# Patent

## Title
Zoom lens

## Abstract

A zoom lens includes, sequentially from an object side, a first lens group having a positive refractive power; a second lens group having negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a positive refractive power. The third lens group includes, sequentially from the object side, a front group having a positive refractive power and a rear group having a negative refractive power. Zoom is performed by moving the second lens group and the third lens group in a direction along an optical axis, and by integrally moving the front group and the rear group in a direction along the optical axis. Focusing is preformed by moving the front group in a direction along the optical axis.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens ideal for digital still and motion cameras (DSMC), single-lens reflex cameras, and the like.
2. Description of the Related Art
Recently, in DSMCs and single-lens reflex cameras, zoom lenses are used in which a portion of the lenses in the first group are moved along the optical axis during focusing (see, for example, Japanese Patent Application Laid-Open Publication Nos. 2002-162564 and 2003-344766). Zoom lenses are also used in which a third or a fourth group configured by plural intermediate diameter lenses is moved along the optical axis during focusing (see, for example, Japanese Patent Application Laid-Open Publication No. 2010-44372).
For example, the zoom lens recited in Japanese Patent Application Laid-Open Publication No. 2002-162564 has, sequentially from an object side, a front lens component that includes plural lenses for performing zoom and focusing; and a rear lens component that includes a relay lens group having an imaging function. The rear lens component has, sequentially from the object side, a lens group A having a positive refractive power, a lens group B having a negative refractive power and movable so as to have a component in a direction orthogonal to the optical axis, and a lens group C having a positive refractive power, where the movement of the lens group B so as to have a component in a direction orthogonal to the optical axis causes displacement of the image.
The zoom lens recited in Japanese Patent Application Laid-Open Publication No. 2003-344766 includes, sequentially from the object side, a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, where the second lens group and the third lens group are moved in a direction along the optical axis to perform zooming. The first lens group is configured sequentially by a positive front group and a rear group having a stronger refractive power than the front group, where the rear group is moved in a direction along the optical axis to perform near focus and the front group is configured sequentially by a negative meniscus lens having a convex surface facing toward the object side, a positive lens whose object-side surface is convex, and a positive lens whose object-side surface is convex. The rear group is configured sequentially by a negative meniscus lens having a convex surface facing toward the object side, and a positive lens component.
The zoom lens recited in Japanese Patent Application Laid-Open Publication No. 2010-44372 has a first lens group having a positive refractive power and disposed farthest on the object side, a second lens group disposed on the image plane side of the first lens group, a Gn lens group disposed farthest on the image plane side, a Gn-1 lens group disposed on the object side of the Gn lens group, and at least 1 lens group disposed between the second lens group and the Gn-1 lens group. When zoom is performed, the first lens group and the Gn lens group are fixed. When focusing is performed, at least 1 lens group disposed between the second lens group and the Gn-1 lens group is moved and at least a portion of the Gn lens group is moved so as to have a component in a direction substantially orthogonal to the optical axis.
Nonetheless, the zoom lenses disclosed in Japanese Patent Application Laid-Open Publication Nos. 2002-162564 and 2003-344766 perform focusing by extending the first group that controls focusing and in which plural large diameter lenses are used. As a result, the first group is heavy and the effective diameter thereof is large, arising in problems of slow focusing speed and large power consumption by the drive apparatus driving the lens group.
Further, the zoom lens disclosed in Japanese Patent Application Laid-Open Publication No. 2010-44372 performs focusing by extending the third or the fourth group and although the third or the fourth group controlling focusing has an intermediate effective diameter, each is configured by intermediate diameter lenses and therefore, it is difficult to say that weight reduction is sufficiently facilitated. Consequently, if zoom and focusing are performed simultaneously, load is placed on the drive apparatus driving the lens group and it is difficult to perform quick zooming and focusing. Additionally, since this zoom lens has an absolute lateral magnification of 5 or less for the focusing group, the distance that the focusing group is extended for focusing from an infinity focus state to a near focus state becomes large, whereby the driving time for focusing increases and the quick capture of an image is difficult.
SUMMARY OF THE INVENTION
It is an object of the present invention to at least solve the above problems in the conventional technologies.
A zoom lens according to one aspect of the present invention includes, sequentially from an object side, a first lens group having a positive refractive power; a second lens group having negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a positive refractive power. The third lens group includes, sequentially from the object side, a front group having a positive refractive power and a rear group having a negative refractive power. Zoom is performed by moving the second lens group and the third lens group in a direction along an optical axis, and by integrally moving the front group and the rear group in a direction along the optical axis. Focusing is preformed by moving the front group in a direction along the optical axis.
The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a cross-sectional view (along the optical axis) of a zoom lens according to a first embodiment;
FIG. 2 is a diagram of various types of aberration at a wide angle edge of the zoom lens of the first embodiment according to the invention;
FIG. 3 is a diagram of various types of aberration at an intermediate position of the zoom lens of the first embodiment according to the invention;
FIG. 4 is a diagram of various types of aberration at a telephoto edge of the zoom lens of the first embodiment according to the invention;
FIG. 5 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a second embodiment;
FIG. 6 is a diagram of various types of aberration at the wide angle edge of the zoom lens of the second embodiment according to the invention;
FIG. 7 is a diagram of various types of aberration at the intermediate position of the zoom lens of the second embodiment according to the invention; and
FIG. 8 is a diagram of various types of aberration at the telephoto edge of the zoom lens of the second embodiment according to the invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the accompanying drawings, exemplary embodiments according to the present invention are explained in detail below.
A zoom lens according to the present invention includes, sequentially from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and fourth lens group having a positive refractive power.
An object of the invention is to provide a zoom lens that has high optical performance and is capable of capturing images quickly by achieving a compact, light-weight focusing group and suppressing the distance that the focusing group is extended for focusing. To achieve this object, various conditions are set as indicated below.
The third lens group includes, sequentially from the object side, a front group having a positive refractive power, and a rear group having a negative refractive power. The zoom lens zooms from a wide angle edge to a telephoto edge by uniformly moving the second lens group and the third lens group along the optical axis, from the object side toward the image side. During this movement, the front group and the rear group constituting the third lens group are moved as a unit along the optical axis. Further, the first lens group and the fourth lens group are fixed. Focusing is performed by moving the front group alone, along the optical axis.
In the invention, a portion of the lens group controlling zoom is used as a focusing group. Consequently, compared to conventional zoom lenses, reductions in the size and weight of the focusing group can be facilitated. As a result, quick focusing becomes possible. Preferably, the front group of the third lens group controlling focusing is configured by 1 positive lens. Such configuration enables further reductions in the size and weight of the focusing group, and quicker focusing.
The front group of the third lens group of the zoom lens according to the present invention is preferably configured by an aspheric lens that satisfies the following conditional expression.

0.1<|100×(ΔS1−ΔS2)/φS|<0.5  (1)

Where, ΔS1 represents the deviation of the paraxial radius of curvature at the height of the effective diameter of the aspheric surface on the object side of the front group and aspheric surface shape, ΔS2 represents the deviation of the paraxial radius of curvature at the height of the effective diameter of the aspheric surface on the image side of the front group and aspheric surface shape, and φS represents the effective diameter of the front group.
Conditional expression (1) prescribes a condition for improving the optical performance of the focusing group. Below the lower limit of conditional expression (1), in a near focus state, the correction of field curvature occurring at the front group of the third lens group becomes difficult. On the other hand, above the upper limit of conditional expression (1), in an infinity focus state, the correction of field curvature occurring at the front group of the third lens group becomes difficult.
More favorable results can be expected if conditional expression (1) is satisfied within the following range.

0.25<|100×(ΔS1−ΔS2)/φS|<0.4  (1)′

By satisfying the range prescribed by conditional expression (1)′, the optical performance of the front group (focusing group) of the third lens group can be further improved.
Further, the zoom lens preferably satisfies the following conditional expression, where F3F is the focal length of the front group of the third lens group and F3 is the focal length of the entire third lens group in the infinity focus state.

0.5<F3F/F3<0.95  (2)

Conditional expression (2) prescribes a condition to favorably correct various types of aberration throughout the entire zoom range, from an infinity focus state to a near focus state. Below the lower limit of conditional expression (2), an imbalance of refractive power between the front group and the rear group of the third lens group occurs. As a result, although the correction of various types of aberration in the infinity focus state is favorable, the correction of the various types of aberration in the near focus state becomes difficult. On the other hand, above the upper limit of conditional expression (2), the refractive power of the third lens group (reciprocal of focal length) becomes too weak. As a result, although aberration variation from the infinity focus state to the near focus state becomes small, the correction of various types of aberration in the infinity focus state becomes difficult.
More favorable results can be expected if conditional expression (2) is satisfied within the following range.

0.75<F3F/F3<0.85  (2)′

By satisfying the range prescribed by conditional expression (2)′, various types of aberration can corrected more favorably.
The zoom lens according to the invention preferably satisfies the following conditional expression, where βFT is the lateral magnification of the front group of the third lens in the infinity focus state at the telephoto edge.

5<|βFT|  (3)

Conditional expression (3) prescribes a condition to both suppress the distance that the focusing group is extended to perform focusing (displacement) and to achieve favorable correction of various types of aberration. If conditional expression (3) is not satisfied, problems arise in that the distance that the focusing group is extended increases, or the correction of various types of aberration becomes difficult. For example, if the value βFT is 0<βFT<5, the refractive power of the front group of the third lens group becomes to weak and although the correction of chromatic aberration in the near focus state is favorable, the distance that the focusing group is extended for focusing in the near focus state becomes large, impeding size reductions of the optical system. On the other hand, if the value of βFT is −5<βFT<0, the refractive power of the front group of the third lens group becomes too strong and although the distance that the focusing group is extended for focusing in the near focus state can be suppressed, the correction of various types of aberration in the infinity focus state and in the near focus state becomes difficult. In particular, if the front group is configured by 1 lens, the correction of various types of aberration becomes difficult.
More favorable results can be expected if conditional expression (3) is satisfied within the following range.

15<|βFT|  (3)′

By satisfying the range prescribed by conditional expression (3)′, the distance that the focusing group is extended for focusing is suppressed while more favorable aberration correction is realized.
As described, in the zoom lens according to the invention, rather than the first lens group, a portion of the third lens group having a small diameter is used as the focusing group, whereby compared to conventional zoom lenses, reductions in the size and weight of the focusing group are achieved. Additionally, since the focusing group is constituted by 1 lens, significant size and weight reductions of the focusing group are achieved. Furthermore, by adopting, as the lens constituting the focusing group, an aspheric lens that satisfies conditional expression (1), field curvature can be favorably corrected throughout the entire zoom range, from the infinity focus state to the near focus state. By satisfying conditional expression (2), various types of aberration can be favorably corrected throughout the entire zoom range, from the infinity focus state to the near focus state. By satisfying conditional expression (3), the distance that the focusing group is extended for focusing can be suppressed, without deterioration in optical performance.
Thus, according to the zoom lens of the invention, reductions in the size and weight of the focusing group as well as suppression of the distance that focusing group is extended for focusing become possible, whereby focusing can be performed quickly. Therefore, images can be captured quickly. Furthermore, the suppression of the distance that focusing group is extended for focusing promotes the shortening of the overall length of the optical system, without deteriorations in optical performance.
FIG. 1 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a first embodiment. The zoom lens includes sequentially from an object side (object not depicted), a first lens group G11 having a positive refractive power, a second lens group G12 having a negative refractive power, a third lens group G13 having a positive refractive power, and a fourth lens group G14 having a positive refractive power. Further, the third lens group G13 includes, sequentially from the object side, a front group G13(F) having a positive refractive power and a rear group G13(R) having a negative refractive power. Both surfaces of the front group G13(F) are formed to be aspheric. Further, in the fourth lens group G14, an aperture stop ST that regulates a given aperture is disposed. At an imaging plane IMG, the light receiving surface of an imaging element, such as a CCD and a CMOS, is disposed.
In the zoom lens, the second lens group G12 and the third lens group G13 are moved integrally along the optical axis, from the object side to the imaging plane IMG side, to zoom from a wide angle edge to a telephoto edge. During this movement, the front group G13(F) and the rear group G13(R) are moved integrally along the optical axis. Furthermore, the first lens group G11 and the fourth lens group G14 do not move. The front group G13 (F) alone is moved in a direction along the optical axis to perform focusing.
Various values related to the zoom lens according to the first embodiment are indicated below.
Focal length of entire zoom lens (mm)=71.5336 (wide angle edge) to 111.5353 (intermediate position) to 194.0928 (telephoto edge)
Focal length of first lens group G11 (mm)=144.469
Focal length of second lens group G12 (mm)=−33.664
Focal length of third lens group G13 (mm)=90.870 (=F3)
Focal length of fourth lens group G14 (mm)=82.798
F number=2.9 (wide angle edge) to 2.9 (intermediate position) to 2.9 (telephoto edge)
Angle of view (2%)=34.66° (wide angle edge) to 20.71° (intermediate position) to 12.48° (telephoto edge)
Zoom ratio=2.713
(Values Related to Conditional Expression (1))
Deviation of paraxial radius of curvature at height of effective diameter of aspheric surface on object side of front group G13(F) and aspheric surface shape (ΔS1)=−0.0453
Deviation of paraxial radius of curvature at height of effective diameter of aspheric surface on image side of front group G13(F) and aspheric shape (ΔS2)=0.0662
Effective diameter of front group G13(F) ((φS)=36.9

|100×(ΔS1−ΔS2)/φS|=0.302

(Values Related to Conditional Expression (2))

Focal length of front group G13(F) of third lens group G13 

(F3F)(mm)=72.385

F3F/F3=0.7966

(Values Related to Conditional Expression (3))

|βFT|=31.838

(Where, βFT is lateral magnification of front group G13(F) of third lens group G13 in infinity focus state, at telephoto edge)

r1=222.8728

d1=2.0000 nd1=1.91082 υd1=35.25
r2=103.7846
d2=10.5000 nd2=1.45860 υd2=90.19
r3=−300.7820
d3=0.2000
r4=88.4625
d4=9.0000 nd3=1.49700 υd3=81.61
r5=−9494.4089
d5=1.3857 (wide angle edge) to 31.7756 (intermediate position) to 54.1401 (telephoto edge)
r6=1908.7598
d5=5.2000 nd4=1.90366 υd4=31.31
r7=−69.4016
d7=1.3500 nd5=1.61800 υd5=63.39
r8=69.4016
d8=3.8890
r9=−141.8243
d9=1.2000 nd6=1.49700 υd6=81.61
r10=52.9228
d10=3.3000 nd7=1.84666 υd7=23.78
r11=111.5902
dn=4.4169
r12=−60.5428
d12=1.2000 nd8=1.88300 υd8=40.80
r13=174.6335
d13=15.4455 (wide angle edge) to 11.6391 (intermediate position) to 1.6000 (telephoto edge)
r14=113.7835 (aspheric surface) (effective diameter φS=36.9)
d14=6.3000 nd9=1.58313 υd9=59.46
r15=−65.7348 (aspheric surface)
d15=13.7091
r16=−163.7249
d16=1.4000 nd10=1.92286 υd10=20.88
r17=−439.7863
d17=40.4088 (wide angle edge) to 13.8254 (intermediate position) to 1.5000 (telephoto edge)
r18=125.0060
d18=5.1184 nd11=1.61800 υd11=63.39
r19=−95.7349
d19=1.7000
r20=∞ (aperture stop)
d20=1.7000
r21=37.0745
d21=9.5000 nd12=1.49700 υd12=81.61
r22=−65.1203
d22=2.5000 nd13=1.71736 υd13=29.50
r23=105.7968
d23=14.2949
r24=88.9484
d24=2.3000 nd14=1.80809 υd14=22.76
r25=−39.7920
d25=1.4000 nd15=1.69350 υd15=53.20
r26=41.7941 (aspheric surface)
d26=10.5167
r27=80.4222
d27=7.5000 nd16=1.61800 υd16=63.39
r28=−44.5394
d28=11.5916
r29=−28.1673
d29=1.5000 nd17=1.71300 υd17=53.94
r30=−64.7347
d30=2.5887
r31=140.8610
d31=5.5000 nd18=1.56883 υd18=56.04
r32=−92.8595
d32=51.37
r33=∞ (imaging plane)
Constant of Cone (K) and Aspheric Coefficients (A, B, C, D, E, F)
(Fourteenth Plane)
K=−11.9954,
A=0, B=2.66677×10−7,
C=1.55243×10−9, D=−2.91159×10−12,
E=2.12840×10−16, F=3.53783×10−18 
(Fifteenth Plane)
K=−0.5492,
A=0, B=1.02602×10−8,
C=1.30471×10−9, D=−6.07503×10−13,
E=−3.7893×10−15, F=5.96719×10−18 
(Twenty-Sixth Plane)
K=−1.4774,
A=0, B=−1.17887×10−6,
C=−1.07533×10−9, D=−3.90498×10−12,
E=8.54852×10−35, F=0
FIG. 2 is a diagram of various types of aberration at the wide angle edge of the zoom lens of the first embodiment according to the invention; FIG. 3 is a diagram of various types of aberration at the intermediate position of the zoom lens of the first embodiment according to the invention; and FIG. 4 is a diagram of various types of aberration at the telephoto edge of the zoom lens of the first embodiment according to the invention. In the diagrams, g, d, and C represent wavelength aberration corresponding to the g-line (λ=435.83 nm), the d-line (λ=587.56 nm), and the c-line (λ=656.27 nm), respectively; and ΔS and ΔM in a portion depicting astigmatism, indicate aberration with respect to a sagittal image plane and a meridional image plane, respectively.
FIG. 5 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a second embodiment. The zoom lens includes, sequentially from an object side (object not depicted), a first lens group G21 having a positive refractive power, a second lens group G22 having a negative refractive power, a third lens group G23 having a positive refractive power, and a fourth lens group G24 having a positive refractive power. Further, the third lens group G23 includes, sequentially from the object side, front group G23(F) having a positive refractive power and rear group G23(R) having a negative refractive power. Both surfaces of the front group G23(F) are formed to be aspheric. Further, in the fourth lens group G24, the aperture stop ST that regulates a given aperture is disposed. At the imaging plane IMG, the light receiving surface of an imaging element, such as a CCD and a CMOS, is disposed.
In the zoom lens, the second lens group G22 and the third lens group G23 are moved integrally along the optical axis, from the object side to the imaging plane IMG side, to zoom from a wide angle edge to a telephoto edge. During this movement, the front group G23(F) and the rear group G23(R) are moved integrally along the optical axis. Furthermore, the first lens group G21 and the fourth lens group G24 do not move. The front group G23(F) alone is moved in a direction along the optical axis to perform focusing.
Various values related to the zoom lens according to the second embodiment are indicated below.
Focal length of entire zoom lens (mm)=71.5209 (wide angle edge) to 117.5105 (intermediate position) to 194.0681 (telephoto edge)
Focal length of first lens group G21 (mm)=144.087
Focal length of second lens group G22 (mm)=−33.304
Focal length of third lens group G23 (mm)=90.526(=F3)
Focal length of fourth lens group G24 (mm)=82.744
F number=2.9 (wide angle edge) to 2.9 (intermediate position) to 2.9 (telephoto edge)
Angle of view (2ω)=34.66° (wide angle edge) to 20.71° (intermediate position) to 12.48° (telephoto edge)
Zoom ratio=2.713
(Values Related to Conditional Expression (1))
Deviation of paraxial radius of curvature at height of effective diameter of aspheric surface on object side of front group G23(F) and aspheric surface shape (ΔS1)=−0.069
Deviation of paraxial radius of curvature at height of effective diameter of aspheric surface on image side of front group G23(F) and aspheric shape (ΔS2)=0.0722
Effective diameter of front group G23(F) ((φS)=37.0

|100×(ΔS1−ΔS2)/(φS|=0.3827

(Values Related to Conditional Expression (2))

Focal length of front group G23(F) of third lens group G23 

(F3F)(mm)=74.084

F3F/F3=0.8184

(Values Related to Conditional Expression (3))

|βFT|=15.232

(Where, βFT is lateral magnification of front group G23(F) of third lens group G23 in infinity focus state, at telephoto edge)

r1=222.2214

d1=2.0000 nd1=1.91082 υd1=35.25
r2=103.4753
d2=10.5000 nd2=1.45860 υd2=90.19
r3=−298.5174
d3=0.2000
r4=88.2603
d4=8.8000 nd3=1.49700 υd3=81.61
r5=−10563.5431
d5=1.2000 (wide angle edge) to 31.5776 (intermediate position) to 53.9008 (telephoto edge)
r6=1009.5052
d6=5.2000 nd4=1.90366 υd4=31.31
r7=−70.2509
d7=1.3500 nd5=1.61800 υd5=63.39
r8=70.2509
d8=3.7528
r9=−156.3665
d9=1.2000 nd6=1.49700 υd6=81.61
r10=49.6801
d10=3.3000 nd7=1.84666 υd7=23.78
r11=95.2516
d11=4.6178
r12=−58.7772
d12=1.2000 nd8=1.88300 υd8=40.80
r13=177.1970
d13=15.1042 (wide angle edge) to 11.3976 (intermediate position) to 1.6000 (telephoto edge)
r14=102.9577 (aspheric surface) (effective diameter φS=37.0)
d14=6.7500 nd9=1.51633 υd9=64.06
r15=−59.5060 (aspheric surface)
d15=13.4571
r16=−137.5555
d16=1.4000 nd10=1.92286 υd10=20.88
r17=−252.3951
d17=40.6970 (wide angle edge) to 14.0261 (intermediate position) to 1.5000 (telephoto edge)
r18=113.0406
d18=5.2838 nd11=1.61800 υd11=63.39
r19=−98.7983
d19=1.7000
r20=∞ (aperture stop)
d20=1.7000
r21=36.3568
d21=9.5000 nd12=1.49700 υd12=81.61
r22=−67.3140
d22=3.0000 nd13=1.71736 υd13=29.50
r23=96.8404
d23=13.5679
r24=−91.3000
d24=2.3000 nd14=1.80809 υd14=22.76
r25=−39.7728
d25=1.4300 nd15=1.69350 υd15=53.20
r26=41.0718 (aspheric surface)
d26=10.0244
r27=75.9182
d27=8.0000 nd16=1.61800 υd16=63.39
r28=−45.6371
d23=12.1798
r29=−27.5459
d29=1.5000 nd17=1.71300 υd17=53.94
r30=−65.7082
d30=3.8260
r31=127.8003
d31=5.5000 nd18=1.56883 υd18=56.04
r32=−96.1226
d32=48.305
r33=∞ (imaging plane)
Constant of Cone (K) and Aspheric Coefficients (A, B, C, D, E, F)
(Fourteenth Plane)
K=−5.6100,
A=0, B=−6.34506×10−9,
C=−2.39141×10−9, D=1.90231×10−11,
E=−4.99934×10−14, F=4.49558×10−17 
(Fifteenth Plane)
K=−0.6649,
A=0, B=2.19580×10−7,
C=−3.02130×10−9, D=2.14493×10−11,
E=−5.34576×10−14, F=4.66441×10−17 
(Twenty-Sixth Plane)
K=−1.3886,
A=0, B=−1.07914×10−6,
C=−2.70333×10−9, D=4.27338×10−12,
E=−7.70848×10−15, F=0
FIG. 6 is a diagram of various types of aberration at the wide angle edge of the zoom lens of the second embodiment according to the invention; FIG. 7 is a diagram of various types of aberration at the intermediate position of the zoom lens of the second embodiment according to the invention; and FIG. 8 is a diagram of various types of aberration at the telephoto edge of the zoom lens of the second embodiment according to the invention. In the diagrams, g, d, and C represent wavelength aberration corresponding to the g-line (λ=435.83 nm), the d-line (λ=587.56 nm), and the c-line (λ=656.27 nm), respectively; and ΔS and ΔM in a portion depicting astigmatism, indicate aberration with respect to a sagittal image plane and a meridional image plane, respectively.
Among the values for each of the embodiments above, r2, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d1, d2, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd1, nd2, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd1, υd2, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens.
Each of the aspheric surfaces above can be expressed by equation [1], where Z is aspheric surface depth, y is the height from the optical axis, and the travel direction of light is positive.




  
    
      
        Z
        =
        
          
            
              y
              2
            
            
              
                R
                ⁡
                
                  (
                  
                    1
                    +
                    
                      
                        1
                        -
                        
                          
                            (
                            
                              1
                              +
                              K
                            
                            )
                          
                          ⁢
                          
                            y
                            /
                            
                              R
                              
                                2
                                ⁢
                                
                                  
                                
                              
                            
                          
                        
                      
                    
                  
                  )
                
              
              2
            
          
          +
          
            Ay
            2
          
          +
          
            By
            4
          
          +
          
            Cy
            6
          
          +
          
            Dy
            8
          
          +
          
            Ey
            10
          
          +
          
            Fy
            12
          
        
      
    
    
      
        [
        1
        ]
      
    
  




Where, R is the paraxial radius of curvature; K is the constant of the cone; and A, B, C, D, E, F are the second, fourth, sixth, eighth, tenth, and twelfth aspheric coefficients, respectively.
In each of the embodiments, the invention is applied and examples configuring a large diameter, internal focusing telephoto zoom lens having at the wide angle edge, a focal length of 72 mm or less, a zoom ratio of 2.7× or more and a F number of 3 or less. As shown in these embodiments, by satisfying each of the conditions above, reductions in the size and weight of the focusing group and suppression of the distance that the focusing group is extended for focusing become possible, enabling focusing to be performed quickly. As a result, a zoom lens capable of quickly capturing images can be provided. Furthermore, suppression of the distance that the focusing group is extended for focusing promotes reductions in the overall length of the optical system and there is no deterioration of optical performance. Moreover, the zoom lens of the embodiments employs a lens having a suitable aspheric surface, whereby favorable optical performance can be maintained with fewer lenses.
According to the present invention, both the suppression of the distance that the focusing group is extended to perform focusing (displacement) and favorable correction of various types of aberration can be achieved. As a result, a compact, high performance zoom lens can be provided.
Further, the present invention effects a zoom lens that has high optical performance and is capable of quickly capturing images by achieving a compact, light weight focusing group and suppressing the distance that the focusing group is moved for focusing.
As described, the zoom lens of the present invention is useful in imaging apparatuses such as DSMCs and single-less reflex cameras, and is particularly ideal when a quick capturing of images is demanded.
Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
The present document incorporates by reference the entire contents of Japanese priority document, 2010-203702 filed in Japan on Sep. 10, 2010.

## Claims

1. A zoom lens comprising sequentially from an object side:
a first lens group having a positive refractive power;
a second lens group having negative refractive power;
a third lens group having a positive refractive power; and
a fourth lens group having a positive refractive power,
wherein the third lens group includes, sequentially from the object side, a front group having a positive refractive power and a rear group having a negative refractive power,
wherein zoom is performed by moving the second lens group and the third lens group in a direction along an optical axis, and by integrally moving the front group and the rear group in a direction along the optical axis, and
focusing is performed by moving the front group in a direction along the optical axis,
wherein the front group of the third lens group is configured by only one positive lens.

2. The zoom lens according to claim 1, wherein the front group of the third lens group is configured by an aspheric lens that satisfies conditional expression 0.1<|100×(ΔS1−ΔS2)/φS|<0.5, where ΔS1 represents deviation of a paraxial radius of curvature at a height of an effective diameter of an aspheric surface on the object side of the front group and aspheric surface shape, ΔS2 represents deviation of the paraxial radius of curvature at a height of an effective diameter of an aspheric surface on an image side of the front group and aspheric shape, and φS represents the effective diameter of the front group.

3. The zoom lens according to claim 1, wherein conditional expression 0.5<F3F/F3<0.95 is satisfied, where F3F is a focal length of the front group of the third lens group and F3 is a focal length of the entire third lens group in an infinity focus state.

4. The zoom lens according to claim 1, wherein conditional expression 5<|βFT| is satisfied, where βFT is lateral magnification of the front group of the third lens group in an infinity focus state at a telephoto edge.

5. A zoom lens comprising sequentially from an object side:
a first lens group having a positive refractive power;
a second lens group having negative refractive power;
a third lens group having a positive refractive power; and
a fourth lens group having a positive refractive power,
wherein the third lens group includes, sequentially from the object side, a front group having a positive refractive power and a rear group having a negative refractive power,
wherein zoom is performed by moving the second lens group and the third lens group in a direction along an optical axis, and by integrally moving the front group and the rear group in a direction along the optical axis, and
focusing is performed by moving the front group in a direction along the optical axis,
wherein conditional expression 5<|βFT| is satisfied, where βFT is lateral magnification of the front group of the third lens group in an infinity focus state at a telephoto edge.

6. The zoom lens according to claim 5, wherein the front group of the third lens group is configured by one positive lens.

7. The zoom lens according to claim 5, wherein the front group of the third lens group is configured by an aspheric lens that satisfies conditional expression 0.1<|100×(ΔS1−ΔS2)/φS|<0.5, where ΔS1 represents deviation of a paraxial radius of curvature at a height of an effective diameter of an aspheric surface on the object side of the front group and aspheric surface shape, ΔS2 represents deviation of the paraxial radius of curvature at a height of an effective diameter of an aspheric surface on an image side of the front group and aspheric shape, and φS represents the effective diameter of the front group.

8. The zoom lens according to claim 5, wherein conditional expression 0.5<F3F/F3<0.95 is satisfied, where F3F is a focal length of the front group of the third lens group and F3 is a focal length of the entire third lens group in an infinity focus state.

9. A zoom lens comprising sequentially from an object side:
a first lens group having a positive refractive power;
a second lens group having negative refractive power;
a third lens group having a positive refractive power; and
a fourth lens group having a positive refractive power,
wherein the third lens group includes, sequentially from the object side, a front group having a positive refractive power and a rear group having a negative refractive power,
wherein zoom is performed by moving the second lens group and the third lens group in a direction along an optical axis, and by integrally moving the front group and the rear group in a direction along the optical axis, and
focusing is performed by moving the front group in a direction along the optical axis,
wherein the first lens group and the fourth lens group are fixed.

10. The zoom lens according to claim 9, wherein the front group of the third lens group is configured by only one positive lens.

11. The zoom lens according to claim 9, wherein the front group of the third lens group is configured by an aspheric lens that satisfies conditional expression 0.1<|100×(ΔS1−ΔS2)/φS|<0.5, where ΔS1 represents deviation of a paraxial radius of curvature at a height of an effective diameter of an aspheric surface on the object side of the front group and aspheric surface shape, ΔS2 represents deviation of the paraxial radius of curvature at a height of an effective diameter of an aspheric surface on an image side of the front group and aspheric shape, and φS represents the effective diameter of the front group.

12. The zoom lens according to claim 9, wherein conditional expression 0.5<F3F/F3<0.95 is satisfied, where F3F is a focal length of the front group of the third lens group and F3 is a focal length of the entire third lens group in an infinity focus state.

13. The zoom lens according to claim 9, wherein conditional expression 5<|βFT| is satisfied, where βFT is lateral magnification of the front group of the third lens group in an infinity focus state at a telephoto edge.