# Patent

## Title
Imaging lens, imaging apparatus, and method for manufacturing imaging lens

## Abstract

An imaging lens SL installed into such as an electronic still camera 1 includes, in order from an object side: a first lens group G1 having positive refractive power; a second lens group G2 having negative refractive power; a third lens group G3 having positive refractive power; and a fourth lens group G4 having positive refractive power; the second lens group G2 and the third lens group G3 move along an optical axis upon focusing from infinity to a close object, and a given conditional expression is satisfied, thereby providing a fast imaging lens capable of taking a picture from infinity to a close object with high optical performance, an optical apparatus equipped with the imaging lens, and a method for manufacturing the imaging lens.

## Background
The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 2009-255596 filed on Nov. 7, 2009.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging lens, an optical apparatus equipped with the imaging lens, and a method for manufacturing the imaging lens.
2. Related Background Art
There has been proposed an imaging lens suitable for a film camera, an electronic still camera and a video camera in such as Japanese Patent Application Laid-Open No. 2001-021798.
However, further high optical performance has been required to the conventional imaging lens.
SUMMARY OF THE INVENTION
The present invention is made in view of the above described desire and has an object to provide a fast imaging lens capable of taking a picture from infinity to a close object with high optical performance, an optical apparatus equipped with the imaging lens, and a method for manufacturing the imaging lens.
According to a first aspect of the present invention, there is provided an imaging lens comprising, in order from an object side: a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having positive refractive power; and a fourth lens group having negative refractive power; the second lens group and the third lens group moving along an optical axis upon focusing from infinity to a close object, and the following conditional expression (1) being satisfied:

1.20<((−β)/FNO)×(f/(−f2))<3.00  (1)

where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes a focal length of the second lens group.

In the first aspect of the present invention, it is preferable that the first lens group includes an object side positive lens disposed to the most object side, and the following conditional expressions (2) and (3) being satisfied:

1.565<nd1  (2)

45.0<νd1  (3)

where nd1 denotes a refractive index of the object side positive lens at d-line (wavelength λ=587.6 nm), and νd1 denotes an Abbe number of the object side positive lens at d-line.

In the first aspect of the present invention, it is preferable that the first lens group includes at least one image side positive lens having positive refractive power disposed to an image side of the object side positive lens, and the following conditional expression (4) is satisfied:

79.0<νd2  (4)

where νd2 denotes an Abbe number of each of the image side positive lens.

In the first aspect of the present invention, it is preferable that the first lens group has at least one lens that is disposed second or later in order from the object side and is the image side positive lens.
In the first aspect of the present invention, it is preferable that the second lens counted in order from the object side of the first lens group is the image side positive lens.
In the first aspect of the present invention, it is preferable that the first lens group includes two image side positive lenses.
In the first aspect of the present invention, it is preferable that the first lens group includes at least one cemented lens.
In the first aspect of the present invention, it is preferable that the cemented lens includes a positive lens and a negative lens.
In the first aspect of the present invention, it is preferable that the first lens group includes, in order from the object side, a first lens having positive refractive power, a second lens group having positive refractive power, a third lens group having negative refractive power, a fourth lens group having positive refractive power, a fifth lens group having negative refractive power, and a sixth lens group having positive refractive power.
In the first aspect of the present invention, it is preferable that the following conditional expression (6) is satisfied:

0.15<fGF/fGR<2.00  (6)

where fGF denotes a focal length of a front lens group, and fGR denotes a focal length of a rear lens group, in which the front lens group includes the first lens, the second lens, and the third lens in the first lens group, and the rear lens group includes the fourth lens, the fifth lens, and sixth lens in the first lens group.

According to a second aspect of the present invention, there is provided an optical apparatus including the imaging lens according to the first aspect.
According to a third aspect of the present invention, there is provided a method for manufacturing an imaging lens including, in order from an object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having negative refractive power, the method comprising steps of: disposing the second lens group and the third lens group movable along an optical axis upon focusing from infinity to a close object; and disposing each lens group with satisfying the following conditional expression (1):

1.20<((−β)/FNO)×(f/(−f2))<3.00  (1)

where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes a focal length of the second lens group.

In the third aspect of the present invention, a following step is preferably included:
disposing an object side positive lens having positive refractive power to the most object side in the first lens group with satisfying the following conditional expressions (2) and (3):

1.565<nd1  (2)

45.0<νd1  (3)

where nd1 denotes a refractive index of the object side positive lens at d-line (wavelength λ=587.6 nm), and νd1 denotes an Abbe number of the object side positive lens at d-line.

In the third aspect of the present invention, a following step is preferably included:
disposing at least one image side positive lens having positive refractive power to the image side of the object side positive lens in the first lens group with satisfying the following conditional expression (4):

79.0<νd2  (4)

where νd2 denotes an Abbe number of each of the image side positive lens.

In the third aspect of the present invention, a following step is preferably included:
disposing at least one image side positive lens to the second or later in order from the object side in the first lens group.
In the third aspect of the present invention, a following step is preferably included:
disposing the image side positive lens to the second, in order from the object side, of the first lens group.
In the third aspect of the present invention, a following step is preferably included:
disposing two image side positive lens in the first lens group.
In the third aspect of the present invention, a following step is preferably included:
disposing at least one cemented lens in the first lens group.
In the third aspect of the present invention, a following step is preferably included:
disposing a positive lens and a negative lens in the cemented lens.
In the third aspect of the present invention, a following step is preferably included:
disposing, in order from the object side, a first lens having positive refractive power, a second lens having positive refractive power, a third lens having negative refractive power, a fourth lens having positive refractive power, a fifth lens having negative refractive power, and a sixth lens having positive refractive power in the first lens group.
In the third aspect of the present invention, a following step is preferably included:
satisfying the following conditional expression (6):

0.15<fGF/fGR<2.00  (6)

where fGF denotes a focal length of a front lens group, and fGR denotes a focal length of a rear lens group, in which the front lens group includes the first lens, the second lens, and the third lens in the first lens group, and the rear lens group includes the fourth lens, the fifth lens, and sixth lens in the first lens group.

With constructing an imaging lens, an optical apparatus equipped with the imaging lens, and a method for manufacturing the imaging lens according to the present invention in this manner, it becomes possible to obtain a fast imaging lens capable of taking a picture from infinity to a close object with high optical performance.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing a lens configuration of an imaging lens focusing on infinity according to Example 1 of the present application.
FIGS. 2A, 2B and 2C are graphs showing various aberrations of the imaging lens according to Example 1, in which FIG. 2A is upon focusing on infinity, FIG. 2B is upon focusing on an intermediate shooting distance, and FIG. 2C is upon focusing on a closest shooting distance.
FIG. 3 is a sectional view showing a lens configuration of an imaging lens focusing on infinity according to Example 2 of the present application.
FIGS. 4A, 4B and 4C are graphs showing various aberrations of the imaging lens according to Example 2, in which FIG. 4A is upon focusing on infinity, FIG. 4B is upon focusing on an intermediate shooting distance, and FIG. 4C is upon focusing on a closest shooting distance.
FIG. 5 is a sectional view showing a lens configuration of an imaging lens focusing on infinity according to Example 3 of the present application.
FIGS. 6A, 6B and 6C are graphs showing various aberrations of the imaging lens according to Example 3, in which FIG. 6A is upon focusing on infinity, FIG. 6B is upon focusing on an intermediate shooting distance, and FIG. 6C is upon focusing on a closest shooting distance.
FIG. 7 is a sectional view showing a lens configuration of an imaging lens focusing on infinity according to Example 4 of the present application.
FIGS. 8A, 8B and 8C are graphs showing various aberrations of the imaging lens according to Example 4, in which FIG. 8A is upon focusing on infinity, FIG. 8B is upon focusing on an intermediate shooting distance, and FIG. 8C is upon focusing on a closest shooting distance.
FIG. 9 is a sectional view showing a lens configuration of an imaging lens focusing on infinity according to Example 5 of the present application.
FIGS. 10A, 10B and 10C are graphs showing various aberrations of the imaging lens according to Example 5, in which FIG. 10A is upon focusing on infinity, FIG. 10B is upon focusing on an intermediate shooting distance, and FIG. 10C is upon focusing on a closest shooting distance.
FIGS. 11A and 11B are diagrams showing an electronic still camera equipped with an imaging lens according to the present embodiment, in which FIG. 11A is a front view, and FIG. 11B is a rear view.
FIG. 12 is a sectional view seen along the AA′ line in FIG. 11A.
FIG. 13 is a flowchart showing a method for manufacturing an imaging lens according to the present embodiment.



DESCRIPTION OF THE MOST PREFERRED EMBODIMENT
A preferred embodiment of the present application is explained below with reference to accompanying drawings. As shown in FIG. 1, an imaging lens SL according to the present embodiment includes, in order from an object side, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
Moreover, in an imaging lens SL according to the present embodiment, the second lens group G2 and the third lens group G3 are moved as focusing lens groups along an optical axis upon focusing from infinity to a close object. The focusing lens groups are suitable for auto focusing, and are suitable for being driven by a motor for auto focusing such as an ultrasonic motor.
In an imaging lens SL according to the present embodiment, the following conditional expression (1) is preferably satisfied:

1.20<((−β)FNO)×(f/(−f2))<3.00  (1)

where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes a focal length of the second lens group G2.

Conditional expression (1) defines an appropriate relation between a shooting magnification, a focal length of the second lens group G2, and an f-number with respect to the focal length of the imaging lens. When the value ((−β)/FNO)×(f/(−f2)) is equal to or exceeds the upper limit of conditional expression (1), refractive power of the second lens group G2 becomes strong, and refractive power of the first lens group G1 becomes weak, so that a total lens length becomes long. Moreover, since refractive power of the second lens group becomes strong, spherical aberration and curvature of field become worse, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the upper limit of conditional expression (1) to 2.50. In order to further secure the effect of the present embodiment, it is most preferable to set the upper limit of conditional expression (1) to 2.00. On the other hand, when the value ((−β)/FNO)×(f/(−f2)) is equal to or falls below the lower limit of conditional expression (1), refractive power of the second lens group G2 becomes weak, so that refractive power of the first lens group G1 becomes strong. As a result, variations in spherical aberration and the image plane upon focusing become large, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (1) to 1.30. In order to further secure the effect of the present embodiment, it is most preferable to set the lower limit of conditional expression (1) to 1.40.
Conditional expressions for constructing such an imaging lens SL are explained. In an imaging lens SL according to the present embodiment, an object side positive lens (L11 in FIG. 1) having positive refractive power is disposed to the most object side of the first lens group G1, and the following conditional expression (2) is satisfied:

1.565<nd1  (2)

where nd1 denotes a refractive index of the object side positive lens L11 at d-line (wavelength λ=587.6 nm).

Conditional expression (2) defines the refractive index of the object side positive lens at d-line. When the value nd1 is equal to or falls below the lower limit of conditional expression (2), variations in spherical aberration and curvature of field upon focusing become large, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (2) to 1.580. Moreover, in order to further secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (2) to 1.600.
Moreover, in an imaging lens SL according to the present embodiment, the following conditional expression (3) is preferably satisfied:

45.0<νd1  (3)

where νd1 denotes an Abbe number of the object side positive lens at d-line.

Conditional expression (3) defines an Abbe number of the object side positive lens. When the value νd1 is equal to or falls below the lower limit of conditional expression (3), the second order aberration of the first lens group G1 becomes large, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (3) to 50.00. In order to further secure the effect of the present embodiment, it is most effective to set the lower limit of conditional expression (3) to 55.00. In order to further secure the effect of the present embodiment, it is most effective to set the lower limit of conditional expression (3) to 60.00.
In an imaging lens SL according to the present embodiment, the first lens group G1 preferably includes at least one image side positive lens (for example, L12 and L14 in FIG. 1) disposed to the image side of the object side positive lens, and the following conditional expression (4) is preferably satisfied:

79.0<νd2  (4)

where νd2 denotes an Abbe number of the image side positive lens at d-line.

Conditional expression (4) defines the Abbe number of the image side positive lens. The image side positive lens is made from an anomalous dispersion glass. Such an image side positive lens is effective when it is disposed in a position where an height of ray is high, so that it is the most effective to be disposed adjoining to the image side of the object side positive lens. When the value νd2 is equal to or falls below the lower limit of conditional expression (4), secondary dispersion generated in the first lens group G1 becomes large, and longitudinal chromatic aberration on the image plane cannot be corrected, so that it is undesirable.
In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (4) to 80.0. In order to further secure the effect of the present embodiment, it is most preferable to set the lower limit of conditional expression (4) to 81.5.
In an imaging lens SL according to the present embodiment, the first lens group G1 preferably has at least one lens (for example, L12 and L14 in FIG. 1) that is disposed second or later in order from the object side and is the image side positive lens. In this case, it is further preferable that the first lens group G1 has two image side positive lenses. Moreover, the second lens counted from the object side (L12 in FIG. 1) is preferably an images side positive lens.
In an imaging lens SL according to the present embodiment, the following conditional expression (5) is preferably satisfied:

0.40<(−β)×(−f2)×FNO/f<0.90  (5)

where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes an focal length of the second lens group.

Conditional expression (5) defines an appropriate relation of the shooting magnification, the focal length of the second lens group G2, and the f-number with respect to the focal length of the imaging lens. When the value (−β)×(−f2)×FNO/f is equal to or exceeds the upper limit of conditional expression (5), refractive power of the second lens group G2 becomes weak, so that refractive power of the first lens group G1 becomes strong. Accordingly, variations in spherical aberration and the image plane upon focusing become large, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the upper limit of conditional expression (5) to 0.80. In order to further secure the effect of the present embodiment, it is most preferable to set the upper limit of conditional expression (5) to 0.70.
On the other hand, when the value (−β)×(−f2)×FNO/f is equal to or falls below the lower limit of conditional expression (5), refractive power of the first lens group G1 becomes weak, so that the total lens length becomes large. Moreover, refractive power of the second lens group G2 becomes strong, and spherical aberration and curvature of field become worse, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (5) to 0.45. In order to further secure the effect of the present embodiment, it is most preferable to set the lower limit of conditional expression (5) to 0.50.
In an imaging lens SL according to the present embodiment, the first lens group G1 preferably includes at least one cemented lens (CL11 in FIG. 1). In this case, the cemented lens preferably has a positive lens (L16 in FIG. 1) and a negative lens (L15 in FIG. 1), thereby carrying out excellent correction of spherical aberration and achromatization.
In an imaging lens SL according to the present embodiment, the first lens group G1 preferably includes, in order from the object side, a first lens L11 having positive refractive power, a second lens L12 having positive refractive power, a third lens L13 having negative refractive power, a fourth lens L14 having positive refractive power, a fifth lens L15 having negative refractive power, and a sixth lens L16 having positive refractive power. With this lens configuration, it becomes possible to keep the f-number small with securing an optical amount.
In an imaging lens SL according to the present embodiment, when a front lens group GF is composed of the first lens L11, the second lens L12, and the third lens L13, and a rear lens group GR is composed of the fourth lens L14, the fifth lens L15, and the sixth lens L16, the following conditional expression (6) is preferably satisfied:

0.15<fGF/fGR<2.00  (6)

where fGF denotes a focal length of the front lens group GF, and fGR denotes a focal length of the rear lens group GR.

conditional expression (6) defines an appropriate focal length of the front lens group GF with respect to the focal length of the rear lens group GR. When the ratio fGF/fGR is equal to or exceeds the upper limit of conditional expression (6), refractive power of the rear lens group GR becomes strong, and variation in longitudinal chromatic aberration upon focusing becomes large, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the upper limit of conditional expression (6) to 1.80. In order to further secure the effect of the present embodiment, it is most preferable to set the upper limit of conditional expression (6) to 1.70.
On the other hand, when the ratio fGF/fGR is equal to or falls below the lower limit of conditional expression (6), refractive power of the front lens group GF becomes strong, and variation in spherical aberration upon focusing becomes large, so that it is undesirable. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (6) to 0.20. In order to further secure the effect of the present embodiment, it is most preferable to set the lower limit of conditional expression (6) to 0.50.
In FIGS. 11A, 11B and 12, construction of an electronic still camera 1 (hereinafter simply shown as a camera) as an optical apparatus equipped with an imaging lens SL according to the present embodiment is shown. In the camera 1, when a power switch button (not shown) is pressed, a shutter (not shown) of an image-taking lens (imaging lens SL) is opened, light from an object (not shown) is converged by the imaging lens SL, and an image is formed on an imaging device C (such as a CCD, or CMOS) disposed on the image plane I. The object image formed on the imaging device C is displayed on a liquid crystal monitor 2 disposed backside of the camera 1. After fixing the image composition of the object image with observing the liquid crystal monitor 2, a photographer depresses a release button 3 to take a picture of the object image by the imaging device C, and stores in a memory (not shown).
In the camera 1, the following members are disposed such as an auxiliary light emitter 4 that emits auxiliary light when the object is dark, a W-T button 5 that makes the zoom lens system carry out zooming between a wide-angle end state (W) and a telephoto end state (T), and a function button 6 that is used for setting various conditions of the camera 1. Although a compact-type camera, in which an imaging lens SL and a camera are formed integrally, is shown in FIG. 11, an optical apparatus may be a single-lens reflex camera that a camera body and a lens barrel including an imaging lens SL are removable.
Then, a method for manufacturing an imaging lens SL according to the present embodiment is explained with reference to FIG. 13.
Step S100:
Each lens group is prepared with disposing each lens into each lens group. In particular, in the present embodiment, for example, in order from an object side, a double convex positive lens L11, a positive meniscus lens L12 having a convex surface facing the object side, a double concave negative lens L13, a double convex positive lens L14, and a cemented lens CL11 constructed by a negative meniscus lens L15 having a convex surface facing the object side cemented with a positive meniscus lens L16 having a convex surface facing the object side are disposed in the first lens group G1. In order from the object side, a negative meniscus lens L21 having a convex surface facing the object side, and a cemented lens CL21 constructed by a double concave negative lens L22 cemented with a positive meniscus lens L23 having a convex surface facing the object side are disposed in the second lens group G2. In order from the object side, a double convex positive lens L31, and a cemented lens CL31 constructed by a double convex positive lens L32 cemented with a negative meniscus lens L33 having a convex surface facing the image side are disposed in the third lens group G3. In order from the object side, a double concave negative lens L41, a double convex positive lens L42, and a double concave negative lens L43 are disposed in the second lens group G4. With disposing each lens group provided in this manner, an imaging lens SL is manufactured.
Step S200:
Disposing the second lens group G2 and the third lens group G3 movably along an optical axis upon focusing from infinity to a close object.
Step S300:
Disposing each lens group with satisfying the following conditional expression (1):

1.20<((−β)/FNO)×(f/(−f2))<3.00  (1)

where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes a focal length of the second lens group G2.

Each example of the present embodiment is explained below with reference to accompanying drawings. FIGS. 1, 3, 5, 7 and 9 are sectional views showing lens configurations of imaging lenses SL (SL1 through SL5) and movement of each lens group upon focusing from infinity to a close object. As shown in each drawing, an imaging lens SL according to each example is composed of, in order from an object side, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. In the first lens group G1, an object side positive lens L11 having positive refractive power is disposed to the most object side thereof, and an image side positive lens L12 having positive refractive power is disposed to the image side of the object side positive lens L11. The second lens group G2 and the third lens group G3 are moved along an optical axis upon carrying out focusing from infinity to a close object. An aperture stop S is disposed between the second lens group G2 and the third lens group G3.
Example 1
FIG. 1 is a sectional view showing a lens configuration of an imaging lens SL1 according to Example 1 of the present application focusing on infinity. In the imaging lens SL1 shown in FIG. 1, the first lens group G1 is composed of, in order from and object side, a double convex positive lens L11, a positive meniscus lens L12 having a convex surface facing the object side, a double concave negative lens L13, a double convex positive lens L14, and a cemented lens CL11 constructed by a negative meniscus lens L15 having a convex surface facing the object side cemented with a positive meniscus lens L16 having a convex surface facing the object side. The second lens group G2 is composed of, in order from the object side, a negative meniscus lens L21 having a convex surface facing the object side, and a cemented lens CL 21 constructed by a double concave negative lens L22 cemented with a positive meniscus lens L23 having a convex surface facing the object side. The third lens group G3 is composed of, in order from the object side, a double convex positive lens L31, and a cemented lens CL 31 constructed by a double convex positive lens L32 cemented with a negative meniscus lens L33 having a convex surface facing the image side. The fourth lens group G4 is composed of, in order from the object side, a double concave negative lens L41, a double convex positive lens L42, and a double concave negative lens L43.
Various values associated with the imaging lens SL1 according to Example 1 are listed in Table 1.
In Table 1, f denotes a focal length of the imaging lens SL1, FNO denotes an f-number, β denotes a shooting magnification, and Bf denotes a distance between an image side surface of the most image side lens and an image plane, TL denotes a total lens length. In [Lens Data], the left most column “i” shows the lens surface number counted in order from the object side, the second column “r” shows a radius of curvature of the lens surface, the third column “d” shows a distance to the next surface, the fourth column “νd” shows an Abbe number at d-line (wavelength λ=587.6 nm), and the fifth column “nd” shows refractive index at d-line (wavelength λ=587.6 nm). In the fifth column “nd” refractive index of the air nd=1.000000 is omitted. In the second column “r”, r=∞ denotes a plane surface. In [Variable Distances], β, variable distances, Bf and a total lens length TL with respect to shooting distance d0 of infinity (INF), an intermediate shooting distance (MID) (β=−0.5), and a closest shooting distance (CLD) (β=−1.0) are shown. In [Values for Conditional Expressions], values for respective conditional expressions are shown.
In respective tables for various values, “mm” is generally used for the unit of length such as the focal length, the radius of curvature and the distance to the next lens surface. However, since similar optical performance can be obtained by an optical system proportionally enlarged or reduced its dimension, the unit is not necessarily to be limited to “mm”, and any other suitable unit can be used.
The explanation of reference symbols is the same in the other Examples.







TABLE 1







[Specifications]












f = 199.96976



FNO = 2.46











[Lens Data]











i
r
d
νd
nd





1
298.1633
10.0000
58.55
1.651597


2
−284.3000
1.0000


3
86.0789
12.5000
82.52
1.497820


4
1451.5525
3.0000


5
−368.5872
3.5000
34.96
1.800999


6
228.3831
10.6267


7
159.9332
8.0000
70.41
1.487490


8
−314.4280
0.1000


9
64.8733
3.5000
41.96
1.667551


10
35.8768
12.0000
82.52
1.497820


11
144.1260
(d11)


12
435.6646
2.8000
65.44
1.603001


13
55.4486
5.0000


14
−455.1677
2.7000
69.98
1.518601


15
36.0015
5.5000
25.43
1.805181


16
52.6821
(d16)











17
∞
(d17)
Aperture Stop S












18
144.7963
5.0000
82.52
1.497820


19
−124.6815
0.5000


20
90.9873
8.0000
60.29
1.620411


21
−68.8895
1.8000
30.13
1.698947


22
−327.4296
(d22)


23
−146.1501
2.0000
23.78
1.846660


24
55.0000
2.8562


25
62.7116
8.0000
23.78
1.846660


26
−109.3188
0.1000


27
−257.3955
2.0000
69.98
1.518601


28
105.3047
57.9810










[Variable Distances]













INF
MID
CLD







β=
0
−0.5
−1.0



d0=
∞
377.60300
235.95060



d11=
4.00000
18.80254
34.77331



d16=
35.77330
20.97077
5.00000



d17=
44.74376
21.33048
5.97949



d22=
4.00000
27.41329
42.76428



Bf=
57.98100
58.24403
58.52183



TL=
249.83294
250.09597
250.37377











[Values for Conditional Expressions]












β = −1.0



FNO = 2.46



f = 199.96976



f2 = −53.900



(1)((−β)/FNO) × (f/(−f2)) = 1.51



(2)nd1 = 1.652



(3)νd1 = 58.55



(4)νd2 = 82.52



(5)(−β) × (−f2) × FNo/f = 0.663



(6)fGF/fGR = 1.452










FIGS. 2A, 2B and 2C are graphs showing various aberrations of the imaging lens according to Example 1, in which FIG. 2A is upon focusing on infinity, FIG. 2B is upon focusing on an intermediate shooting distance (β=−0.5), and FIG. 2C is upon focusing on a closest shooting distance (β=−1.0).
In respective graphs, FNO denotes an f-number, NA denotes a numerical number, Y denotes an image height, and ω denotes a half angle of view (unit: degrees). In respective graphs, D denotes an aberration curve at d-line (wavelength λ=587.6 nm), and G denotes an aberration curve at g-line (wavelength λ=435.8 nm). In graphs showing astigmatism and distortion, the maximum value of the image height Y is shown. In graphs showing coma, value of each image height is shown. In the graph showing astigmatism, a solid line indicates a sagittal image plane, and a broken line indicates a meridional image plane. The above-described explanations regarding various aberration graphs are the same as the other Examples.
As is apparent from FIGS. 2A, 2B and 2C, the imaging lens according to Example 1 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity to a close object.
Example 2
FIG. 3 is a sectional view showing a lens configuration of an imaging lens SL2 according to Example 2 of the present application focusing on infinity. In the imaging lens SL2 shown in FIG. 3, the first lens group G1 is composed of, in order from and object side, a double convex positive lens L11, a positive meniscus lens L12 having a convex surface facing the object side, a double concave negative lens L13, a double convex positive lens L14, and a cemented lens CL11 constructed by a negative meniscus lens L15 having a convex surface facing the object side cemented with a positive meniscus lens L16 having a convex surface facing the object side. The second lens group G2 is composed of, in order from the object side, a double concave negative lens L21, and a cemented lens CL 21 constructed by a double concave negative lens L22 cemented with a positive meniscus lens L23 having a convex surface facing the object side. The third lens group G3 is composed of, in order from the object side, a double convex positive lens L31, and a cemented lens CL 31 constructed by a double convex positive lens L32 cemented with a negative meniscus lens L33 having a convex surface facing the image side. The fourth lens group G4 is composed of, in order from the object side, a double concave negative lens L41, a double convex positive lens L42, and a double convex positive lens L43.
Various values associated with the imaging lens SL2 according to Example 2 are listed in Table 2.







TABLE 2







[Specifications]












f = 169.98735



FNO = 2.50











[Lens Data]











i
r
d
νd
nd





1
211.2162
10.5000
55.40
1.677900


2
−384.5087
0.9043


3
74.2569
14.0000
82.52
1.497820


4
1721.7830
1.8085


5
−789.7207
3.1649
35.04
1.749500


6
257.0044
11.8180


7
201.5450
5.0000
91.20
1.456000


8
−760.5152
0.1000


9
60.3722
3.1649
41.17
1.701540


10
31.6491
10.0000
82.52
1.497820


11
89.1239
(d11)


12
−649.9643
2.5319
65.44
1.603001


13
53.1965
5.0000


14
−125.0621
2.4415
62.06
1.588245


15
33.6580
4.0000
25.43
1.805181


16
55.7399
(d16)











17
∞
(d17)
Aperture Stop S












18
69.9907
7.5000
63.37
1.618000


19
−104.2598
0.4521


20
211.9953
6.5106
91.20
1.456000


21
−60.8154
1.6277
23.78
1.846660


22
−195.3139
(d22)


23
−66.8968
1.8085
35.04
1.749500


24
55.0000
2.8101


25
262.6091
3.8000
27.51
1.755199


26
−543.4102
0.0904


27
66.7147
5.5000
25.43
1.805181


28
−135.4887
43.9357










[Variable Distances]













INF
MID
CLD







β=
0
−0.5
−1.0



d0=
∞
466.79610
297.31720



d1=
4.00001
26.37250
45.56607



d16=
46.56607
24.19357
5.00000



d17=
8.20000
3.90000
3.00000



d22=
34.70030
39.00030
39.90030



Bf=
43.93570
43.73004
44.50869



TL=
241.93545
241.72979
242.50844











[Values for Conditional Expressions]












β = −1.0



FNO = 2.42



f = 169.98735



f2 = −40.801



(1)((−β)/FNO) × (f/(−f2)) = 1.72



(2)nd1 = 1.678



(3)νd1 = 55.40



(4)νd2 = 82.52



(5)(−β) × (−f2) × FNo/f = 0.580



(6)fGF/fGR = 0.221










FIGS. 4A, 4B and 4C are graphs showing various aberrations of the imaging lens according to Example 2, in which FIG. 4A is upon focusing on infinity, FIG. 4B is upon focusing on an intermediate shooting distance (β=−0.5), and FIG. 4C is upon focusing on a closest shooting distance (β=−1.0).
As is apparent from FIGS. 4A, 4B and 4C, the imaging lens according to Example 2 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity to a close object.
Example 3
FIG. 5 is a sectional view showing a lens configuration of an imaging lens SL3 according to Example 3 of the present application focusing on infinity. In the imaging lens SL3 shown in FIG. 5, the first lens group G1 is composed of, in order from and object side, a double convex positive lens L11, a positive meniscus lens L12 having a convex surface facing the object side, a double concave negative lens L13, a double convex positive lens L14, and a cemented lens CL11 constructed by a negative meniscus lens L15 having a convex surface facing the object side cemented with a positive meniscus lens L16 having a convex surface facing the object side. The second lens group G2 is composed of, in order from the object side, a negative meniscus lens L21 having a convex surface facing the object side, and a cemented lens CL 21 constructed by a double concave negative lens L22 cemented with a positive meniscus lens L23 having a convex surface facing the object side. The third lens group G3 is composed of, in order from the object side, a positive meniscus lens L31 having a concave surface facing the object side, and a cemented lens CL 31 constructed by a double convex positive lens L32 cemented with a negative meniscus lens L33 having a convex surface facing the image side. The fourth lens group G4 is composed of, in order from the object side, a cemented lens CL 41 constructed by a positive meniscus lens L41 having a convex surface facing the image side cemented with a double concave negative lens L42, a double convex positive lens L43, and a double concave negative lens L44.
Various values associated with the imaging lens SL3 according to Example 3 are listed in Table 3.







TABLE 3







[Specifications]












f = 198.00003



FNO = 3.21











[Lens Data]











i
r
d
νd
nd





1
129.7596
9.3000
55.52
1.696797


2
−731.2617
1.0000


3
76.9560
9.2000
82.52
1.497820


4
259.2427
4.2000


5
−301.5232
3.5000
33.89
1.803840


6
206.8971
0.1000


7
118.7575
8.5000
82.52
1.497820


8
−274.2081
0.1952


9
52.1747
3.5000
45.29
1.794997


10
31.0793
11.2000
82.52
1.497820


11
152.0694
(d11)


12
388.1555
2.5000
47.38
1.788001


13
34.2544
4.3000


14
−296.4558
2.3000
60.29
1.620410


15
26.4313
5.5000
31.59
1.756920


16
96.9634
(d16)











17
∞
(d17)
Aperture Stop S












18
−1970.5204
3.5000
82.52
1.497820


19
−78.6469
0.5000


20
50.2171
8.0000
82.52
1.497820


21
−32.7001
1.8000
33.89
1.803840


22
−56.2120
(d22)


23
−106.7266
5.0000
25.68
1.784723


24
−34.9543
1.8000
55.48
1.638540


25
35.7172
12.3237


26
45.5316
7.0000
46.57
1.804000


27
−1028.7209
0.5000


28
−1610.3480
2.0000
23.78
1.846660


29
85.1001
41.2322










[Variable Distances]













INF
MID
CLD







β=
0
−0.5
−1.0



d0=
∞
385.26190
238.37790



d11=
9.72009
19.44212
30.03625



d16=
25.44056
15.71854
5.12440



d17=
27.52794
12.21742
1.81442



d22=
3.05288
18.36340
28.76640



Bf=
41.23220
41.26834
41.23514



TL=
214.69255
214.72869
214.69549











[Values for Conditional Expressions]












β = −1.0



FNO = 3.21



f = 198.00003



f2 = −37.640



(1)((−β)/FNO) × (f/(−f2)) = 1.64



(2)nd1 = 1.697



(3)νd1 = 55.52



(4)νd2 = 82.52



(5)(−β) × (−f2) × FNo/f = 0.609



(6)fGF/fGR = 1.690










FIGS. 6A, 6B and 6C are graphs showing various aberrations of the imaging lens according to Example 3, in which FIG. 6A is upon focusing on infinity, FIG. 6B is upon focusing on an intermediate shooting distance (β=−0.5), and FIG. 6C is upon focusing on a closest shooting distance (β=−1.0).
As is apparent from FIGS. 6A, 6B and 6C, the imaging lens according to Example 3 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity to a close object.
Example 4
FIG. 7 is a sectional view showing a lens configuration of an imaging lens SL4 according to Example 4 of the present application focusing on infinity. In the imaging lens SL4 shown in FIG. 7, the first lens group G1 is composed of, in order from and object side, a double convex positive lens L11, a positive meniscus lens L12 having a convex surface facing the object side, a double concave negative lens L13, a double convex positive lens L14, and a cemented lens CL11 constructed by a negative meniscus lens L15 having a convex surface facing the object side cemented with a positive meniscus lens L16 having a convex surface facing the object side. The second lens group G2 is composed of, in order from the object side, a double concave negative lens L21, and a cemented lens CL 21 constructed by a double concave negative lens L22 cemented with a positive meniscus lens L23 having a convex surface facing the object side. The third lens group G3 is composed of, in order from the object side, a double convex positive lens L31, and a cemented lens CL 31 constructed by a double convex positive lens L32 cemented with a negative meniscus lens L33 having a convex surface facing the image side. The fourth lens group G4 is composed of, in order from the object side, a double concave negative lens L41, a double convex positive lens L42, and a double concave negative lens L43.
Various values associated with the imaging lens SL4 according to Example 4 are listed in Table 4.







TABLE 4







[Specifications]












f = 219.98771



FNO = 2.04











[Lens Data]











i
r
d
νd
nd





1
258.3171
15.0000
54.66
1.729157


2
−463.5027
0.3000


3
118.3970
16.0000
67.90
1.593190


4
911.5383
3.8000


5
−639.7392
3.8506
32.35
1.850260


6
223.5223
9.1199


7
125.4664
12.8000
91.20
1.456000


8
−684.1438
0.1594


9
92.1119
3.8506
47.38
1.788001


10
49.9130
14.5000
82.52
1.497820


11
210.9790
(d11)


12
−4316.8254
3.0805
55.52
1.696797


13
81.5657
5.6000


14
−477.8628
2.9704
55.52
1.696797


15
45.3786
7.5000
23.78
1.846660


16
85.1484
(d16)











17
∞
(d17)
Aperture Stop S












18
146.1050
6.5000
82.52
1.497820


19
−134.0472
0.3000


20
96.7435
12.0000
65.46
1.603001


21
−60.5750
1.9803
35.04
1.749500


22
−183.0858
(d22)


23
−118.1879
2.2003
30.13
1.698950


24
50.6161
10.7983


25
84.1488
11.5000
35.04
1.749500


26
−60.1396
0.1100


27
−71.7556
2.2003
70.45
1.487490


28
444.5196
58.6015










[Variable Distances]













INF
MID
CLD







β=
0
−0.5
−1.0



d0=
∞
461.83610
310.25050



d11=
8.93482
32.30173
53.30294



d16=
48.67817
25.31126
4.31005



d17=
38.99927
18.58195
3.58851



d22=
9.87060
30.28791
45.28135



Bf=
58.60150
58.51021
57.65735



TL=
311.20498
311.11369
310.26083











[Values for Conditional Expressions]












β = −1.0



FNO = 2.04



f = 219.98771



f2 = −56.841



(1)((−β)/FNO) × (f/(−f2)) = 1.90



(2)nd1 = 1.729



(3)νd1 = 54.66



(4)νd2 = 91.20



(5)(−β) × (−f2) × FNo/f = 0.527



(6)fGF/fGR = 1.152










FIGS. 8A, 8B and 8C are graphs showing various aberrations of the imaging lens according to Example 4, in which FIG. 8A is upon focusing on infinity, FIG. 8B is upon focusing on an intermediate shooting distance (β=−0.5), and FIG. 8C is upon focusing on a closest shooting distance (β=−1.0).
As is apparent from FIGS. 8A, 8B and 8C, the imaging lens according to Example 4 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity to a close object.
Example 5
FIG. 9 is a sectional view showing a lens configuration of an imaging lens SL5 according to Example 5 of the present application focusing on infinity. In the imaging lens SL5 shown in FIG. 9, the first lens group G1 is composed of, in order from and object side, a double convex positive lens L11, positive meniscus lens L12 having a convex surface facing the object side, a double concave negative lens L13, a positive meniscus lens L14 having a convex surface facing the object side, and a cemented lens CL11 constructed by a negative meniscus lens L15 having a convex surface facing the object side cemented with a positive meniscus lens L16 having a convex surface facing the object side. The second lens group G2 is composed of, in order from the object side, a negative meniscus lens L21 having a convex surface facing the object side, and a cemented lens CL 21 constructed by a double concave negative lens L22 cemented with a positive meniscus lens L23 having a convex surface facing the object side. The third lens group G3 is composed of, in order from the object side, a double convex positive lens L31, and a cemented lens CL 31 constructed by a double convex positive lens L32 cemented with a negative meniscus lens L33 having a convex surface facing the image side. The fourth lens group G4 is composed of, in order from the object side, a double concave negative lens L41, a double convex positive lens L42, and a double concave negative lens L43.
Various values associated with the imaging lens SL5 according to Example 5 are listed in Table 5.







TABLE 5







[Specifications]












f = 160.01928



FNO = 2.87











[Lens Data]











i
r
d
νd
nd





1
189.3966
11.5000
65.47
1.603000


2
−256.5570
0.2182


3
66.3234
14.0000
91.38
1.456000


4
488.9346
2.5000


5
−800.0000
3.5000
32.35
1.850260


6
181.6514
7.0000


7
76.7085
7.0000
82.56
1.497820


8
477.4907
0.1000


9
44.6509
2.8004
47.38
1.788000


10
27.8882
11.5000
91.20
1.456000


11
140.9438
(d11)


12
177.9173
2.2404
57.34
1.670000


13
37.6470
4.5000


14
−235.9017
2.1603
55.52
1.696797


15
29.1580
4.4442
23.78
1.846660


16
58.5189
(d16)











17
∞
(d17)
Aperture Stop S












18
84.6544
4.5000
82.52
1.497820


19
−66.7276
0.2182


20
56.2011
8.0000
82.52
1.497820


21
−43.4875
1.4402
35.04
1.749500


22
−113.1902
(d22)


23
−88.9707
2.0000
32.11
1.672700


24
38.1868
5.0000


25
53.0515
10.0000
34.96
1.801000


26
−43.0712
0.5000


27
−45.8497
1.6003
65.47
1.603000


28
76.3227
37.2903










[Variable Distances]













INF
MID
CLD







β=
0
−0.5
−1.0



d0=
∞
309.71390
196.91690



d11=
1.43518
13.21354
25.04256



d16=
26.05596
14.27760
2.44858



d17=
29.17815
15.21786
5.25829



d22=
2.24602
16.20631
26.16588



Bf=
37.29034
37.42219
37.29068



TL=
202.92784
203.05973
202.92822











[Values for Conditional Expressions]












β = −1.0



FNO = 2.87



f = 160.01928



f2 = −36.317



(1)((−β)/FNO) × (f/(−f2)) = 1.54



(2)nd1 = 1.603



(3)νd1 = 65.47



(4)νd2 = 91.38



(5)(−β) × (−f2) × FNo/f = 0.651



(6)fGF/fGR = 1.301










FIGS. 10A, 10B and 10C are graphs showing various aberrations of the imaging lens according to Example 5, in which FIG. 10A is upon focusing on infinity, FIG. 10B is upon focusing on an intermediate shooting distance (β=−0.5), and FIG. 10C is upon focusing on a closest shooting distance (β=−1.0).
As is apparent from FIGS. 10A, 10B and 10C, the imaging lens according to Example 5 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity to a close object.
Incidentally, the following description may suitably be applied within limits that do not deteriorate optical performance.
Although an imaging lens with a four-lens-group configuration is shown as each Example of the present application, the lens-group configuration according to the present application is not limited to this, other lens-group configurations such as a five-lens-group configuration or a six-lens-group configuration is possible. Moreover, a lens configuration that a lens or a lens group is added to the object side thereof is possible, and a lens configuration that a lens or a lens group is added to the image side thereof is also possible. Incidentally, a lens group means a portion that includes at least one lens and is separated by air spaces that vary upon focusing.
In an imaging lens according to the present application, a lens group or a portion of a lens group may be shifted in a direction including a component perpendicular to the optical axis as a vibration reduction lens group, or tilted (swayed) in a direction including the optical axis for correcting an image blur caused by a camera shake. In an imaging lens according to the present application, it is particularly preferable that at least a portion of the fourth lens group G4 is used as a vibration reduction lens group.
A lens surface of a lens composing an imaging lens according to the present application may be a spherical surface, a plane surface, or an aspherical surface. When a lens surface is a spherical surface or a plane surface, lens processing, assembling and adjustment become easy, and deterioration in optical performance caused by lens processing, assembling and adjustment errors can be prevented, so that it is preferable. Moreover, even if the surface is shifted, deterioration in optical performance is little, so that it is preferable. When a lens surface is an aspherical surface, the aspherical surface may be fabricated by a fine grinding process, a glass molding process that a glass material is formed into an aspherical shape by a mold, or a compound type process that a resin material is formed into an aspherical shape on a glass lens surface. A lens surface may be a diffractive optical surface, and a lens may be a graded-index type lens (GRIN lens) or a plastic lens.
In an imaging lens according to the present application, although an aperture stop is preferably provided between the second lens group G2 and the third lens group G3, the function may be substituted by a lens frame without disposing a member as an aperture stop.
An antireflection coating having high transmittance over a broad wavelength range may be applied to each lens surface of an imaging lens according to the present application to reduce flare or ghost images, so that high optical performance with high contrast can be attained.
In an imaging lens SL according to the present application, the first lens group G1 preferably includes three positive lens components and one negative lens component. The first lens group G1 preferably disposes these lens components, in order from the object side, positive-positive-negative-positive with an air space between each of them.
In an imaging lens SL according to the present application, the second lens group G2 preferably includes two negative lens components. In an imaging lens SL according to the present application, the third lens group G3 preferably includes two positive lens components.
In an imaging lens SL according to the present application, the fourth lens group G4 preferably includes one positive lens component and one negative lens component. The fourth lens group G4 preferably disposes these lens components, in order from the object side, negative-positive with an air space between each of them.
Above-described each example only shows a specific example for the purpose of better understanding of the present invention. Accordingly, it is needless to say that the invention in its broader aspect is not limited to the specific details and representative devices shown and described herein.

## Claims

1. An imaging lens comprising, in order from an object side:
a first lens group having positive refractive power;
a second lens group having negative refractive power;
a third lens group having positive refractive power; and
a fourth lens group having negative refractive power;
the second lens group and the third lens group moving along an optical axis upon focusing from infinity to a close object, and
the following conditional expression being satisfied:


1.20<((−β)/FNO)×(f/(−f2))<3.00



where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes a focal length of the second lens group.

2. The imaging lens according to claim 1, wherein
the first lens group includes an object side positive lens disposed to the most object side, and the following conditional expressions being satisfied:


1.565<nd1



45.0<νd1



where nd1 denotes a refractive index of the object side positive lens at d-line (wavelength λ=587.6 nm), and νd1 denotes an Abbe number of the object side positive lens at d-line.

3. The imaging lens according to claim 2, wherein the first lens group includes at least one image side positive lens having positive refractive power disposed to an image side of the object side positive lens, and the following conditional expression is satisfied:


79.0<νd2


where νd2 denotes an Abbe number of each of the image side positive lens.

4. The imaging lens according to claim 3, wherein the first lens group has at least one lens that is disposed second or later in order from the object side and is the image side positive lens.

5. The imaging lens according to claim 3, wherein the second lens counted in order from the object side of the first lens group is the image side positive lens.

6. The imaging lens according to claim 4, wherein the first lens group includes two image side positive lenses.

7. The imaging lens according to claim 1, wherein the first lens group includes at least one cemented lens.

8. The imaging lens according to claim 7, wherein the cemented lens includes a positive lens and a negative lens.

9. The imaging lens according to claim 1, wherein the first lens group includes, in order from the object side, a first lens having positive refractive power, a second lens group having positive refractive power, a third lens group having negative refractive power, a fourth lens group having positive refractive power, a fifth lens group having negative refractive power, and a sixth lens group having positive refractive power.

10. The imaging lens according to claim 9, wherein the following conditional expression is satisfied:


0.15<fGF/fGR<2.00


where fGF denotes a focal length of a front lens group, and fGR denotes a focal length of a rear lens group, in which the front lens group includes the first lens, the second lens, and the third lens in the first lens group, and the rear lens group includes the fourth lens, the fifth lens, and sixth lens in the first lens group.

11. An optical apparatus including the imaging lens according to claim 1.

12. A method for manufacturing an imaging lens including, in order from an object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having negative refractive power, the method comprising steps of:
disposing the second lens group and the third lens group movable along an optical axis upon focusing from infinity to a close object; and
disposing each lens group with satisfying the following conditional expression:


1.20<((−β)/FNO)×(f/(−f2))<3.00



where β (negative) denotes an available shooting magnification whose absolute value gives the maximum value, f denotes a focal length of the imaging lens, FNO denotes an f-number, and f2 denotes a focal length of the second lens group.

13. The method according to claim 12, further comprising a step of:
disposing an object side positive lens having positive refractive power to the most object side in the first lens group with satisfying the following conditional expressions:


1.565<nd1



45.0<νd1



where nd1 denotes a refractive index of the object side positive lens at d-line (wavelength λ=587.6 nm), and νd1 denotes an Abbe number of the object side positive lens at d-line.

14. The method according to claim 13, further comprising a step of:
disposing at least one image side positive lens having positive refractive power to the image side of the object side positive lens in the first lens group with satisfying the following conditional expression:


79.0<νd2



where νd2 denotes an Abbe number of each of the image side positive lens.

15. The method according to claim 14, further comprising a step of:
disposing at least one image side positive lens to the second or later in order from the object side in the first lens group.

16. The method according to claim 14, further comprising a step of:
disposing the image side positive lens to the second, in order from the object side, of the first lens group.

17. The method according to claim 15, further comprising a step of:
disposing two image side positive lens in the first lens group.

18. The method according to claim 12, further comprising a step of:
disposing at least one cemented lens in the first lens group.

19. The method according to claim 18, further comprising a step of:
disposing a positive lens and a negative lens in the cemented lens.

20. The method according to claim 12, further comprising a step of:
disposing, in order from the object side, a first lens having positive refractive power, a second lens having positive refractive power, a third lens having negative refractive power, a fourth lens having positive refractive power, a fifth lens having negative refractive power, and a sixth lens having positive refractive power in the first lens group.

21. The method according to claim 20, further comprising a step of:
satisfying the following conditional expression:


0.15<fGF/fGR<2.00



where fGF denotes a focal length of a front lens group, and fGR denotes a focal length of a rear lens group, in which the front lens group includes the first lens, the second lens, and the third lens in the first lens group, and the rear lens group includes the fourth lens, the fifth lens, and sixth lens in the first lens group.