (see Patent Document 3 listed below).

PATENT DOCUMENT 1

Japanese Patent Publication of Unexamined Application No. 2005-292338

PATENT DOCUMENT 2

Japanese Patent Publication of Unexamined Application No. H10-133107

PATENT DOCUMENT 3

Japanese Patent Publication of Unexamined Application No. 2000-47107

The zoom lens as disclosed in Patent Document 1 has a problem of significant variation in curved image plane at the wide-angle end due to a negative lens piece closest to the photoshot subject in the second lens group that has its surface closer to the subject shaped in concave.

In the zoom lens as disclosed in Patent Document 2, the first or the leading to the fourth of the lens groups underdevelop the varied spherical aberration during the focusing for the shift from the long distance zooming to the short distance zooming; however, the fifth lens group permits the variations in spherical aberration to be overdone and prevalent by means of connecting a single negative lens piece and another lens piece into a composite lens with its junction serving to diverge incident beams, so as to countervail the variations in the spherical aberration in the lens optics, as a whole. Configured in this manner, however, the zoom lens fails to sufficiently adjust the varied spherical aberration as desired in the comprehensive lens optics, especially, at the telephoto end, due to the insufficient diverging effects at the junction of the composite lenses.

Patent Document 2 also teaches the fifth and the fourth of the lens groups behave in the floating manner for the focusing. This is especially for adjusting and controlling both the curved image plane at the wide-angle end and the varied spherical aberration at the telephoto end. In contrast with the one that carries out the focusing simply relying on the inner-focusing system, however, this prior art embodiment becomes more complicated in structure because of additional coupling components to a lens barrel to cope with the floating.

According to the disclosure of Patent Document 3, the first to the fourth of the lens groups underdevelop the varied spherical aberration, and instead, the fifth overturns the same during the focusing, which permit the variations in spherical aberration to be reduced in the lens optics, as a whole. However, the negative lens of the composite lens insufficiently diverges the incident beam thereon to satisfactorily compensate for the varied spherical aberration at the telephoto end. Moreover, the teachings about the focusing where the fourth and the six of the lens groups behave in the floating manner bring about another problem of the more complicated structure due to the coupling components added to the lens barrel to conduct the floating.

During the focusing for the shift from the long distance zooming to the short distance zooming, in general, the zoom lens is prone to underdevelop the variations in spherical aberration unless it is specifically modified to address the desired reduction in displacement of the focusing lens groups or modified in some other ways, and this tendency is conspicuous especially at the telephoto end. As to the zoom lenses designed specifically for the telephoto shooting where a focal length is as long as 400 mm, degradation in optical performance is considerable during the focusing for the short distance zooming.

The present invention is made to overcome the aforementioned disadvantages in the prior art where the zoom lenses employing the focusing system such as inner-focusing, the rear-focusing, or the like, allow for rapid focusing in contrast with the front lens focusing system but compromise with difficulty in well compensating for the varied aberration, and accordingly, it is an object of the present invention to provide a zoom lens that has merely a single group of lens pieces among other groups utilized for the inner-focusing or the rear-focusing to well compensate for the varied spherical aberration at the telephoto end during the focusing.

SUMMARY OF THE INVENTION

In an aspect of the present invention, there is provided an inner-focusing zoom lens that includes multi groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole. At least one group of positive refractivity among the trailing groups includes two or more negative lens pieces, at least one of which is connected on its surface to another lens piece to form a duplicated composite lens so that the junction between two of them functions to diverge incident beams. Assuming now that a sum of the refractivities of all the junctions of the composite lenses can be expressed as Σφ=Σ|(N1−N2)/R| where N1 and N2 are refractivities that substances before and after the junction of the composite lenses respectively have, R is a radius of curvature of the junction, ft is a focal length of the comprehensive lens optics of the zoom lens at the telephoto end, and φt=1/ft is the refractivity of the comprehensive lens optics at the telephoto end, the requirements as defined in the following formula are satisfied:


2<Σφ/φt<10 (1)

The inner-focusing zoom lens of the present invention can be described in more detailed as follows:

The zoom lens is comprised of at least five of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, and the fifth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis, and the fourth lens group includes two or more negative lens pieces, at least one of which is connected on its surface to another lens piece into a duplicated composite lens so that the junction between two of them functions to diverge incident beams.

The zoom lens is modified to include at least six of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, the fifth lens group of positive refractivity, and the six lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis, and the fifth lens group includes two or more negative lens pieces, at least one of which is connected on its surface to another lens piece into a duplicated composite lens so that the junction between two of them functions to diverge incident beams.

The zoom lens in the present aspect is characterized in that the rearmost lens group closest to the image plane are used for the focusing.

The zoom lens in the present aspect is also characterized in that the rearmost lens of positive refractivity and closest to the image plane is used for the focusing.

The zoom lens is also characterized in that at least one group of positive refractivity among the third lens group and all the succeeding lens groups includes a triplicated composite lens of negative-positive-negative power configuration of three of the lens pieces.

In another aspect of the present invention, the zoom lens is comprised of at least five of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, and the fifth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis, or is modifiably comprised of at least six of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, the fifth lens group of positive refractivity, and the six lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis. In either of such zoom lenses, the first lens group at least includes one negative lens piece and two or more positive lens pieces, and the first lens group provides optical properties as expressed by the following formulae:


40<vd1<55 (2)


20<vd2<35 (3)

where vd1 is an Abbe number of the negative lens piece(s) in the first lens group, and vd2 is the Abbe number of the positive lens piece(s) in any one of the lens groups of positive refractivity succeeding to the first lens group.

In still another aspect of the present invention, the inner-focusing zoom lens that does not conduct the floating focusing includes multi groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole. The second lens group includes a meniscus negative lens having its convex surface faced toward the photoshot subject, and another composite lens having negative and positive lens pieces connected to one another, and the second lens group provides optical properties as expressed by the following formulae:


1<R/f(wide)<8 (4)


2ω(wide)<40 (50)

where R is a radius of curvature of an objective surface of the foremost lens piece in the second lens group, f(wide) is a focal length of the comprehensive lens optics at the wide-angle end, and ω(wide) is a half field angle of the comprehensive lens optics at the wide-angle end.

The zoom lens in the present aspect is characterized in that at least one group of positive refractivity among the third lens group and all the succeeding lens groups includes a triplicated composite lens of negative-positive-negative power configuration of three of the lens pieces.

The zoom lens is comprised of the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole. At least one group of positive refractivity among the trailing groups includes two or more negative lens pieces, at least one of which is connected on its surface to another lens piece to form a duplicated composite lens so that the junction between two of them functions to diverge incident beams. Assuming now that a sum of the refractivities of all the junctions of the composite lenses can be expressed as Σφ=Σ|(N1−N2)/R| where N1 and N2 are refractivities that substances before and after the junction of the composite lenses respectively have, R is a radius of curvature of the junction, f(tele) is a focal length of the comprehensive lens optics of the zoom lens at the telephoto end, and φ(tele)=1/f(tele) is the refractivity of the comprehensive lens optics at the telephoto end, the following formula is given:


2<Σφ/φ(tele)<10 (6)

The zoom lens can be modified to be characterized in that at least one group of positive refractivity among the third lens group and all the succeeding lens groups includes a triplicated composite lens of negative-positive-negative power configuration of three of the lens pieces.

Details of the Required Components

The zoom lens of the present invention satisfies the conditions as given by the following formula:


2<Σφ/φt<10 (1)

with the assumptions that the sum of the refractivities of all the junctions of the composite lenses can be expressed as Σφ=Σ|(N1−N2)/R| where N1 and N2 are the refractivities that substances before and after the junction of the composite lenses respectively have, R is the radius of curvature of the junction, ft is the focal length of the comprehensive lens optics of the zoom lens at the telephoto end, and φt=1/ft is the refractivity of the comprehensive lens optics at the telephoto end.

The formula (1) defines a range of the sum of the refractivities of all the junctions of the composite lenses in any of the lens group(s) (only the 4th in this case) including two or more negative lenses and one or more composite lenses, relative to the refractivity of the comprehensive lens optics. As the sum of the refractivities of the junctions exceeds the lower limit, the zoom lens tends to lose more the effects of varying the spherical aberration to be overdone and prevalent, resulting in the comprehensive lens optics hardly countervailing the spherical aberration, especially, at the telephoto end.

As the sum exceeds the upper limit defined in the formula (1), the zoom lens tends to have the spherical aberration excessively overdone and prevalent during the fourth lens group's focusing for the long distance zooming, resulting in the zoom lens hardly countervailing the spherical aberration throughout the entire variable power range unless the lens group(s) displaced ahead of the fourth lens group, especially, the third lens group adjusts the spherical aberration to be emphatically underdeveloped.

In an embodiment of the present invention, a zoom lens comprising at least five of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, and the fifth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis, or comprising at least six of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, the fifth lens group of positive refractivity, and the six lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis,

the first lens group at least includes one negative lens piece and two or more positive lens pieces, and the first lens group provides optical properties as expressed by the following formulae:


40<vd1<55 (2)


20<vd2<35 (3)

where vd1 is an Abbe number of the negative lens piece(s) in the first lens group, and vd2 is the Abbe number of the positive lens piece(s) in any one of the lens groups of positive refractivity succeeding to the first lens group.

The formula (2) defines a range of the Abbe number of the negative lens piece in the first lens group. As the Abbe number exceeds the lower limit, the zoom lens tends to encounter difficulties in well compensating for chromatic aberration of magnification of the g-line and comatic aberration in an area below the g-line, especially, at the telephoto end. As the Abbe number exceeds the upper limit, the zoom lens tends to get hard to well compensate for the chromatic aberration of magnification of the c-line, especially, at the telephoto end.

The formula (3) defines a range of the Abbe number of the positive lens piece(s) in any one of the lens group(s) of positive refractivity succeeding to the aperture stop.

As the Abbe number exceeds the lower limit, the zoom lens tends to encounter difficulties in well compensating for the chromatic aberration of magnification of the g-line, especially, at the telephoto end. As the Abbe number exceeds the upper limit, the zoom lens tends to get hard to well compensate for axial chromatic aberration of the c-line and the chromatic aberration of magnification of the same, especially, at the telephoto end.

The zoom lens according to the present invention applies the formula (2) to well compensate for the chromatic aberration of magnification of the g-line, especially at the telephoto end. With an adjustment by simply applying the formula (2), it becomes hard instead to satisfactorily correct the chromatic aberration of magnification of the c-line, especially at the telephoto end. Hence, such adjustment along with the application of the formula (3) enables the zoom lens to successfully correct the chromatic aberration of magnification of the c-line as well.

In another embodiment of the present invention, the inner-focusing zoom lens comprises at least five of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, and the fifth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis; the fourth lens group including two or more negative lens pieces, at least one of which is connected on its surface to another lens piece into a duplicated composite lens so that the junction between two of them functions to diverge incident beams. This zoom lens is advantageous in that its entire length can be downsized. To depart from the above-defined requirements would undesirably increase the dimensions of the zoom lens.

In another embodiment of the invention, the inner-focusing zoom lens comprises at least six of the groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, the fifth lens group of positive refractivity, and the sixth lens group of negative refractivity all arranged in this order on the closest to the photoshot subject foremost basis; the fifth lens group including two or more negative lens pieces, at least one of which is connected on its surface to another lens piece into a duplicated composite lens so that the junction between two of them functions to diverge incident beams. This zoom lens is preferred in that its entire length can be downsized and that it can well compensate for astigmatism throughout the entire zoom range. To departing from the above-defined requirements would lead to the zoom lens resulting in undesirably increased dimensions as well as difficulties in flattening the image plane throughout the entire zoom range.

In the zoom lens of the present invention, it is desirable that the rearmost lens group closest to the image plane primarily conducts the focusing. This is desirable in successfully reducing the variations in the spherical aberration, as a whole, in the comprehensive lens optics. When the rearmost lens group closest to the image plane is not used primarily to conduct the focusing, the zoom lens is unable to increase a height of the incident beams upon the lens group including at least two negative lens pieces and the composite lens during the focusing for the short distance zooming, and consequently, it fails to reduce the variations in the spherical aberration in the comprehensive lens optics.

In the zoom lens of the present invention, it is advantageous when the rearmost lens group of positive refractivity and closest to the image plane primarily conducts the focusing. This is advantageous in that the variations in the spherical aberration in the comprehensive lens optics can be reduced. When the rearmost lens group of positive refractivity and closest to the image plane is not used primarily to conduct the focusing, the zoom lens is unable to increase a height of the incident beams upon the lens group including two negative lens pieces and the composite lens during the focusing for the short distance zooming, and therefore, it fails to reduce the variations in the spherical aberration in the comprehensive lens optics.

The best mode to adjust and control the varied spherical aberration during the focusing, is when at least one group of positive refractivity among the third lens group and all the succeeding lens groups includes a triplicated composite lens of negative-positive-negative power configuration of three of the lens pieces. When this is not the case, the zoom lens encounters significantly frequented and increased higher-order aberration and adverse sensitivity that means considerable development of various types of the aberration due to manufacturing tolerance, eventually resulting in inability to design for fine products.

In an inner-focusing zoom lens that does not conduct the floating focusing and that comprises multi groups of lens pieces, namely, the first lens group or the leading lens group of positive refractivity, the second lens group of negative refractivity, and the remaining trailing groups of positive refractivity, as a whole,

the second lens group includes a meniscus negative lens having its convex surface faced toward the photoshot subject, and a composite lens having negative and positive lens pieces connected to one another, and the second lens group provides optical properties as expressed by the following formulae:


1<R/f(wide)<8 (4)


2ω(wide)<40 (5)

where R is a radius of curvature of an objective surface of the foremost lens piece in the second lens group, f(wide) is a focal length of the comprehensive lens optics at the wide-angle end, and ω(wide) is a half field angle of the comprehensive lens optics at the wide-angle end. The formula 1<R/f(wide)<8 set forth above provides a range of the radius of curvature of an objective surface of the negative lens piece closest to the photoshot subject in the second lens group, relative to the focal length at the wide-angel end. As the radius of curvature exceeds the upper limit, it is hard to well compensate for the variations in curvature of field during the focusing, especially, at the wide-angle end. As the radius of curvature exceeds the lower limit, the desired anti-aberration effectiveness imposed on the negative lens piece closest to the photoshot subject must be taken over and countervailed by another negative lens piece(s) in the same lens group, which cannot be attained with the triplicated composite power configuration of three of the lens pieces in the second lens group.

The formula 2ω(wide)<40 set forth above provides a range of the field angle at the wide-angle end.

As the field angle exceeds the upper limit, the zoom lens loses its merits as a telephoto-shooting zooming lens.

The formula 2<Σφ/φ(tele)<10 provides a range of the sum of the refractivities of all the junctions of the composite lenses disposed along with two or more negative lens pieces in the same lens group, relative to the refractivity of the comprehensive lens optics at the telephoto end.

As the sum of the refractivities exceeds the upper limit, the zoom lens tends to have the spherical aberration excessively overdone and prevalent during such a lens group's focusing for the long distance zooming, resulting in the zoom lens hardly countervailing the spherical aberration throughout the entire variable power range unless any one(s) displaced ahead of that lens group, especially, the third lens group adjusts the spherical aberration to be emphatically underdeveloped.

As the sum of the refractivities exceeds the lower limit, that lens group tends to lose more the effects of varying the spherical aberration to be overdone and prevalent, resulting in the comprehensive lens optics hardly countervailing the spherical aberration, especially, at the telephoto end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a first embodiment of a zoom lens in accordance with the present invention.

FIG. 2 is a sectional view illustrating a second embodiment of the zoom lens in accordance with the present invention.

FIG. 3 is a sectional view illustrating a third embodiment of the zoom lens in accordance with the present invention.

FIG. 4 is a sectional view illustrating a fourth embodiment of the zoom lens in accordance with the present invention.

FIG. 5 is a sectional view illustrating a fifth embodiment of the zoom lens in accordance with the present invention.

FIG. 6 is a sectional view illustrating a sixth embodiment of the zoom lens in accordance with the present invention.

FIG. 7 depicts aberration at the wide-angle end under an assumption of the infinitely far imaging plane in the first exemplary zoom lens.

FIG. 8 depicts the aberration at the wide-angle end under another assumption of 1.6 meters in distance from the leading or foremost lens piece to a photoshot subject in the first exemplary zoom lens.

FIG. 9 depicts aberration at the intermediate focal length under the assumption of the infinitely far imaging plane in the first exemplary zoom lens.

FIG. 10 depicts the aberration at the intermediate focal length under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the first exemplary zoom lens.

FIG. 11 depicts aberration at the telephoto end under the assumption of the infinitely far imaging plane in the first exemplary zoom lens.

FIG. 12 depicts the aberration at the telephoto end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the first exemplary zoom lens.

FIG. 13 depicts aberration at the wide-angle end under the assumption of the infinitely far imaging plane in the second exemplary zoom lens.

FIG. 14 depicts the aberration at the wide-angle end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the second exemplary zoom lens.

FIG. 15 depicts aberration at the intermediate focal length under the assumption of the infinitely far imaging plane in the second exemplary zoom lens.

FIG. 16 depicts the aberration at the intermediate focal length under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the second exemplary zoom lens.

FIG. 17 depicts aberration at the telephoto end under the assumption of the infinitely far imaging plane in the second exemplary zoom lens.

FIG. 18depicts the aberration at the telephoto end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the second exemplary zoom lens.

FIG. 19 depicts aberration at the wide-angle end under the assumption of the infinitely far imaging plane in the third exemplary zoom lens.

FIG. 20 depicts the aberration at the wide-angle end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the third exemplary zoom lens.

FIG. 21 depicts aberration at the intermediate focal length under the assumption of the infinitely far imaging plane in the third exemplary zoom lens.

FIG. 22 depicts the aberration at the intermediate focal length under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the third exemplary zoom lens.

FIG. 23 depicts aberration at the telephoto end under the assumption of the infinitely far imaging plane in the third exemplary zoom lens.

FIG. 24 depicts the aberration at the telephoto end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the third exemplary zoom lens.

FIG. 25 depicts aberration at the wide-angle end under the assumption of the infinitely far imaging plane in the fourth exemplary zoom lens.

FIG. 26 depicts the aberration at the wide-angle end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the fourth exemplary zoom lens.

FIG. 27 depicts aberration at the intermediate focal length under the assumption of the infinitely far imaging plane in the fourth exemplary zoom lens.

FIG. 28 depicts the aberration at the intermediate focal length under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the fourth exemplary zoom lens.

FIG. 29 depicts aberration at the telephoto end under the assumption of the infinitely far imaging plane in the fourth exemplary zoom lens.

FIG. 30 depicts the aberration at the telephoto end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the fourth exemplary zoom lens.

FIG. 31 depicts aberration at the wide-angle end under the assumption of the infinitely far imaging plane in the fifth exemplary zoom lens.

FIG. 32 depicts the aberration at the wide-angle end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the fifth exemplary zoom lens.

FIG. 33 depicts aberration at the intermediate focal length under the assumption of the infinitely far imaging plane in the fifth exemplary zoom lens.

FIG. 34 depicts the aberration at the intermediate focal length under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the fifth exemplary zoom lens.

FIG. 35 depicts aberration at the telephoto end under the assumption of the infinitely far imaging plane in the fifth exemplary zoom lens.

FIG. 36 depicts the aberration at the telephoto end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the fifth exemplary zoom lens.

FIG. 37 depicts aberration at the wide-angle end under the assumption of the infinitely far imaging plane in the sixth exemplary zoom lens.

FIG. 38 depicts the aberration at the wide-angle end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the sixth exemplary zoom lens.

FIG. 39 depicts aberration at the intermediate focal length under the assumption of the infinitely far imaging plane in the sixth exemplary zoom lens.

FIG. 40 depicts the aberration at the intermediate focal length under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the sixth exemplary zoom lens.

FIG. 41 depicts aberration at the telephoto end under the assumption of the infinitely far imaging plane in the sixth exemplary zoom lens.

FIG. 42 depicts the aberration at the telephoto end under the assumption of 1.6 meters in distance from the leading or foremost lens piece to the photoshot subject in the sixth exemplary zoom lens.

DETAILED DESCRIPTION OF THE INVENTION

Zoom lenses according to the present invention, including their respective variations and modifications are adapted to conduct the focusing such as inner-focusing, rear-focusing, and the like, more briskly and quickly in comparison with the front lens focusing system, and the improved zoom lenses of groups of lens pieces merely displace a single group of lens pieces to well compensate for variations in spherical aberration, especially, at the telephoto end.

Embodiment 1

f = 72.0785~390.0019
FNo. = 4.11~5.78
(Data on Surfaces)
rdndvd
 0 (Objective Surface)
 1425.39052.30001.8042046.50
 2105.03878.19551.4970081.61
 3−562.00440.3000
 4110.78717.38671.4970081.61
−1221.5706Variable
 6256.49141.50001.4874970.44
38.56315.8780
 8−89.51710.90001.4874970.44
 945.08113.45311.8061033.27
10120.9491Variable
1174.04903.70001.6700347.20
12−131.14561.9246
13−47.64231.30001.8061040.73
14−280.57700.2000
1543.72303.50001.7200050.34
1664.07684.3536
17 (Aperture Stop)Variable
18−141.98673.50001.8051825.46
19−51.76640.1500
2048.62091.30001.8340037.34
2124.618310.78801.5168064.20
22−27.62491.20001.8466623.78
2396.44842.2370
2481.03054.33051.8466623.78
25−68.8511Variable
26191.93511.20001.7725049.62
2747.76632.5284
28482.33844.00001.8466623.78
29−76.84785.0000
30−69.13581.50001.8348142.72
31200.0000Variable
32 (Image Plane)
(Various Data)
Zoom Ratio 5.411
WIDEMIDDLETELE
f72.0785167.1066390.0019
FNo.4.115.565.78
2ω°33.21214.4546.234
Image Height21.63321.63321.633
Full Lens Length225.142275.999311.667
Variable Interval
(Focused at the Infinity-Point)
d0
d51.500054.4500114.8693
d1055.512330.01661.5000
d1716.864710.04841.5000
d2521.730514.76681.0000
BF46.909084.0920110.1720
Variable Interval
(Focused with 1.6-meter Distance to the Subject)
d01374.861324.001288.33
d51.500054.4500114.8693
d1055.512330.01661.5000
d1716.864710.04841.5000
d2523.397018.984217.1782
BF45.243079.875093.9940
(Data on the Zoom Lens Groups)
GroupInitial Surfacef
11210.0756
26−61.8149
311165.5535
41860.9734
526−63.5770
(Values of the Primary Term in the Formulae)
Formula (1)9.68
Formula (2)46.50
Formula (3)23.78

Embodiment 2

f = 72.0702~387.8665
FNo. = 4.15~5.74
(Data on the Surfaces)
rdndvd
 0 (Objective Surface)
 1464.30372.30001.8040046.58
 2117.27767.63041.4970081.61
 3−668.15230.3000
 4125.07436.90361.4970081.61
 5−1086.8029Variable
 6400.00001.70001.5168064.20
 762.98432.9660
 8−209.88811.30001.4874970.21
 943.12212.56621.7552027.51
1070.2788Variable
1177.18593.25831.5891361.18
12−142.47202.4834
13−43.67741.50001.7234237.95
14−360.16440.2000
1548.17904.16041.7552027.51
16102.93963.0000
17 (Aperture Stop)Variable
18221.34971.50001.7618226.55
1955.03933.5987
20176.60144.94031.5163364.15
21−40.68790.2000
22132.62695.95831.5688356.34
23−33.22351.30001.7859044.19
24−266.00500.2000
2552.19844.06871.4970081.61
26−458.0671Variable
27103.51931.50001.8348142.72
2843.09872.3941
291915.86833.73711.7618226.55
30−44.96121.50001.4874970.21
31−66.56553.1137
32−50.28011.20001.8040046.58
33200.0000Variable
34 (Image Plane)
(Various Data)
Zoom Ratio 5.382
WIDEMIDDLETELE
f72.0702167.0620387.8665
FNo.4.155.125.74
2ω°33.21414.3906.260
Image Height21.63321.63321.633
Full Lens Length230.089283.557320.126
Variable Interval
(Focused at the Infinity-Point)
d0
d57.717976.4777132.8660
d1062.988233.71141.5000
d1715.926410.25971.5000
d2620.888314.71321.5000
BF47.089072.9160107.2810
Variable Interval
(Focused with 1.6-meter Distance to the Subject)
d01369.911316.441279.87
d57.717976.4777132.8660
d1062.988233.71141.5000
d1715.926410.25971.5000
d2622.344919.003015.1222
BF45.633068.627093.6590
(Data on the Zoom Lens Groups)
GroupInitial Surfacef
11232.6486
26−71.5603
311165.8567
41856.5968
527−57.0465
(Values of the Primary Term in the Formulae)
Formula (1)2.53
Formula (2)46.58
Formula (3)27.51

Embodiment 3

f = 71.7730~291.9632
FNo. = 4.59~5.79
(Data on the Surfaces)
rdndvd
 0 (Objective Surface)
 175.03925.68331.4874970.44
 2617.32940.2000
 388.88791.40001.7859043.93
 444.02788.09311.4970081.61
 5526.7875Variable
 6275.58401.00001.7725049.62
 732.93672.9002
 8−251.03571.00001.4874970.44
 925.38414.06751.6727032.17
10135.9493Variable
11 (Aperture Stop)2.0000
1229.88545.02411.4874970.44
13−268.44372.0014
14−34.60791.00001.7725049.62
15−94.7884Variable
16−29.84682.71031.7282528.32
17−23.84870.1000
1870.84481.00001.8061033.27
1928.88528.29771.4874970.44
20−20.25041.00001.8061033.27
21−42.26050.1000
2237.75703.50001.7725049.62
23199.3442Variable
2497.69151.00001.6204160.34
2526.31762.9258
26−153.44592.59531.8466623.78
27−40.05471.20001.7725049.62
28200.0000Variable
34 (Image Plane)
(Various Data)
Zoom Ratio 4.068
WIDEMIDDLETELE
f71.7730140.0942291.9632
FNo.4.595.605.79
2ω°32.82417.0548.246
Image Height21.63321.63321.633
Full Lens Length175.925206.840235.878
Variable Interval
(Focused at Infinity-Point)
d0
d51.650030.417665.7475
d1028.903915.77233.6150
d1520.349314.95217.8202
d2313.63159.76601.5000
BF52.592077.133098.3970
Variable Interval
(Focused with 1.6-meter Distance to the Subject)
d01424.071393.161364.12
d51.650030.417665.7475
d1028.903915.77233.6150
d1520.349314.95217.8202
d2314.359711.38896.3196
BF51.864075.510093.5770
(Data on the Zoom Lens Groups)
GroupInitial Surfacef
11139.7971
26−49.1963
311193.8488
41636.0311
524−39.7982
(Values of the Primary Term in the Formulae)
Formula (1)7.81
Formula (2)43.93
Formula (3)28.32

Embodiment 4

f = 72.0790~388.0549
FNo. = 3.85~6.35
(Data on the Surfaces)
rdndvd
 0 (Objective Surface)
 1241.21702.30001.8340037.34
 2111.07976.76491.4970081.61
 3−695.27990.3000
 4137.53955.83351.4970081.61
 5−754.2861Variable
 6350.00001.50001.6228057.06
 740.82844.5778
 8−86.65121.20001.4874970.44
 947.94023.39891.8061033.27
10266.2338Variable
11171.99733.37321.6584450.85
12−112.61531.9131
13−42.74541.20001.8348142.72
14−261.93020.2000
1574.04023.84581.6031160.69
16−179.31585.0000
17 (Aperture Stop)Variable
18−414.62253.15681.7847225.72
19−78.32443.4069
2078.39561.50001.7995042.34
2134.878610.14231.4874970.44
22−34.88081.50001.8466623.78
23−96.83110.3338
2460.72704.50001.7200050.34
25−392.9783Variable
26132.91111.20001.8042046.50
2736.52872.8896
28−489.48543.92591.8466623.78
29−36.80541.20001.8348142.72
30200.0000Variable
31 (Image Plane)
(Various Data)
Zoom Ratio 5.384
WIDEMIDDLETELE
f72.0790167.0937388.0549
FNo.3.855.266.35
2ω°33.13014.4006.248
Image Height21.63321.63321.633
Full Lens Length235.118276.672320.213
Variable Interval
(Focused at the Infinity-Point)
d0
d52.500046.829792.3228
d1054.001726.17962.0000
d1732.986825.716719.6391
d2516.679411.77383.0000
BF53.788091.0100128.0890
Variable Interval
(Focused with 1.6-meter Distance to the Subject)
d01364.881323.331279.79
d52.500046.829792.3228
d1054.001726.17962.0000
d1732.273923.671212.9801
d2517.392313.81939.6590
BF53.788091.0100128.0890
(Data on the Zoom Lens Groups)
GroupInitial Surfacef
11185.9118
26−64.1404
311199.4687
41849.3767
526−45.6959
(Values of the Primary Term in the Formulae)
Formula (1)7.47
Formula (2)37.34
Formula (3)25.72

Embodiment 5

f = 70.7231~388.1393
FNo. = 4.05~5.87
(Data on the Surfaces)
rdndvd
 0 (Objective Surface)
 1341.48652.30001.8042046.50
 2100.07498.67881.4970081.61
 3−580.61820.3000
 4102.63787.95441.4970081.61
 5−1349.2717Variable
 6250.00001.70001.6385455.45
 738.50995.5546
 8−113.58201.30001.4874970.44
 950.74133.18841.8061033.27
10154.1035Variable
1183.83533.72211.6385455.45
12−166.12241.8375
13−44.97791.30781.8348142.72
14232.97220.2000
1559.84073.97201.8466623.78
16−354.68102.0000
17 (Aperture Stop)Variable
18204.17031.30001.8466623.78
1934.66216.49631.6476933.84
20−98.0309Variable
21−317.93911.30001.8466623.78
2236.544710.87071.6385455.45
23−25.84011.30001.8061033.27
24−60.56100.2000
2559.55974.61311.8466623.78
26−533.7403Variable
2763.60491.20001.7200050.34
2831.87463.6172
29−235.13994.07561.7847225.72
30−37.45521.20001.8042046.50
31237.3893Variable
32 (Image Plane)
(Various Data)
Zoom Ratio 5.488
WIDEMIDDLETELE
f70.7231167.0889388.1393
FNo.4.055.325.87
2ω°33.84814.4686.256
Image Height21.63321.63321.633
Full Lens Length235.174278.686320.220
Variable Interval
(Focused at the Infinity-Point)
d0
d52.500049.1230100.5475
d1051.303121.46222.0000
d1718.805510.75260.9318
d201.40497.748412.6161
d2618.960815.52244.1107
BF62.011093.8890119.8250
Variable Interval
(Focused with 1.6-meter Distance to the Subject)
d01364.821321.311279.78
d52.500049.1230100.5475
d1051.303121.46222.0000
d1718.805510.75260.9318
d201.40497.748412.6161
d2619.842818.283214.1726
BF61.129091.1280109.7630
(Data on the Zoom Lens Groups)
GroupInitial Surfacef
11186.6475
26−56.9423
311164.6565
418197.9375
52163.1960
627−53.3835
(Values of the Primary Term in the Formulae)
Formula (1)4.73
Formula (2)46.50
Formula (3)23.78

Embodiment 6

f = 72.0562~388.0927
FNo. = 4.11~5.79
(Data on the Surfaces)
rdndvd
 0 (Objective Surface)
 1322.45992.30001.8042046.50
 291.01039.21921.4970081.61
 3−569.01880.3000
 493.46158.60411.4970081.61
 5−1008.031Variable
 6198.43621.70001.6180063.39
 737.60628.4309
 8−70.07481.30001.4874970.44
 952.54353.22191.8061033.27
10190.1372Variable
1173.67673.88641.6180063.39
12−176.63822.1013
13−42.34391.78861.8348142.72
14196.04110.2000
1569.64024.50001.8466623.78
16−174.30322.0000
17 (Aperture Stop)Variable
18−5925.00491.30001.8466623.78
1939.57477.50041.5955139.22
20−56.4431Variable
21174.57241.30001.8466623.78
2247.33258.87341.6180063.39
23−33.63931.30001.8061033.27
24−147.22320.2000
2581.28104.00001.8466623.78
26−208.9178Variable
27145.40291.20001.6180063.39
2841.15852.8866
29−934.53064.04101.7407727.76
30−46.38791.20001.8042046.50
31390.3333Variable
32 (Image Plane)
(Various Data)
Zoom Ratio 5.386
WIDEMIDDLETELE
f72.0562167.0619388.0927
FNo.4.115.185.79
2ω°33.61014.5306.268
Image Height21.63321.63321.633
Full Lens Length240.127284.816330.165
Variable Interval
(Focused at the Infinity-Point)
d0
d52.500047.295892.4906
d1046.161218.46552.0000
d1717.129410.94306.3591
d202.851911.581113.0322
d2621.933918.25722.5000
BF66.196394.9192130.4291
Variable Interval
(Focused with 1.6-meter Distance to the Subject)
d01359.871315.181269.84
d52.500047.295892.4906
d1046.161218.46552.0000
d1717.129410.94306.3591
d201.97688.46041.9920
d2622.809021.377913.5402
BF66.196394.9192130.4291
(Data on the Zoom Lens Groups)
GroupInitial Surfacef
11172.0317
26−52.5524
311189.0866
418228.1124
52169.8075
627−65.9427
(Values of the Primary Term in the Formulae)
Formula (1)4.04
Formula (2)46.50
Formula (3)23.78

Detailed Description of Embodiment 1 to 4

The preferred embodiments disclosed as Embodiment 1 to Embodiment 4 have five groups of lens pieces, namely, the first or foremost lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, and the fifth lens group of negative refractivity, and in order to alter the variable power from the wide-angle view to the telephoto view, at least the first, the third, the fourth, the fifth of the lens groups are moved toward the photoshot subject so that the first and the second of the lens groups to move to have an increased interval therebetween, the second and the third of them to move to have the decreased interval therebetween, the third and the fourth of them to move to have the reduced interval therebetween, and the fourth and the fifth of them to move to have the reduced interval therebetween.

The aperture stop is disposed in position closer to either the image plane or the photoshot subject in the third lens group, and it is displaced along with the lens pieces of the third lens group.

In order to focus from the long distance zooming to the short distance zooming, there are two ways, namely, the rear-focusing (as set forth in Embodiment 1 to Embodiment 3) where the rearmost lens group closest to the image plane (the fifth in this case) is displaced, and the inner-focusing (as set forth in Embodiment 4) where the rearmost lens group of positive refractivity and closest to the image plane is displaced. Either of the inner- and rear-focusing systems can attain more brisk and quick focusing, compared with the front lens focusing, and employing these focusing systems enables the zoom lenses to be downsized without degradation in ensuring a sufficient amount of spherical light. A displacement of the lens groups relative to the identical photoshot subject becomes greater as the focal length increases.

The lens group(s) (the fourth in this case) of positive refractivity among the third lens group and all the succeeding lens groups has two or more negative lens pieces, and at least one of the negative lens pieces has its one major surface connected to another lens piece to form the composite lens of which junction serves to diverge beams incident thereon. In general, the zoom lens is prone to underdevelop variations in spherical aberration unless it is specifically modified in structure to reduce the displacement of the lens groups used for the focusing. The zoom lens according to the present invention also has the first to the third of the lens groups adapted to underdevelop the varied spherical aberration. With the fourth lens group of positive refractivity designed to have two negative lens pieces at least one of which is the composite lens having the beam diverging junction, the fourth lens group causes the varied spherical aberration to be overdone and prevalent, thereby countervailing the varied spherical aberration, as a whole, in the comprehensive lens optics, especially at the telephoto end.

The overdone variations in the spherical aberration by the fourth lens group is by virtue of excessively developing the spherical aberration as a result of inducing the beams to be incident upon the fourth lens group at a higher point during the short distance zooming relative to the long distance zooming. In this way, the lens groups dedicated to the focusing does not have its performance design limited to the rear-focusing but have a choice of the inner-focusing by the rearmost lens group of positive refractivity and closest to the image plane (as in Embodiment 4).

However, any of the focusing lens groups other than the one including two negative lens pieces at least one of which is the composite lens serving to diverge beams at its junction with the remaining component lens piece(s) cannot induce the incident beams thereon to enter at the higher point during the short distance focusing, and therefore, the varied spherical aberration cannot be reduced.

To satisfy requirements of the composite lenses, it is desirable that the negative lens component should be made of glass showing a high refractive index while the positive lens component is made of glass showing a low refractive index, so as to enhance a radius of curvature at the junction between them. In the present invention, however, the requirements are defined to meet those given in the formula (1).

The best mode for the focusing with the reduced variations in the spherical aberration desirably includes a triplicated composite lens of negative-positive-negative power configuration of three lens pieces in at least one lens group (in this case, the fourth) of positive refractivity among the third and the remaining succeeding ones. Although a single lens is also capable of inducing the spherical aberration to be overdone, such a mono-lens configuration is liable to cause high-order aberration as well as enhanced sensitivity, which resultantly obstructs the way of enhancing the radius of curvature.

Detailed Description of Embodiment 5 and Embodiment 6

The preferred embodiments disclosed as Embodiment 5 and Embodiment 6 have six groups of lens pieces, namely, the first or foremost lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, the fourth lens group of positive refractivity, the fifth lens group of positive refractivity, and the six lens group of negative refractivity, and in order to alter the variable power from the wide-angle view to the telephoto view, at least the first, the third, the fourth, the fifth and the sixth of the lens groups are moved toward the photoshot subject so that the first and the second of the lens groups to move to have an increased interval therebetween, the second and the third of them to move to have the decreased interval therebetween, the third and the fourth of them to move to have the reduced interval therebetween, and the fourth and the fifth of them to move to have the increased interval therebetween, and the fifth and the sixth of them to move to have the reduced interval therebetween. The second lens group may keep static during altering the power ratio.

The aperture stop is disposed in position closer to either the image plane or the photoshot subject in the third lens group, and it is displaced along with the lens pieces of the third lens group.

In order to focus from the long distance zooming to the short distance zooming, there are two ways, namely, the rear-focusing (as set forth in Embodiment 5) where the rearmost lens group closest to the image plane (the sixth in this case) is displaced, and the inner-focusing (as set forth in Embodiment 6) where the rearmost lens group of positive refractivity and closest to the image plane is displaced.

During altering the variable power from the wide-angle view to the telephoto view, the fourth lens group and the fifth lens group are displaced to come farther from each other. In this way, the zoom lens is satisfactorily compensated for astigmatism.

The fifth lens group includes two or more negative lens pieces one of which is a component of the composite lens that has its junction serving to diverge beams, thereby inducing the spherical aberration to be overdone and prevalent in the fifth lens group during the focusing so as to countervail the varied spherical aberration in the comprehensive lens optics, especially, at the telephoto end.

Moreover, the fourth lens group includes the additional composite lens that has a junction between the component lens pieces serving to diverge beams incident thereon, thereby functioning as an auxiliary effect of inducing the overdone spherical aberration. In this way, the fourth lens group in addition to the fifth cooperatively work to reduce the varied spherical aberration during the focusing.