# Patent

## Title
Zoom lens

## Abstract

The invention relates to a zoom lens that can achieve higher performance easily. In order from the object side of the zoom lens, it comprises a positive first lens group G1, a negative second lens group G2, a positive third lens group G3 and a positive fourth lens group G4, wherein zooming is implemented by a change in the space between the respective lens groups. Upon zooming from the wide-angle end to the telephoto end, while the space between the first lens group G1 and the second lens group G2 grows wide, the space between the second lens group G2 and the third lens group G3 becomes narrow and the space between the third lens group G3 and the fourth lens group G4 becomes narrow, the first, the third, and the fourth lens group moves toward the object side only, and the second lens group G2 moves in a convex orbit toward the image plane side. The second lens group G2, and the fourth lens group G4 has at least one aspheric surface, and the zoom lens satisfies the following conditions: 0.1<|f2/f1|<0.14 and 0.1<|f4/f3|<0.6 where fi is the focal length of an i-th lens group.

## Background
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens, and more particularly to a zoom lens that covers a mid-to-full telephoto area used for interchangeable lenses for single-lens reflex cameras, etc., and has a relatively high zoom ratio.
Among zoom lenses that cover a focal length region from the wide-angle area to the mid-to-full telephoto area used for interchangeable lenses for single-lens reflex cameras or the like and have a zoom ratio of 3 or more, there is already a zoom lens comprising, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power, as known from Patent Publications 1, 2 and 3 or the like. Possible reasons for such arrangement are that higher zoom rations are easily achievable; the positive group in front permits the total length of the zoom lens to be curtailed; and so on.
Patent Publication 1
JP(A)11-142740
Patent Publication 2
JP(A)2000-321497
Patent Publication 3
JP(A)2001-208970
With the spread of digital single-lens reflex cameras, on the other hand, there is now a growing demand for an interchangeable lens optimized to cameras that comprise an imaging device having an image circle smaller than that of silver halide 35-mm film or the like. The interchangeable lens for such digital cameras should have a resolving power higher than demanded for interchangeable lenses for conventional silver halide 35-mm film and a more limited tolerance for chromatic aberrations. Moreover, the proportion of the back focus relative to the focal length of the whole system must be large: generally, there must be a lot more of retrofocus power profile.
As for zoom lens specifications, there are mounting demands for higher zoom ratios, larger aperture ratios, wider-angle arrangement, reductions in the closest object distance, etc., and with them, a sensible tradeoff between size reductions and cost reductions is in great need as well.
The situation being like such, the invention has for its object to the provision of a zoom lens that can easily achieve higher performance than ever before.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is a zoom lens provided, which comprises, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power, wherein zooming is implemented by changing the space between the respective lens groups, characterized in that:
upon zooming from the wide-angle end to the telephoto end, while the space between the first lens group and the second lens group grows wide, the space between the second lens group and the third lens group becomes narrow and the space between the third lens group and the fourth lens group becomes narrow, the first, the third, and the fourth lens group moves toward the object side only, the second lens group moves in a convex orbit toward the image plane side, and the second lens group, and the fourth lens group has at least one aspheric surface, with the satisfaction of the following conditions:

0.1<|f2/f1|<0.14  (1)

0.1<|f4/f3|<0.6  (2)

where fi is the focal length of an i-th lens group.

The advantages of, and the requirements for, such arrangement are now explained.
For a zoom lens for digital cameras wherein the proportion of the back focus needed relative to the focal length remains large as already noted, it is required that the power profile for the so-called retrofocus be tighter. At the same time, to offer a sensible tradeoff between having high zoom ratios and wider-angle arrangement at the wide-angle end and demands for size reductions, etc., it is necessary to increase the power of each lens group. However, there are large amounts of aberrations occurring, too. For instance, when there is a wide-angle zone included in the focal length range, it is difficult to correct distortion and astigmatism in particular. The aspheric surface in the second lens group is primarily effective for the correction of distortion and astigmatism at the wide-angle end, and the aspheric surface in the fourth lens group is effective for the correction of astigmatism and coma all over the zooming zone.
Condition (1) defines the ratio of the power of the first lens group to the power of the second lens group, and as the power of the first lens group grows stronger than the upper limit of 0.14 to condition (1), it renders it difficult to make sure the necessary back focus. It also causes the distance of an entrance pupil from an entrance surface in the first lens group to grow long, resulting in much difficulty in balancing the wide-angle arrangement at the wide-angle end against the size reductions of the front lens. As the power of the first lens group becomes less than the lower limit of 0.1 to condition (1), it causes the zoom ratio of the second lens group to become low, rendering it difficult to make sure the zoom ratio of the whole system and curtail the total length of the lens arrangement.
Condition (2) defines the ratio of the power of the third lens group to the power of the fourth lens, and as the power of the third lens group grows stronger than the upper limit of 0.6 to condition (2), it renders it difficult to make sure the necessary back focus. As the power of the third lens group becomes smaller than the lower limit of 0.1 to condition (2), it is not preferable because astigmatism and coma occur in too large amounts at the fourth lens group, and there are large astigmatism fluctuations during zooming as well.
According to another aspect of the invention, there is a zoom lens provided, which comprises, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power, wherein zooming is implemented by changing the space between the respective lens groups, characterized in that:
upon zooming from the wide-angle end to the telephoto end, while the space between the first lens group and the second lens group grows wide, the space between the second lens group and the third lens group becomes narrow and the space between the third lens group and the fourth lens group becomes narrow, the first, the third, and the fourth lens group moves toward the object side only, the second lens group moves in a convex orbit toward the image plane side; the second lens group, and the fourth lens group has at least one aspheric surface; and at least one negative lens in the second lens group, and at least one positive lens in the fourth lens group is made of a material that satisfies the following conditions:

nd>1.4  (3)

νd>80  (4)

where nd is a d-line refractive index, and

νd is an Abbe constant.
The advantages of, and the requirements for, such arrangement are now explained.
As described above, there is a mounting demand for having a wide-angle arrangement at the zoom wide-angle end; however, that wide-angle arrangement renders the correction of chromatic aberration of magnification difficult. What is particularly difficult to correct is the so-called secondary spectrum that, upon achromatization at two certain wavelengths, remains at other wavelengths. It is well known that much difficulty is encountered in the correction of this residual chromatic aberration by use of a combination of vitreous materials having ordinary partial dispersion ratios, and that correction in particular gets more difficult by reason of an increase in the power of each lens group incidental with size reductions and the wide-angle arrangement, as described above. The vitreous material that satisfies conditions (3) and (4) simultaneously has low dispersion and anomalous dispersion; if the negative lens in the second lens group, and the positive lens in the fourth lens group is made of such a vitreous material, it is then possible to correct the secondary spectrum of the chromatic aberration of magnification produced at the wide-angle end while minimizing chromatic aberrations occurring at each lens group.
According to yet another aspect of the invention, there is a zoom lens provided, which comprises, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power, wherein zooming is implemented by changing the space between the respective lens groups, characterized in that:
the second lens group comprises a front unit having negative refracting power and a rear unit having negative refracting power, and focusing on a nearby subject is implemented by allowing the second lens group to move in an optical axis direction with a change in the space between said front unit and said rear unit.
The advantages of, and the requirements for, such arrangement are now explained.
As described above, reductions in the closest object distance are desired for the specifications of recently developed zoom lenses. Generally for faster focusing, more reduced lens diameters, further reductions in the closest object distance, etc., the so-called inner focusing mode involving the movement of lens groups within a lens system is often used. A zoom lens comprising a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power as contemplated herein relies on a focusing mode involving the movement of the second lens group in the optical axis direction. In this case, however, not only is there a large fluctuation of astigmatism, etc. in association with focusing on a nearby distance, which may otherwise render near distance performance worse, but also there is a certain limit on the reductions in the closest object distance.
In the invention, the second lens group is built up of a front unit having negative refracting power and a rear unit having negative refracting power, and focusing is implemented by moving the whole second lens group in the optical axis direction while there is a change in the space between the front unit and the rear unit, thereby minimizing fluctuations of astigmatism in association with focusing on a nearby subject.
Such arrangement as contemplated herein is more effective for a zoom lens having an angle of view of 80° or greater and a zoom ratio of 4 or higher.
If there is a negative lens having at least one aspheric surface on the object side nearest to the second lens group and there is a negative lens having at least one aspheric surface nearest to the image plane side, it is desirous because of further reductions in fluctuations of distortion and astigmatism at the wide-angle end, and fluctuations of astigmatism, etc. in association with focusing on a nearby subject.
Preferably, the first lens group comprises, in order from its object side, a cemented positive lens of negative and positive lens elements, and a positive meniscus lens convex on the object side.
It is then easy to correct chromatic aberrations, off-axis aberrations on the wide-angle side, and spherical aberrations on the telephoto side while making sure the first lens group has positive refracting power.
Preferably, the second lens group comprises a double-convex positive lens, a plurality of negative lenses located more on the object side than that double-convex positive lens, and a negative lens located more on the image side than that double-convex positive lens.
It is then easy to correct chromatic aberrations, off-axis aberrations on the wide-angle side, and spherical aberrations while making sure the second lens group has negative refracting power.
More preferably, that double-convex positive lens should be cemented to the object-side negative lens, and that negative lens should be a double-concave negative lens.
This works more favorably for the correction of chromatic aberrations, and facilitates holding back aberration fluctuations due to the decentration of the double-concave negative lens and double-convex positive lens in the second lens group.
In view of the correction of aberrations, that double-concave negative lens in particular satisfies the aforesaid conditions (3) and (4).
Preferably, the third lens group comprises a positive lens and a negative lens.
It is then easy to hold back the occurrence of chromatic aberrations at the third lens group.
Preferably, the fourth lens group comprises a double-convex positive lens, an object-side positive lens located on the object side of that double-convex positive lens, and a cemented positive lens located on the image side of that double-convex positive lens.
It is then easy to make strong the refracting power of the fourth lens group, and that works for holding back the occurrence of spherical aberrations, coma and chromatic aberrations.
More preferably, that cemented positive lens should comprise, in order from its object side, a negative lens and a double-convex positive lens.
This works favorably for reducing chromatic aberrations at the fourth lens group and aberration fluctuations due to decentration.
More preferably for the correction of aberrations, the double-convex position lens in the cemented positive lens should satisfy the aforesaid conditions (3) and (4).
More preferably for the correction of aberrations, the object-side positive lens in the fourth lens group should be configured in the form of a double-convex positive lens that satisfies the aforesaid conditions (3) and (4).
Of course, satisfying some or all of the aforesaid arrangements of the invention simultaneously achieves a zoom lens that works more favorably for ever higher performance.
More preferably, conditions (1) and (2) should be narrowed down:
the lower limit to condition (1) should be set at 0.11;
the upper limit to condition (1) should be set at 0.135;
the lower limit to condition (2) should be set at 0.13; and
the upper limit to condition (2) should be set at 0.5.
More preferably, conditions (3) and (4) should be narrowed down:

1.7>nd>1.4  (3)′

120>νd>80  (4)′

Exceeding the upper limits to the respective conditions is preferable for the correction of aberrations; however, the materials are difficult to fabricate, and less available as well.
According to the invention, it is possible to obtain a zoom lens suitable for interchangeable lenses, etc. for single-lens reflex cameras that have high performances: longer back focuses, high zoom rations and large aperture ratios, and reductions in the closest object distance.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is illustrative in lens arrangement section of Example 1 of the zoom lens according to the invention at the wide-angle end (a), in an intermediate state (b), and at the telephoto end (c), respectively.
FIG. 2 is illustrative, as in FIG. 1, of Example 2 of the zoom lens according to the invention.
FIG. 3 is illustrative, as in FIG. 1, of Example 3 of the zoom lens according to the invention.
FIG. 4 is illustrative, as in FIG. 1, of Example 4 of the zoom lens according to the invention.
FIG. 5 is an aberration diagram for Example 1 upon focusing on an object point at infinity.
FIG. 6 is an aberration diagram for Example 1 upon focusing on a subject distance of 25 cm.
FIG. 7 is an aberration diagram for Example 2 upon focusing on an object point at infinity.
FIG. 8 is an aberration diagram for Example 2 upon focusing on a subject distance of 25 cm.
FIG. 9 is an aberration diagram for Example 3 upon focusing on an object point at infinity.
FIG. 10 is an aberration diagram for Example 3 upon focusing on a subject distance of 25 cm.
FIG. 11 is an aberration diagram for Example 4 upon focusing on an object point at infinity.
FIG. 12 is an aberration diagram for Example 4 upon focusing on a subject distance of 25 cm.
FIG. 13 is a sectional view of a single-lens reflex camera with the zoom lens of the invention used as an interchangeable lens.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Examples 1, 2, 3 and 4 of the inventive zoom lens are given below. FIGS. 1, 2, 3 and 4 are illustrative in lens arrangement section of Examples 1, 2, 3 and 4 at their wide-angle ends (a), in their intermediate states (b) and at their telephoto ends (c), respectively, upon focusing on an object point at infinity. In these figures, G1 stands for the first lens group, G2 the second lens group, S an aperture stop, G3 the third lens group, G4 the fourth lens group, C a plane-parallel plate for the cover glass, etc. of an electronic imaging device, and I an image plane.
EXAMPLE 1
As shown in FIG. 1, this example is directed to a zoom lens made up of, in order from its object side, the first lens group G1 having positive refracting power, the second lens group G2 having negative refracting power, the aperture stop S, the third lens group G3 having positive refracting power, and the fourth lens group G4 having positive refracting power. Upon zooming from the wide-angle end to the telephoto end of the zoom lens, the first lens group G1 moves toward the object side, and the second lens group G2 moves in a convex locus toward the image plane side and is positioned a bit more on the object side at the telephoto end than at the wide-angle end. The aperture stop S moves toward the object side while the space between it and the second lens group G2 becomes narrow, the third lens group G3 moves toward the object side while the space between it and the aperture stop S grows first wide and then narrow and the space between it and the second lens group G2 becomes narrow, and the fourth lens group G4 moves toward the object side while the space between it and the third lens group G3 gets narrow.
In order from the object side, the first lens group G1 is made up of a cemented lens of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and a positive meniscus lens convex on its object side, and the second lens group G2 is made up of two negative meniscus lenses, each convex on its object side, a cemented lens of a double-concave negative lens and a double-convex positive lens, and a negative meniscus lens convex on its image plane side, wherein the first mentioned negative meniscus lens and the cemented lens constitute together the front unit of the second lens group G2 while the one negative meniscus lens located nearest to the image plane side forms the rear unit of the second lens group G2. The third lens group G3 is made up of a double-convex positive lens and a negative meniscus lens convex on its image plane side, and the fourth lens group G4 is made up of a cemented lens of a double-convex positive lens and a negative meniscus lens convex on its image plane side, a double-convex positive lens, a double-concave negative lens, and a cemented lens of a negative meniscus lens convex on its object side and a double-convex positive lens.
Six aspheric surfaces are used: two at both surfaces of the second negative meniscus in the second lens group G2 as counted from its object side, two at both surfaces of the negative meniscus lens located in the second lens group G2 and nearest to its image plane side, and two at both surface of the single double-convex positive lens in the fourth lens group G4.
Focusing on a nearby subject (object point) is implemented by moving the whole second lens group G2 toward the object side while the space between the front unit and the rear unit of the second lens group G2 gets narrow.
EXAMPLE 2
As shown in FIG. 2, this example is directed to a zoom lens made up of, in order from its object side, the first lens group G1 having positive refracting power, the second lens group G2 having negative refracting power, the aperture stop S, the third lens group G3 having positive refracting power, and the fourth lens group G4 having positive refracting power. Upon zooming from the wide-angle end to the telephoto end of the zoom lens, the first lens group G1 moves toward the object side, and the second lens group G2 moves in a convex locus toward the image plane side and is positioned a bit more on the object side at the telephoto end than at the wide-angle end. The aperture stop S moves toward the object side while the space between it and the second lens group G2 becomes narrow, the third lens group G3 moves toward the object side while the space between it and the aperture stop S grows first wide and then narrow and the space between it and the second lens group G2 gets narrow, and the fourth lens group G4 moves toward the object side while the space between it and the third lens group G3 gets narrow.
In order from the object side, the first lens group G1 is made up of a cemented lens of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and a positive meniscus lens convex on its object side, and the second lens group G2 is made up of two negative meniscus lenses, each convex on its object side, a cemented lens of a double-concave negative lens and a double-convex positive lens, and a negative meniscus lens convex on its image plane side, wherein the first mentioned negative meniscus lens and the cemented lens constitute together the front unit of the second lens group G2 while the one negative meniscus lens located nearest to the image plane side forms the rear unit of the second lens group G2. The third lens group G3 is made up of a double-convex positive lens and a cemented lens of a double-convex positive lens and a double-concave negative lens, and the fourth lens group G4 is made up of a cemented lens of a double-convex positive lens and a negative meniscus lens convex on its image plane side, a double-convex positive lens, and a cemented lens of a double-concave negative lens and a double-convex positive lens.
Five aspheric surfaces are used: on at the object-side surface of the negative meniscus lens located in, and nearest to the object side of, the second lens group G2, two at both surfaces of the negative meniscus lens located in, and nearest to the image plane side of, the second lens group G2, and two at both surfaces of the single double-convex positive lens in the fourth lens group G4.
Focusing on a nearby subject (object point) is implemented by moving the whole second lens group G2 toward the object side while the space between the front unit and the rear unit of the second lens group G2 gets narrow.
EXAMPLE 3
As shown in FIG. 3, this example is directed to a zoom lens made up of, in order from its object side, the first lens group G1 having positive refracting power, the second lens group G2 having negative refracting power, the aperture stop S, the third lens group G3 having positive refracting power, and the fourth lens group G4 having positive refracting power. Upon zooming from the wide-angle end to the telephoto end of the zoom lens, the first lens group G1 moves toward the object side, and the second lens group G2 moves in a convex locus toward the image plane side and is positioned a bit more on the object side at the telephoto end than at the wide-angle end. The aperture stop S moves toward the object side while the space between it and the second lens group G2 becomes narrow, the third lens group G3 moves toward the object side while the space between it and the aperture stop S grows first wide and then narrow and the space between it and the second lens group G2 gets narrow, and the fourth lens group G4 moves toward the object side while the space between it and the third lens group G3 gets narrow.
In order from the object side, the first lens group G1 is made up of a cemented lens of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and a positive meniscus lens convex on its object side, and the second lens group G2 is made up of a cemented lens of a thin negative meniscus lens convex on its object side and a negative meniscus lens convex on its object side, a negative meniscus lens convex on its image plane side, a double-convex positive lens, and a negative meniscus lens convex on its image plane side, wherein the cemented lens, the first mentioned negative meniscus lens and the double-convex positive lens constitute together the front unit of the second lens group G2 and the one negative meniscus lens nearest to the image plane side forms the rear unit of the second lens group G2. The third lens group G3 is made up of a double-convex positive lens, and a cemented lens of a double-convex positive lens and a double-concave negative lens, and the fourth lens group G4 is made up of a cemented lens of a double-convex positive lens and a negative meniscus lens convex on its image plane side, a double-convex positive lens, and a cemented lens of a double-concave negative lens and a double-convex positive lens.
Five aspheric surfaces are used: one at the surface of the cemented lens in the second lens group G2, two at both surfaces of the negative meniscus lens located in, and nearest to the image plane side of, the second lens group G2, and two both surfaces of the single double-convex positive lens in the fourth lens group G4.
Focusing on a nearby subject (object point) is implemented by moving the whole second lens group G2 toward the object side while the space between the front unit and the rear unit of the second lens group G2 gets narrow.
EXAMPLE 4
As shown in FIG. 4, this example is directed to a zoom lens made up of, in order from its object side, the first lens group G1 having positive refracting power, the second lens group G2 having negative refracting power, the aperture stop S, the third lens group G3 having positive refracting power, and the fourth lens group G4 having positive refracting power. Upon zooming from the wide-angle end to the telephoto end of the zoom lens, the first lens group G1 moves toward the object side, and the second lens group G2 moves in a convex locus toward the image plane side and is positioned a bit more on the object side at the telephoto end than at the wide-angle end. The aperture stop S moves toward the object side while the space between it and the second lens group G2 becomes narrow, the third lens group G3 moves together with the aperture stop S toward the object side, and the fourth lens group G4 moves toward the object side while the space between it and the third lens group G3 gets narrow.
In order from the object side, the first lens group G1 is made up of a cemented lens of a negative meniscus lens convex on its object side and a plano-convex positive lens convex on its object side, and a positive meniscus lens convex on its object side, and the second lens group G2 is made up of a thin negative meniscus lens convex on its object side and a negative meniscus lens convex on its object side, a cemented lens of a double-concave negative lens and a double-convex positive lens, and a negative meniscus lens convex on its image plane side, wherein the two cemented lenses constitute together the front unit of the second lens group G2 having negative refracting power and the one negative meniscus lens nearest to the image plane side forms the rear unit of the second lens group G2. The third lens group G3 is made up of a double-convex positive lens and a cemented lens of a double-convex positive lens and a double-concave negative lens, and the fourth lens group G4 is made up of two double-convex positive lenses, and a cemented lens of a double-concave negative lens and a double-convex positive lens.
Five aspheric surfaces are used: one at the surface nearest to the object side of the object-side cemented lens in the second lens group G2, two at both surfaces of the negative meniscus lens located in, and nearest to the image plane side of, the second lens group G2, and two both surfaces of the single double-convex positive lens in the fourth lens group G4.
Focusing on a nearby subject (object point) is implemented by moving the whole second lens group G2 toward the object side while the space between the front unit and the rear unit of the second lens group G2 gets narrow.
Set out below are the numerical data on each example. The symbols used hereinafter but not hereinbefore are indicative of:
f: the focal length of the whole system,
FNO: an F-number,
WE: the wide-angle end,
ST: an intermediate state,
TE: the telephoto end,
r1, r2, . . . : the radius of curvature of each lens surface,
d1, d2, . . . : the space between the respective lens surfaces,
nd1, nd2, . . . : the d-line refractive index of each lens,
νd1, νd2, . . . : the Abbe constant of each lens, and
OD is a subject distance as measured from the image plane. Suppose here that x is an optical axis with the direction of travel of light taken as positive and y is a direction orthogonal to the optical axis. Then, aspheric configuration is given by

x=(y2/r)/[1+{1−(K+1)(y/r)2}1/2]+A4y4+A6y6+A8y8+A10y10+A12y12 

where r is a paraxial radius of curvature, and A4, A6, A8, A10 and A12 are the 4th, 6th, 8th, 10th, and 12th-order aspheric coefficients, respectively.

EXAMPLE 1















r1 = 109.4949
d1 = 2.5500
nd1 = 1.84666
νd1 = 23.78


r2 = 65.6064
d2 = 6.5166
nd2 = 1.56384
νd2 = 60.67


r3 = 250.3611
d3 = 0.2000


r4 = 56.3041
d4 = 5.2365
nd3 = 1.77250
νd3 = 49.60


r5 = 129.4446
d5 = (Variable)


r6 = 56.6646
d6 = 2.0000
nd4 = 1.88300
νd4 = 40.76


r7 = 15.7727
d7 = 2.9353


r8 = 59.6791
d8 = 1.9684
nd5 = 1.80610
νd5 = 40.88


(Aspheric)


r9 = 20.8286
d9 = 6.1861


(Aspheric)


r10 = −31.0694
d10 = 1.1413
nd6 = 1.43875
νd6 = 94.93


r11 = 15.7046
d11 = 6.7088
nd7 = 1.61293
νd7 = 37.00


r12 = −29.7268
d12 = (Variable)


r13 = −24.5029
d13 = 1.2783
nd8 = 1.80610
νd8 = 40.88


(Aspheric)


r14 = −92.4430
d14 = (Variable)


(Aspheric)


r15 = ∞ (Stop)
d15 = (Variable)


r16 = 67.0118
d16 = 2.4442
nd9 = 1.76182
νd9 = 26.52


r17 = −69.5017
d17 = 0.9379


r18 = −27.8030
d18 = 1.1994
nd10 = 1.75500
νd10 = 52.32


r19 = −81.0550
d19 = (Variable)


r20 = 35.1459
d20 = 6.7687
nd11 = 1.43875
νd11 = 94.93


r21 = −17.3775
d21 = 1.2000
nd12 = 1.88300
νd12 = 40.76


r22 = −30.3574
d22 = 0.2000


r23 = 26.3652
d23 = 7.7834
nd13 = 1.49700
νd13 = 81.54


(Aspheric)


r24 = −24.7855
d24 = 0.1500


(Aspheric)


r25 = −96.4756
d25 = 1.2057
nd14 = 1.69680
νd14 = 55.53


r26 = 31.6869
d26 = 0.8737


r27 = 52.5247
d27 = 1.3000
nd15 = 1.88300
νd15 = 40.76


r28 = 19.1738
d28 = 7.3100
nd16 = 1.49700
νd16 = 81.54


r29 = −32.1161
d29 = (Variable)


r30 = ∞
d30 = 4.6000
nd17 = 1.51633
νd17 = 64.14


r31 = ∞
d31 = 1.0603


r32 = ∞


(Imaging plane)










Aspherical Coefficients












8th surface



K = 0



A4 = 1.5703 × 10−4



A6 = −6.2535 × 10−7



A8 = 1.8023 × 10−9



A10 = −4.7976 × 10−12



9th surface



K = 0



A4 = 1.8640 × 10−4



A6 = −3.0654 × 10−7



A8 = 1.6790 × 10−9



A10 = −1.1240 × 10−11



13th surface



K = 0



A4 = 6.4830 × 10−5



A6 = −1.4322 × 10−7



A8 = 4.0045 × 10−11



A10 = 0.0000



14th surface



K = 0



A4 = 5.2909 × 10−5



A6 = −1.6207 × 10−7



A8 = 0.0000



A10 = 0.0000



23th surface



K = 0



A4 = −1.5009 × 10−5



A6 = 3.8481 × 10−8



A8 = −1.6118 × 10−10



A10 = 0.0000



24th surface



K = 0



A4 = 2.6325 × 10−5



A6 = −7.9830 × 10−9



A8 = −7.1375 × 10−11



A10 = 0.0000











Zooming Data (∞)













WE
ST
TE







f (mm)
12.31
24.42
49.14



FNO
2.88
3.20
3.57



d5
0.80000
18.51118
37.33684



d12
2.69354
2.69354
2.69354



d14
16.00294
5.07006
2.15164



d15
1.37426
3.59390
1.70000



d19
10.99914
4.54263
0.60000



d29
29.59504
41.08752
55.16241




(OD = 250 mm)



d5
0.58742
17.63032
35.17989



d12
0.84541
0.84541
0.84541



d14
18.06366
7.79904
6.15671



d15
1.37426
3.59390
1.70000



d19
10.99914
4.54263
0.60000



d29
29.59504
41.08752
55.16241










EXAMPLE 2















r1 = 133.9297
d1 = 2.5500
nd1 = 1.84666
νd1 = 23.78


r2 = 70.8155
d2 = 5.5842
nd2 = 1.56384
νd2 = 60.67


r3 = 628.1229
d3 = 0.2000


r4 = 55.7629
d4 = 4.3250
nd3 = 1.75500
νd3 = 52.32


r5 = 132.6592
d5 = (Variable)


r6 = 83.7872
d6 = 1.8000
nd4 = 1.88300
νd4 = 40.76


(Aspheric)


r7 = 12.8779
d7 = 5.8688


r8 = 91.7309
d8 = 1.5925
nd5 = 1.80610
νd5 = 40.88


r9 = 40.0967
d9 = 3.8406


r10 = −23.9023
d10 = 1.1413
nd6 = 1.43875
νd6 = 94.93


r11 = 29.7208
d11 = 4.6330
nd7 = 1.74000
νd7 = 28.30


r12 = −35.1879
d12 = (Variable)


r13 = −22.3039
d13 = 1.3000
nd8 = 1.80610
νd8 = 40.88


(Aspheric)


r14 = −40.3842
d14 = (Variable)


(Aspheric)


r15 = ∞ (Stop)
d15 = (Variable)


r16 = 63.8808
d16 = 2.2014
nd9 = 1.74320
νd9 = 49.34


r17 = −114.0569
d17 = 0.0000


r18 = 25.8855
d18 = 4.2726
nd10 = 1.51742
νd10 = 52.43


r19 = −43.9674
d19 = 1.1922
nd11 = 1.77250
νd11 = 49.60


r20 = 32.3633
d20 = (Variable)


r21 = 26.1059
d21 = 5.9106
nd12 = 1.43875
νd12 = 94.93


r22 = −20.6230
d22 = 1.1678
nd13 = 1.81600
νd13 = 46.62


r23 = −37.8449
d23 = 0.2000


r24 = 77.1275
d24 = 9.1919
nd14 = 1.43875
νd14 = 94.93


(Aspheric)


r25 = −27.8225
d25 = 0.1500


(Aspheric)


r26 = −171.1239
d26 = 1.2667
nd15 = 1.88300
νd15 = 40.76


r27 = 21.2550
d27 = 5.9748
nd16 = 1.51823
νd16 = 58.90


r28 = −39.0618
d28 = (Variable)


r29 = ∞
d29 = 4.6000
nd17 = 1.51633
νd17 = 64.14


r30 = ∞
d30 = 1.0600


r31 = ∞


(Imaging plane)










Aspherical Coefficients












6th surface



K = 1.7635



A4 = 1.3806 × 10−5



A6 = −2.6968 × 10−8



A8 = −8.3717 × 10−12



A10 = 1.9486 × 10−13



A12 = −3.5163 × 10−16



13th surface



K = 0



A4 = 1.1284 × 10−5



A6 = −2.7446 × 10−8



A8 = 2.1064 × 10−9



A10 = −1.4063 × 10−11



14th surface



K = 0



A4 = 7.3332 × 10−6



A6 = −1.3581 × 10−8



A8 = 9.8131 × 10−10



A10 = −7.0268 × 10−12



24th surface



K = 0



A4 = −1.7023 × 10−5



A6 = 4.2058 × 10−9



A8 = 3.0899 × 10−10



A10 = 0.0000



25th surface



K = 0



A4 = 1.4658 × 10−5



A6 = −1.2664 × 10−8



A8 = 3.4364 × 10−10



A10 = 0.0000











Zooming Data (∞)













WE
ST
TE







f (mm)
12.36
26.05
58.82



FNO
2.88
3.50
4.08



d5
0.91498
19.02982
40.39278



d12
4.09389
4.09389
4.09389



d14
21.63568
4.45715
2.15164



d15
3.31209
6.81666
1.70000



d20
10.35886
4.00218
0.60000



d28
29.23134
43.21124
61.28523




(OD = 250 mm)



d5
0.80288
18.18846
37.81587



d12
1.38885
1.38885
1.38885



d14
24.45282
8.00354
7.43359



d15
3.31209
6.81666
1.70000



d20
10.35886
4.00218
0.60000



d28
29.23134
43.21124
61.28523










EXAMPLE 3















r1 = 149.4865
d1 = 2.5500
nd1 = 1.84666
νd1 = 23.78


r2 = 72.9114
d2 = 6.3970
nd2 = 1.56384
νd2 = 60.67


r3 = 6612.0179
d3 = 0.2000


r4 = 50.4880
d4 = 4.6358
nd3 = 1.77250
νd3 = 49.60


r5 = 101.1472
d5 = (Variable)


r6 = 231.2367
d6 = 0.1500
nd4 = 1.52288
νd4 = 52.50


(Aspheric)


r7 = 62.9706
d7 = 1.8000
nd5 = 1.88300
νd5 = 40.76


r8 = 12.1142
d8 = 9.7070


r9 = −20.0260
d9 = 1.5836
nd6 = 1.71300
νd6 = 53.87


r10 = −82.8026
d10 = 0.1132


r11 = 94.2202
d11 = 3.6706
nd7 = 1.84666
νd7 = 23.78


r12 = −32.9015
d12 = (Variable)


r13 = −26.1797
d13 = 1.2879
nd8 = 1.69680
νd8 = 55.53


(Aspheric)


r14 = −62.2624
d14 = (Variable)


(Aspheric)


r15 = ∞ (Stop)
d15 = (Variable)


r16 = 72.0562
d16 = 1.9100
nd9 = 1.67790
νd9 = 55.34


r17 = −153.1599
d17 = 0.1000


r18 = 27.3638
d18 = 4.6088
nd10 = 1.51823
νd10 = 58.90


r19 = −25.5166
d19 = 1.1965
nd11 = 1.69680
νd11 = 55.53


r20 = 38.1737
d20 = (Variable)


r21 = 27.4467
d21 = 4.9545
nd12 = 1.43875
νd12 = 94.93


r22 = −31.9955
d22 = 1.1402
nd13 = 1.69680
νd13 = 55.53


r23 = −56.9889
d23 = 0.2000


r24 = 62.6056
d24 = 8.3345
nd14 = 1.49700
νd14 = 81.54


(Aspheric)


r25 = −40.5654
d25 = 0.1500


(Aspheric)


r26 = −172.9586
d26 = 1.2621
nd15 = 1.88300
νd15 = 40.76


r27 = 21.3503
d27 = 6.4097
nd16 = 1.49700
νd16 = 81.54


r28 = −30.7497
d28 = (Variable)


r29 = ∞
d29 = 4.6000
nd17 = 1.51633
νd17 = 64.14


r30 = ∞
d30 = 1.0600


r31 = ∞


(Imaging plane)










Aspherical Coefficients












6th surface



K = 159.4608



A4 = 4.3367 × 10−5



A6 = −1.4020 × 10−7



A8 = 3.7624 × 10−10



A10 = −6.6989 × 10−13



A12 = 3.8476 × 10−16



13th surface



K = 0



A4 = −2.1382 × 10−5



A6 = 3.8665 × 10−7



A8 = −2.2226 × 10−10



A10 = −1.6238 × 10−11



14th surface



K = 0



A4 = −1.8267 × 10−5



A6 = 2.5498 × 10−7



A8 = 8.8866 × 10−10



A10 = −2.1610 × 10−11



24th surface



K = 0



A4 = −1.1052 × 10−5



A6 = 2.2345 × 10−9



A8 = 2.4337 × 10−10



A10 = 0.0000



25th surface



K = 0



A4 = 1.6900 × 10−5



A6 = −1.0380 × 10−8



A8 = 2.8435 × 10−10



A10 = 0.0000











Zooming Data (∞)













WE
ST
TE







f (mm)
12.37
26.82
58.74



FNO
2.88
3.50
4.08



d5
0.74536
19.84896
40.78501



d12
4.08951
4.08951
4.08951



d14
21.74410
10.75495
2.15164



d15
5.62433
1.41631
1.70000



d20
6.74840
2.16306
0.60000



d28
29.49174
42.81400
58.04971




(OD = 250 mm)



d5
0.75192
18.94355
37.83580



d12
0.92556
0.92556
0.92556



d14
24.90149
14.82432
8.26481



d15
5.62433
1.41631
1.70000



d20
6.74840
2.16306
0.60000



d28
29.49174
42.81400
58.04971










EXAMPLE 4















r1 = 144.2266
d1 = 2.5500
nd1 = 1.84666
νd1 = 23.78


r2 = 80.6172
d2 = 6.2616
nd2 = 1.56384
νd2 = 60.67


r3 = ∞
d3 = 0.1000


r4 = 52.3050
d4 = 5.2077
nd3 = 1.60311
νd3 = 60.64


r5 = 116.6412
d5 = (Variable)


r6 = 113.5593
d6 = 0.0880
nd4 = 1.51940
νd4 = 51.94


(Aspheric)


r7 = 56.8420
d7 = 1.5000
nd5 = 1.88300
νd5 = 40.76


r8 = 12.0420
d8 = 8.7845


r9 = −23.6760
d9 = 1.2000
nd6 = 1.49700
νd6 = 81.54


r10 = 18.7895
d10 = 6.6093
nd7 = 1.64769
νd7 = 33.79


r11 = −29.8761
d11 = (Variable)


r12 = −23.7859
d12 = 1.3635
nd8 = 1.69300
νd8 = 53.14


(Aspheric)


r13 = −60.9085
d13 = (Variable)


(Aspheric)


r14 = ∞ (Stop)
d14 = 1.7000


r15 = 50.0908
d15 = 2.2951
nd9 = 1.58913
νd9 = 61.14


r16 = −140.7841
d16 = 0.1500


r17 = 22.8280
d17 = 4.6397
nd10 = 1.54814
νd10 = 45.79


r18 = −44.0128
d18 = 1.0008
nd11 = 1.80400
νd11 = 46.57


r19 = 27.3163
d19 = (Variable)


r20 = 24.8862
d20 = 6.5073
nd12 = 1.49650
νd12 = 81.53


(Aspheric)


r21 = −50.5293
d21 = 0.1500


(Aspheric)


r22 = 135.3961
d22 = 3.7881
nd13 = 1.43875
νd13 = 94.93


r23 = −44.8085
d23 = 0.1500


r24 = −180.8473
d24 = 1.2340
nd14 = 1.88300
νd14 = 40.76


r25 = 19.4304
d25 = 11.2743
nd15 = 1.49700
νd15 = 81.54


r26 = −27.2992
d26 = (Variable)


r27 = ∞
d27 = 4.6000
nd16 = 1.51633
νd16 = 64.14


r28 = ∞
d28 = 1.0586


r29 = ∞


(Imaging plane)










Aspherical Coefficients












6th surface



K = 34.8871



A4 = 3.0812 × 10−5



A6 = −9.6291 × 10−8



A8 = 1.8012 × 10−10



A10 = −2.2992 × 10−13



A12 = −1.4937 × 10−16



12th surface



K = 0.0251



A4 = −3.4769 × 10−5



A6 = 6.1141 × 10−7



A8 = −3.8025 × 10−9



A10 = 1.0400 × 10−11



13th surface



K = −18.7766



A4 = −4.3652 × 10−5



A6 = 5.5882 × 10−7



A8 = −3.6933 × 10−9



A10 = 9.8343 × 10−12



20th surface



K = 0



A4 = −1.2143 × 10−5



A6 = −7.3098 × 10−9



A8 = 2.7747 × 10−10



A10 = −3.6706 × 10−12



21th surface



K = 0



A4 = 2.0138 × 10−5



A6 = −3.6119 × 10−8



A8 = 3.7265 × 10−10



A10 = −3.9715 × 10−12











Zooming Data (∞)













WE
ST
TE







f (mm)
12.33
26.62
58.81



FNO
2.88
3.50
4.08



d5
0.84223
20.86435
43.41045



d11
3.18840
3.18840
3.18840



d13
24.08524
9.77242
2.15164



d19
8.09555
3.06658
0.80000



d26
29.58052
43.73719
60.80026




(OD = 250 mm)



d5
0.59917
19.79172
40.54294



d11
0.77704
0.77704
0.77704



d13
26.74520
13.25433
7.42963



d19
8.09555
3.06658
0.80000



d26
29.58052
43.73719
60.80026










FIG. 5 is an aberration diagram for Example 1 upon focusing on an object point at infinity, and FIG. 6 is an aberration diagram for Example 1 upon focusing on a subject distance of 25 cm as measured from the image plane. FIGS. 7 and 8 are similar aberration diagrams for Example 2; FIGS. 9 and 10 are similar aberration diagrams for Example 3; and FIGS. 11 and 12 are similar aberration diagrams for Example 4. These aberration diagrams are indicative of spherical aberration (SA), astigmatism (AS), distortion (DT) and chromatic aberration of magnification (CC) at the wide-angle end (a), in the intermediate setting (b), and at the telephoto end (c). In each diagram, 1.000 at ordinate for spherical aberration (SA) is indicative of the maximum aperture, and “FIY” in other aberrations is indicative of an image height in mm.
Set out below are the values of conditions (1) and (2) in each example.















Condition
f2/f1
f4/f3









Ex. 1
−0.114
0.145



Ex. 2
−0.121
0.416



Ex. 3
−0.131
0.345



Ex. 4
−0.119
0.301










FIG. 13 is illustrative in section of a single-lens reflex camera operating as an electronic imaging apparatus with the inventive zoom lens incorporated in it and a small-format CCD, CMOS or the like used as an imaging device. In FIG. 13, reference numeral 1 is a single-lens reflex camera, 2 a taking lens system located within a lens barrel comprising a zooming mechanism and a focusing mechanism, and 3 a lens barrel mount that makes the taking lens system 2 attachable to or detachable from the single-lens reflex camera 1. For this mount, a mount of the screw type or a mount of the bayonet type may be used. In the example here, the screw type mount is used.
Reference numeral 4 is indicative of an image device plane, 5 a quick return mirror interposed between the lens system and the image device plane 4 on a light path 6 through the taking lens system 2, 7 a finder screen located in a path of light reflected off the quick return mirror 5, 8 a penta prism, 9 a finder, and E the eye of an observer (eye point).
For the taking lens system 2 of the single-lens reflex camera 1 of such construction, for instance, the inventive zoom lens shown in each of Examples 1 to 4 is used.
According to the present invention as described above, it is possible to achieve a zoom lens that has such higher performance as represented by a longer back focus, a higher zoom ratio, a larger aperture ratio, and a shorter closest object distance as an interchangeable lens for a single-lens reflex type digital camera.

## Claims

1. A zoom lens which comprises, in order from an object side thereof,
a first lens group having positive refracting power,
a second lens group having negative refracting power,
a third lens group having positive refracting power, and
a fourth lens group having positive refracting power, wherein zooming from a wide-angle end to a telephoto end is implemented by changing a space between the respective lens groups;
upon zooming from said wide-angle end to said telephoto end,
a space between said first lens group and said second lens group grows wide,
a space between said second lens group and said third lens group becomes narrow,
a space between said third lens group and said fourth lens group becomes narrow,
said first lens group, said third lens group, and said fourth lens group moves toward the object side only, and
said second lens group moves toward the object side after moving to an image plane side;
said second lens group and said fourth lens group each have at least one aspheric surface; and
said second lens group has at least one negative lens made of a material that satisfies the following multiple conditions, and said fourth lens group has at least one positive lens that satisfies the following multiple conditions:


nd>1.4  (3)



νd>80  (4)



where nd is a d-line refractive index, and
νd is an Abbe constant.

2. A zoom lens which comprises, in order from an object side thereof,
a first lens group having positive refracting power,
a second lens group having negative refracting power,
a third lens group having positive refracting power, and
a fourth lens group having positive refracting power, wherein zooming from a wide-angle end to a telephoto end is implemented by changing a space between the respective lens groups;
upon zooming from said wide-angle end to said telephoto end,
a space between said first lens group and said second lens group grows wide,
a space between said second lens group and said third lens group becomes narrow,
a space between said third lens group and said fourth lens group becomes narrow,
said first lens group, said third lens group, and said fourth lens group moves toward the object side only,
said second lens group moves toward the object side after moving to an image plane side; and
said second lens group and said fourth lens group each have at least one aspheric surface, with satisfaction of the following conditions:


0.1<|f2/f1|<0.14  (1)



0.1<|f4/f3|<0.6  (2)


where fi is a focal length of an i-th lens group;
wherein the zoom lens has an angle of view of 80° or greater at the wide-angle end and a zoom ratio of 4 or higher.

3. The zoom lens according to claim 1, which has an angle of view of 80° or greater at the wide-angle end and a zoom ratio of 4 or higher.

4. A zoom lens which comprises, in order from an object side thereof,
a first lens group having positive refracting power,
a second lens group having negative refracting power,
a third lens group having positive refracting power, and
a fourth lens group having positive refracting power, wherein zooming from a wide-angle end to a telephoto end is implemented by changing a space between the respective lens groups;
said second lens group comprises a front unit having negative refracting power and a rear unit having negative refracting power; and
focusing on a nearby subject is implemented by movement of the second lens group in an optical axis direction with a change in a space between said front unit and said rear unit;
wherein the zoom lens has an angle of view of 80° or greater at the wide-angle end and a zoom ratio of 4 or higher.

5. The zoom lens according to claim 2, wherein said second lens group comprises a negative lens that is located nearest to the object side and has at least one aspheric surface, and a negative lens that is located nearest to the image plane side and has at least one aspheric surface.

6. The zoom lens according to claim 1, wherein said second lens group comprises a negative lens that is located nearest to the object side and has at least one aspheric surface, and a negative lens that is located nearest to the image plane side and has at least one aspheric surface.

7. The zoom lens according to claim 4, wherein said second lens group comprises a negative lens that is located nearest to the object side and has at least one aspheric surface, and a negative lens that is located nearest to the image plane side and has at least one aspheric surface.

8. A zoom lens which comprises, in order from an object side thereof,
a first lens group having positive refracting power,
a second lens group having negative refracting power,
a third lens group having positive refracting power, and
a fourth lens group having positive refracting power, wherein zooming from a wide-angle end to a telephoto end is implemented by changing a space between the respective lens groups;
upon zooming from said wide-angle end to said telephoto end,
a space between said first lens group and said second lens group grows wide,
a space between said second lens group and said third lens group becomes narrow,
a space between said third lens group and said fourth lens group becomes narrow,
said first lens group, said third lens group, and said fourth lens group moves toward the object side only,
said second lens group moves toward the object side after moving to an image plane side; and
said second lens group and said fourth lens group each have at least one aspheric surface, with satisfaction of the following conditions:


0.1<|f2/f1|<0.14  (1)



0.1<|f4/f3|<0.6  (2)


where fi is a focal length of an i-th lens group;
wherein said second lens group has at least one negative lens made of a material that satisfies the following multiple conditions, and said fourth lens group has at least one positive lens that satisfies the following multiple conditions:


nd>1.4  (3)



νd>80  (4)



where nd is a d-line refractive index, and
νd is an Abbe constant.

9. The zoom lens according to claim 2, wherein said second lens group comprises a front unit having negative refracting power and a rear unit having negative refracting power; and
focusing on a nearby subject is implemented by movement of the second lens group in an optical axis direction with a change in a space between said front unit and said rear unit.

10. The zoom lens according to claim 1, wherein
said second lens group comprises a front unit having negative refracting power and a rear unit having negative refracting power; and
focusing on a nearby subject is implemented by movement of the second lens group in an optical axis direction with a change in a space between said front unit and said rear unit.

11. The zoom lens according to claim 8, wherein
said second lens group comprises a front unit having negative refracting power and a rear unit having negative refracting power; and
focusing on a nearby subject is implemented by movement of the second lens group in an optical axis direction with a change in a space between said front unit and said rear unit.