JP7792782B2 - Optical system and imaging device - Google Patents

Description

This invention relates to an optical system and an imaging device.

In recent years, imaging devices using solid-state imaging elements, such as digital still cameras, have become increasingly popular. As a result, optical systems have become more compact and sophisticated, leading to a rapid rise in the popularity of compact imaging systems. With conventional lenses, miniaturizing the optical system while maintaining high optical performance has become a challenge, particularly for surveillance lenses, video camera lenses, digital still camera lenses, single-lens reflex camera lenses, and mirrorless single-lens camera lenses, which require a short overall length and compact optical system.

Patent Document 1 discloses an optical system that is composed of, from the object side, a first lens group with positive refractive power, a second lens group with positive refractive power, and a third lens group with negative refractive power, with the second lens group moving in the optical axis direction during focusing. However, in the lenses described in Examples 2 to 5, the refractive power of the lens group located closer to the object than the aperture stop is weaker than the refractive power of the first lens group, resulting in larger lens diameters and hindering efforts to make the lens barrel more compact.

JP 2015-001641 A

The objective of this invention is to provide a compact optical system that has high optical performance.

In order to solve the above problem, the optical system of the present invention is composed of, in order from the object side to the image side, a first lens group having positive refractive power, a second lens group having positive refractive power, and a third lens group having negative refractive power, and during focusing, the first lens group and the third lens group are fixed in the optical axis direction with respect to the image plane, and the second lens group moves along the optical axis, and the first lens group is composed of, in order from the object side to the image side, a 1a group having negative refractive power, an aperture stop, and a 1b group having positive refractive power, and satisfies the following formula:
-1.10 ≦ f1a/f1 ≦ -0.05 (1)
however,
f1a: focal length of the 1a-th lens group f1: focal length of the 1st lens group

Furthermore, in order to solve the above problem, the imaging device according to the present invention is characterized by comprising the above optical system and an imaging element that converts the optical image formed by the optical system into an electrical signal.

This invention makes it possible to provide a compact optical system that has high optical performance.

FIG. 1 is a cross-sectional view of an optical system according to a first embodiment. FIG. 4 is a longitudinal aberration diagram of the optical system of Example 1 in a state where the optical system is focused at infinity. FIG. 10 is a cross-sectional view of an optical system according to a second embodiment. FIG. 10 is a longitudinal aberration diagram of the optical system of Example 2 in an infinity focused state. FIG. 10 is a cross-sectional view of an optical system according to a third embodiment. FIG. 10 is a longitudinal aberration diagram of the optical system of Example 3 in a state where the optical system is focused at infinity. FIG. 10 is a cross-sectional view of an optical system according to a fourth embodiment. FIG. 10 is a longitudinal aberration diagram of the optical system of Example 4 in a state where the optical system is focused at infinity. FIG. 10 is a cross-sectional view of an optical system according to a fifth embodiment. 10A and 10B are aberration diagrams of the optical system of Example 5 in a state where the optical system is focused at infinity. 1 is a diagram schematically illustrating an example of the configuration of an imaging device according to an embodiment of the present invention.

The following describes embodiments of the optical system and imaging device according to the present invention. However, the optical system and imaging device described below are one aspect of the optical system and imaging device according to the present invention, and the optical system and imaging device according to the present invention are not limited to the following aspects.

1. Optical System 1-1. Optical Configuration The optical system according to the present invention is composed of, in order from the object side, a first lens group having positive refractive power, a second lens group having positive refractive power, and a third lens group having negative refractive power. This configuration makes it easy to reduce the size.

(1) First Lens Group The specific configuration of the first lens group is not particularly limited, as long as it has positive refractive power and is fixed relative to the image plane during focusing. The first lens group is composed of, in order from the object side to the image side, a first a group having negative refractive power, an aperture stop, and a first b group having positive refractive power. This configuration, particularly the configuration in which the first a group (i.e., all lenses included in the first lens group and arranged before the aperture stop) have negative refractive power, reduces various aberrations and facilitates compactness.

Here, a "lens group" is defined as consisting of one or more adjacent lenses, and the distance between adjacent lens groups along the optical axis changes during focusing. When a lens group is composed of multiple lenses, the distance on the optical axis between the lenses included in that lens group does not change during focusing.

There are no particular limitations on the 1a group of the first lens group as long as it has negative refractive power, but it is preferable that the lens component located closest to the object in 1a group (hereinafter also referred to as the "most object-side lens component") have negative refractive power. Here, "lens component" refers to a single lens, a cemented lens in which multiple single lenses are integrated with no air gap between them, or a compound lens in which a single lens and resin are integrated with no air gap between them. If the most object-side lens component is a cemented lens or compound lens, there are no particular limitations on the configuration, as long as the cemented lens or compound lens as a whole has negative refractive power.

(2) Second Lens Group The second lens group is not particularly limited in its specific configuration, as long as it has positive refractive power and moves along the optical axis during focusing. It is preferable that the second lens group be composed of a single lens component, since it moves quickly along the optical axis during focusing. It is also preferable that the second lens group have a biconvex lens closest to the image side. This configuration allows for easy size reduction while still providing aberration correction.

(3) Third Lens Group The specific configuration of the third lens group is not particularly limited, as long as it has negative refractive power and is fixed relative to the image plane during focusing. It is preferable that the third lens group has a biconcave lens positioned closest to the image. This configuration makes it easy to reduce the overall length while correcting distortion. Furthermore, in order to achieve compactness, it is preferable that the third lens group be composed of only one lens component.

(4) Aperture Stop In the optical system, the specific configuration of the aperture stop is not particularly limited as long as it is located within the first lens group. By locating the aperture stop within the first lens group, aberrations before and after the aperture stop can be efficiently canceled out, which is preferable for obtaining an optical system with high optical performance.

1-2. Operation (1) Focusing The specific operation of the optical system is not particularly limited as long as the second lens group moves on the optical axis when focusing from infinity to a close distance. Furthermore, a configuration in which the second lens group moves on the optical axis toward the object side when focusing from infinity to a close distance is preferable.

1-3. Formula It is desirable that the optical system employs the above-described configuration and satisfies at least one of the following formulas.

1-3-1. Formula (1)
-1.10 ≦ f1a/f1 ≦ -0.05 (1)
however,
f1a: focal length of the 1a group f1: focal length of the 1st lens group

Equation (1) defines the ratio between the focal length of the first lens group and the focal length of the 1a group, which is located closer to the object than the aperture stop. Satisfying equation (1) facilitates compactness while providing good correction for various aberrations.

Below the lower limit of formula (1), the refractive power of group 1a becomes weak, making it impossible to sufficiently reduce the diameter of the lens group located closer to the object than the aperture stop. This also hinders efforts to make the lens barrel more compact. On the other hand, above the upper limit of formula (1), the refractive power of group 1a becomes strong, making it difficult to correct aberrations such as coma and distortion.

To achieve the above effect, the lower limit of formula (1) is preferably -1.00, and more preferably -0.95. Furthermore, the upper limit of formula (1) is preferably -0.10, and more preferably -0.20. When adopting these preferred lower or upper limits, the inequality sign (≦) in formula (1) may be replaced with an inequality sign (<). The same principle applies to other formulas.

1-3-2. Formula (2)
0.05 ≦ f1/f ≦ 4.30 (2)
however,
f: focal length of the optical system when focused at infinity

Equation (2) defines the ratio of the focal length of the first lens group to the focal length of the optical system when focused at infinity. Satisfying equation (2) makes it easy to reduce the size of the first lens group while effectively correcting various aberrations.

Below the lower limit of formula (2), the refractive power of the first lens group becomes too strong, making it difficult to correct various aberrations such as spherical aberration and coma. On the other hand, above the upper limit of formula (2), the refractive power of the first lens group becomes too weak, making it difficult to reduce the diameter of the second lens group. This also leads to an increase in the size of the drive unit, making it difficult to reduce the size of the lens barrel.

To achieve the above effect, the lower limit of formula (2) is preferably 0.10, more preferably 0.20, and even more preferably 1.00. Furthermore, the upper limit of formula (2) is preferably 4.00, and even more preferably 3.70.

1-3-3. Formula (3)
1.10 ≦ (1-β2 ² )×β3 ² ≦ 5.00 (3)
however,
β2: Lateral magnification of the second lens group when focusing at infinity β3: Lateral magnification of the third lens group when focusing at infinity

Equation (3) defines the absolute value of the focus sensitivity of the second lens group, which moves along the optical axis during focusing; in other words, the amount of image plane movement when the second lens group moves a unit amount. By satisfying equation (3), the amount of movement during focusing can be reduced, making it easier to reduce the size of the lens barrel.

On the other hand, if the value of equation (3) falls below the lower limit, the amount of movement of the second lens group along the optical axis during focusing becomes large, making it difficult to reduce the overall optical length. On the other hand, if the value of equation (3) exceeds the upper limit, the amount of movement of the second lens group required to correct a misalignment of the focus position becomes too small, requiring highly accurate control, which is undesirable.

To achieve the above effect, the lower limit of formula (3) is preferably 1.20, and more preferably 1.30. Furthermore, the upper limit of formula (3) is preferably 4.50, and more preferably 4.00.

1-3-4. Formula (4)
1.60 ≦ Nd11 ≦ 2.15 (4)
however,
Nd11: refractive index at d-line of the lens component arranged closest to the object side in the first lens group (lens component closest to the object side)

Equation (4) defines the refractive index at the d-line of the lens component located closest to the object. Satisfying equation (4) facilitates compactness while effectively correcting various aberrations.

Below the lower limit of equation (4), the refractive index of the lens component closest to the object is small, which increases the diameter of the front lens, making it difficult to reduce the diameter of the lens barrel. On the other hand, exceeding the upper limit of equation (4) makes it difficult to correct field curvature aberration, making it difficult to achieve high optical performance. When the lens component is a cemented lens, it may be made up of two lenses, or three or more lenses. Furthermore, when the lens component located closest to the object in the first lens group is a cemented lens, it is preferable that the lens closest to the object in the cemented lens satisfies equation (4). Furthermore, it is preferable that the negative lens in the cemented lens satisfies equation (4). Furthermore, when the lens component located closest to the object in the first lens group is a compound lens, it is preferable that the base lens in the compound lens satisfies equation (4).

To achieve the above effect, the lower limit of formula (4) is preferably 1.65, and more preferably 1.70. The upper limit of formula (4) is preferably 2.10, and more preferably 2.05.

1-3-4. Formula (5)
0.50 ≦ f11/f1a ≦ 1.70 (5)
however,
f11: focal length of the lens component arranged closest to the object in the first lens group (the lens component closest to the object)

Equation (5) defines the ratio of the focal length of the lens component located closest to the object in the first lens group to the focal length of group 1a. Satisfying equation (5) facilitates compactness while providing good correction for various aberrations.

Below the lower limit of formula (5), the refractive power of the lens component located closest to the object in the first lens group becomes too strong, making it difficult to correct various aberrations such as astigmatism and chromatic aberration of magnification. On the other hand, above the upper limit of formula (5), the refractive power of the lens component located closest to the object in the first lens group becomes too weak, making it difficult to reduce the diameter of the front lens element. Furthermore, it becomes difficult to reduce the size of the lens barrel.

To achieve the above effect, the lower limit of formula (5) is preferably 0.55, more preferably 0.60, and even more preferably 0.80. Furthermore, the upper limit of formula (5) is preferably 1.65, and even more preferably 1.60.

1-3-6. Formula (6)
-1.25 ≦ f11/f1 ≦ -0.35 (6)

Equation (6) defines the ratio of the focal length of the first lens group to the lens component located closest to the object in the first lens group. Satisfying equation (6) facilitates compactness while providing good correction for various aberrations.

Below the lower limit of equation (6), the refractive power of the lens component located closest to the object in the first lens group becomes weak, making it difficult to reduce the diameter of the front lens insufficiently. It also makes it difficult to reduce the size of the lens barrel. On the other hand, above the upper limit of equation (6), the refractive power of the lens component located closest to the object in the first lens group becomes strong, making it difficult to correct various aberrations such as astigmatism and chromatic aberration of magnification.

To achieve the above effect, the lower limit of formula (6) is preferably -1.20, more preferably -1.15, and even more preferably -1.00. Furthermore, the upper limit of formula (6) is preferably -0.40, and more preferably -0.45.

1-3-7. Formula (7)
-3.45 ≦ f3/f ≦ -1.35 (7)
however,
f3: focal length of the third lens group

Equation (7) defines the ratio of the focal length of the third lens group, which is located closest to the object, to the focal length of the optical system when focused at infinity. Satisfying equation (7) facilitates compactness while providing good correction for various aberrations.

Below the lower limit of formula (7), the refractive power of the third lens group becomes weak, making it difficult to reduce the diameter of the rear lens element sufficiently. It also becomes difficult to reduce the size of the lens barrel. On the other hand, above the upper limit of formula (7), the refractive power of the third lens group becomes strong, making it difficult to correct various aberrations such as astigmatism and chromatic aberration of magnification.

To achieve the above effect, the lower limit of formula (7) is preferably -3.40, and more preferably -3.30. Furthermore, the upper limit of formula (7) is preferably -1.40, and more preferably -1.50, and even more preferably -2.00.

2. Imaging Device Next, we will explain the imaging device according to the present invention. The imaging device according to the present invention is characterized by comprising the optical system according to the present invention described above and an imaging element that converts an optical image formed by the optical system into an electrical signal. It is preferable that the imaging element is provided on the image side of the optical system.

Here, the imaging element is not particularly limited, and solid-state imaging elements such as CCD (Charge Coupled Device) sensors and CMOS (Complementary Metal Oxide Semiconductor) sensors can also be used. The imaging device of the present invention is suitable for imaging devices using these solid-state imaging elements, such as digital cameras and video cameras. The imaging device can also be applied to various imaging devices, such as single-lens reflex cameras, mirrorless single-lens cameras, digital still cameras, surveillance cameras, in-vehicle cameras, and drone-mounted cameras. These imaging devices may be interchangeable-lens imaging devices or fixed-lens imaging devices in which the lens is fixed to the housing. The optical system of the present invention is particularly suitable for imaging devices equipped with large-sized imaging elements, such as full-frame cameras. Because the optical system is compact and lightweight overall and has high optical performance, it can produce high-quality images even when used as an optical system for such imaging devices.

Figure 11 is a diagram schematically illustrating an example of the configuration of an imaging device according to this embodiment. As shown in Figure 11, imaging device 1 has a camera 2 and a lens 3 that is detachable from camera 2. Imaging device 1 is one form of imaging device.

The camera 2 has a CCD sensor 21 as an imaging element and a cover glass 22. The CCD sensor 21 is positioned within the camera 2 at a position where the optical axis of the optical system within the lens 3 attached to the camera 2 is the central axis. The camera 2 may have an IR cut filter or the like instead of the cover glass 22.

Next, the present invention will be explained in detail using examples. However, the present invention is not limited to the following examples.

(1) Optical Configuration Figure 1 is a cross-sectional view of an optical system of Example 1 according to the present invention when focused at infinity. The optical system is composed of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having negative refractive power.

When focusing from an object at infinity to a close object, the second lens group G2 moves from the image side to the object side along the optical axis.

The first lens group G1 is composed of, from the object side, a 1a group G1a, an aperture stop S, and a 1b group G1b. The 1a group G1a is composed of, from the object side, a negative meniscus lens (the lens component closest to the object), a negative meniscus lens, and a cemented lens formed by cementing a biconcave lens and a biconvex lens, while the 1b group G1b is composed of, from the object side, a cemented lens formed by cementing a negative meniscus lens and a biconvex lens, a positive meniscus lens, a biconvex lens, and a biconcave lens.

The second lens group G2 is composed only of biconvex lenses.

The third lens group G3 consists solely of a biconcave hybrid aspherical lens with an aspherical surface on the object side.

In Figure 1, "IP" stands for image plane, and specifically refers to the imaging surface of a solid-state imaging device such as a CCD sensor or CMOS sensor, or the film surface of a silver halide film. A cover glass CG is provided on the object side of the image plane IP. This is the same for the lens cross-sectional views shown in other embodiments, so further explanation will be omitted.

(2) Numerical Examples Next, numerical examples to which specific numerical values of the optical system are applied will be described. The "lens data,""specificationtable,""variablespacing,""asphericalcoefficients," and "focal lengths of each lens group" are shown below. The values of each formula (Table 1) are also summarized after Example 5. In the following numerical examples, all length units are "mm," and all angle units are "°."

In (Lens Data), "Surface No." indicates the order of the lens surface counted from the object side, "r" indicates the radius of curvature of the lens surface, "D" indicates the lens thickness or air space on the optical axis, "Nd" indicates the refractive index at the d-line (wavelength λ=587.56 nm), and "vd" indicates the Abbe number at the d-line. In the "Surface No." column, an "*" next to a number indicates that the lens surface is aspherical, and "S" indicates that the surface is an aperture stop S. In the "D" column, "D(7)," "D(10)," etc. indicate that the spacing on the optical axis of the lens surface is a variable spacing that changes when focusing. In the "Radius of Curvature" column, "∞" indicates that the lens surface is flat.

In the specifications table, "f" is the focal length of the optical system, "Fno." is the F-number, and "ω" is the half angle of view. These values are for infinity and close-up focusing, respectively.

(Variable interval) shows the values when focused at infinity and when focused at close range.

(Aspherical coefficients) indicate aspherical coefficients when the aspherical shape is defined as follows: where x is the amount of displacement from the reference surface in the optical axis direction, r is the paraxial radius of curvature, H is the height from the optical axis in a direction perpendicular to the optical axis, K is a conical coefficient, and An is an n-th order aspherical coefficient. In the "Aspherical coefficients" table, "E±XX" represents exponential notation, meaning "×10 ^±XX ."

The details of these numerical examples are similar to those of other examples, so further explanation will be omitted.

Figure 2 also shows longitudinal aberration diagrams of this optical system when focused on an object at infinity. The longitudinal aberration diagrams shown in each diagram, from left to right, represent spherical aberration (mm), astigmatism (mm), and distortion (%). In the spherical aberration diagram, the solid line represents spherical aberration at the d-line (wavelength 587.56 nm), the long dashed line represents spherical aberration at the F-line (wavelength 486.13 nm), and the short dashed line represents spherical aberration at the C-line (wavelength 656.28 nm). In the astigmatism diagram, the vertical axis represents half angle of view (ω) and the horizontal axis represents defocus. The solid line represents the sagittal image plane (S) for the d-line, and the dashed line represents the meridional image plane (T) for the d-line. In the distortion diagram, the vertical axis represents half angle of view (ω) and the horizontal axis represents distortion. These details are the same for the aberration diagrams shown in other examples, so further explanation will be omitted.

(lens data)
Surface No. r D Nd vd
1 23.0814 1.3000 1.69680 55.46
2 9.5000 3.8122
3* 23.4834 0.9000 1.59201 67.02
4* 10.4049 3.3950
5 -77.3884 0.8100 1.49700 81.61
6 22.7686 2.8852 1.91268 31.83
7 -112.9005 2.5831
8S∞2.0049
9 27.2055 0.8000 1.76729 25.24
10 9.2640 3.5569 1.51641 77.12
11 -47.6755 3.2862
12 -249.3209 2.7156 1.92042 21.15
13 -19.8093 0.5217
14* 51.7169 4.5570 1.49710 81.56
15* -12.1735 0.1000
16 -21.1008 0.8000 1.71309 27.86
17 20.1206 D(17)
18 33.4718 5.7074 1.49700 81.61
19 -22.6116 D(19)
20* -138.8796 0.2000 1.53610 41.21
21 -66.7308 1.0000 1.90043 37.37
22 70.0000 15.0744
23 ∞ 2.5000 1.51680 64.20
24∞1.0000

(Specifications table)
f 16.4814 15.8322
Fno. 2.8843 2.8961
ω 54.9227 54.8843

(variable interval)
Magnification ∞ -0.1154
D(17) 3.7929 2.7838
D(19) 2.4975 3.5066

(aspherical coefficients)
Surface No. K A4 A6 A8 A10
3 0.00000E+00 6.82780E-05 -9.99113E-07 5.12613E-09 0.00000E+00
4 0.00000E+00 7.14175E-05 -1.14562E-06 -5.34864E-09 0.00000E+00
14 0.00000E+00 -2.92014E-05 -2.49463E-07 -6.03768E-11 0.00000E+00
15 0.00000E+00 9.17837E-05 -3.87076E-07 2.46877E-09 0.00000E+00
20 0.00000E+00 -3.02348E-05 -1.12435E-07 1.17164E-10 0.00000E+00

(focal length of each lens group)
G1 30.706
G2 28.103
G3 -44.933

(1) Optical Configuration Figure 3 is a cross-sectional view of an optical system according to Example 2 of the present invention when focused at infinity. This optical system is composed of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having negative refractive power.

When focusing from an object at infinity to a close object, the second lens group G2 moves from the image side to the object side along the optical axis.

The first lens group G1 is composed of, from the object side, a 1a group G1a, an aperture stop S, and a 1b group G1b. The 1a group G1a is composed of, from the object side, a negative meniscus lens (the lens component closest to the object side), a negative meniscus lens, and a cemented lens formed by cementing a biconcave lens and a biconvex lens. The 1b group G1b is composed of, from the object side, a cemented lens formed by cementing a negative meniscus lens and a biconvex lens, a biconvex lens, a biconvex lens, a biconvex lens, and a biconcave lens.

The second lens group G2 is composed only of biconvex lenses.

The third lens group G3 consists solely of a biconcave hybrid aspherical lens with an aspherical surface on the object side.

(2) Numerical Examples Numerical examples using specific numerical values of the optical system will now be described. Also, Fig. 4 shows longitudinal aberration diagrams of the optical system when focused at infinity.

(lens data)
Surface No. r D Nd vd
1 22.7424 1.3000 1.69680 55.46
2 8.8352 4.6915
3* 32.5332 0.9000 1.59201 67.02
4* 11.2658 2.8359
5 -406.9572 0.8100 1.49700 81.61
6 20.6064 3.2305 1.91082 35.25
7 -94.4209 3.5321
8S∞2.0000
9 41.7689 0.8000 1.75211 25.05
10 10.3064 3.6808 1.49700 81.61
11 -27.2522 2.3793
12 110.4840 2.7446 1.86966 20.02
13 -22.5696 0.6460
14* 74.7953 3.8332 1.49710 81.56
15* -13.1969 0.2000
16 -23.4567 0.8000 1.78472 25.72
17 21.8059 D(17)
18 48.2189 5.1869 1.49700 81.61
19 -19.6894 D(19)
20* -176.9605 0.2000 1.53610 41.21
21 -70.9987 1.0000 1.90043 37.37
22 73.9218 15.0000
23 ∞ 2.5000 1.51680 64.20
24∞1.0000

(Specifications table)
f 15.8110 15.2446
Fno. 2.8840 2.8995
ω 56.0468 56.1871

(variable interval)
Magnification ∞ -0.1107
D(17) 4.0290 3.0446
D(19) 2.5004 3.4848

(aspherical coefficients)
Surface No. K A4 A6 A8 A10
3 0.00000E+00 7.26547E-05 -1.02670E-06 4.33587E-09 0.00000E+00
4 0.00000E+00 6.23251E-05 -1.15432E-06 -1.27734E-08 0.00000E+00
14 0.00000E+00 -2.26052E-05 -1.58588E-07 -8.32590E-10 0.00000E+00
15 0.00000E+00 8.36388E-05 -4.47202E-07 1.13438E-09 0.00000E+00
20 0.00000E+00 -3.87171E-05 -9.81562E-08 -3.87217E-10 0.00000E+00

(focal length of each lens group)
G1 28.312
G2 28.862
G3 -49.017

(1) Optical Configuration Figure 5 is a cross-sectional view of an optical system according to Example 3 of the present invention when focused at infinity. This optical system is composed of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having negative refractive power.

When focusing from an object at infinity to a close object, the second lens group G2 moves from the image side to the object side along the optical axis.

The first lens group G1 is composed of, from the object side, a 1a group G1a, an aperture stop S, and a 1b group G1b. The 1a group G1a is composed of, from the object side, a negative meniscus lens (the lens component closest to the object), a negative meniscus lens, and a cemented lens formed by cementing a negative meniscus lens and a positive meniscus lens. The 1b group G1b is composed of, from the object side, a cemented lens formed by cementing a negative meniscus lens and a biconvex lens, a biconvex lens, a biconvex lens, and a biconcave lens.

The second lens group G2 is composed only of biconvex lenses.

The third lens group G3 consists solely of a biconcave hybrid aspherical lens with an aspherical surface on the object side.

(2) Numerical Examples Numerical examples using specific numerical values of the optical system will now be described. Also, Fig. 6 shows longitudinal aberration diagrams of the optical system when focused at infinity.

(lens data)
Surface No. r D Nd vd
1 22.4358 1.3000 1.77250 49.62
2 8.2652 4.1325
3* 20.7394 0.9000 1.61881 63.86
4* 11.4094 2.5630
5 140.2929 0.8100 1.49700 81.61
6 17.3243 4.3457 1.90366 31.32
7 44.2243 2.5000
8S∞2.4758
9 18.9894 0.8000 1.84666 23.78
10 10.9150 4.0175 1.49700 81.61
11 -26.7127 2.3720
12 49.2918 2.8745 1.86966 20.02
13 -26.5307 0.2000
14* 47.0076 2.9932 1.49710 81.56
15* -18.5096 0.2000
16 -26.0067 0.8000 1.84666 23.78
17 18.5284 D(17)
18 34.4262 5.4142 1.49700 81.61
19 -18.5986 D(19)
20* -1251.3915 0.2000 1.53610 41.21
21 -88.7095 1.0000 1.90043 37.37
22 49.2248 16.8333
23 ∞ 2.5000 1.51680 64.20
24∞1.0000

(Specifications table)
f 16.1597 15.6244
Fno. 2.8840 2.9208
ω 55.2222 55.2293

(variable interval)
Magnification ∞ -0.1154
D(17) 3.6817 2.8587
D(19) 3.0869 3.9100

(aspherical coefficients)
Surface No. K A4 A6 A8 A10
3 0.00000E+00 1.27560E-04 -8.33243E-07 -9.28556E-09 0.00000E+00
4 0.00000E+00 9.45114E-05 -5.25690E-07 -4.74923E-08 0.00000E+00
14 0.00000E+00 -4.15675E-05 -3.56142E-07 -9.80924E-10 0.00000E+00
15 0.00000E+00 2.82561E-05 -4.19613E-07 1.24271E-12 0.00000E+00
20 0.00000E+00 -6.36104E-05 -1.38232E-07 2.14449E-10 0.00000E+00

(focal length of each lens group)
G1 42.008
G2 25.149
G3 -43.721

7 is a cross-sectional view of an optical system according to Example 4 of the present invention when focused at infinity. The optical system is composed of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having negative refractive power.

When focusing from an object at infinity to a close object, the second lens group G2 moves from the image side to the object side along the optical axis.

The first lens group G1 is composed of, from the object side, a 1a group G1a, an aperture stop S, and a 1b group G1b. The 1a group G1a is composed of, from the object side, a negative meniscus lens (the lens component closest to the object), a negative meniscus lens, and a biconvex hybrid aspherical lens with an aspherical surface on the object side, while the 1b group G1b is composed of, from the object side, a biconvex lens and a cemented lens formed by cementing a positive meniscus lens and a negative meniscus lens.

The second lens group G2 is composed of, from the object side, a biconcave aspherical lens and a biconvex aspherical lens.

The third lens group G3 consists solely of a biconcave hybrid aspherical lens with an aspherical surface on the object side.

(2) Numerical Examples Numerical examples using specific numerical values of the optical system will now be described. Also, Fig. 8 shows longitudinal aberration diagrams of the optical system when focused at infinity.

(lens data)
Surface No. r D Nd vd
1 19.8611 1.2000 1.88300 40.81
2 9.4923 5.8751
3 156.9035 1.0000 1.49700 81.61
4 10.4878 2.6977
5* 55.2977 0.2000 1.53610 41.21
6 29.4087 2.3472 1.91082 35.25
7 -84.5403 6.0300
8S∞1.5000
9 25.4275 3.7629 1.49700 81.61
10 -14.7214 0.6836
11 -32.6671 4.0004 1.78590 43.93
12 -8.2866 0.7000 1.88100 40.13
13 -30.6235 D(13)
14* -64.1141 0.8000 1.80139 45.45
15* 81.9216 0.7144
16* 29.8488 6.7390 1.49700 81.61
17* -10.1153 D(17)
18* -53.8636 0.2000 1.53610 41.21
19 -40.2005 1.0000 1.64769 33.84
20 23.5243 20.2092
21 ∞ 2.5000 1.51680 64.20
22∞1.0000

(Specifications table)
f 16.4811 15.8986
Fno. 2.8878 2.9113
ω 54.3806 54.6280

(variable interval)
Magnification ∞ -0.0905
D(13) 2.6917 2.2614
D(17) 1.9992 2.4296

(aspherical coefficients)
Surface No. K A4 A6 A8 A10
5 0.00000E+00 4.45639E-05 8.12432E-07 -1.14394E-08 1.96460E-10
14 0.00000E+00 1.44370E-04 -2.84799E-06 -5.84227E-08 5.20571E-10
15 0.00000E+00 1.85613E-04 1.88726E-06 -1.35773E-07 1.20959E-09
16 0.00000E+00 -1.27270E-04 5.40504E-06 -9.21760E-08 4.95535E-10
17 0.00000E+00 9.44297E-05 -4.03657E-07 7.01258E-09 0.00000E+00
18 0.00000E+00 5.77689E-06 -4.71936E-07 0.00000E+00 0.00000E+00

(focal length of each lens group)
G1 22.363
G2 21.72
G3 -24.683

9 is a cross-sectional view of an optical system according to Example 5 of the present invention when focused at infinity. The optical system is composed of, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having negative refractive power.

When focusing from an object at infinity to a close object, the second lens group G2 moves from the image side to the object side along the optical axis.

The first lens group G1 is composed of, from the object side, a 1a group G1a, an aperture stop S, and a 1b group G1b. The 1a group G1a is composed of, from the object side, a negative meniscus lens (the lens component closest to the object side), a negative meniscus lens, and a biconvex hybrid aspherical lens with an aspherical surface on the object side, while the 1b group G1b is composed of, from the object side, a biconvex lens and a cemented lens formed by cementing a biconvex lens and a biconcave lens.

The second lens group G2 is composed of, from the object side, a biconcave aspherical lens and a biconvex aspherical lens.

The third lens group G3 consists solely of a biconcave hybrid aspherical lens with an aspherical surface on the object side.

(2) Numerical Examples Numerical examples using specific numerical values of the optical system will now be described. Also, Fig. 10 shows longitudinal aberration diagrams of the optical system when focused at infinity.

(lens data)
Surface No. r D Nd vd
1 19.1175 1.2000 1.88300 40.81
2 9.1370 6.0287
3 215.1384 1.0000 1.49700 81.61
4 10.9693 2.5426
5* 59.9342 0.2000 1.53610 41.21
6 34.0236 2.3466 1.91082 35.25
7 -59.3684 6.0321
8S∞1.5000
9 35.1168 3.5416 1.49700 81.61
10 -14.9146 0.6046
11 309.6062 4.9668 1.80611 40.73
12 -8.5126 0.7000 1.88202 37.22
13 108.2794 D(13)
14* -125.0396 0.8000 1.76802 49.24
15* 115.7335 0.5563
16* 29.9394 6.8106 1.49700 81.61
17* -10.0903 D(17)
18* -57.3545 0.2000 1.53610 41.21
19 -41.4948 1.0000 1.64769 33.84
20 26.2967 19.6246
21 ∞ 2.5000 1.51680 64.20
22∞1.0000

(Specifications table)
f 16.4815 15.9592
Fno. 2.8840 2.8956
ω 54.3231 54.4370

(variable interval)
Magnification ∞ -0.0909
D(13) 2.6973 2.2663
D(17) 1.9988 2.4298

(aspherical coefficients)
Surface No. K A4 A6 A8 A10
5 0.00000E+00 3.92816E-05 9.14059E-07 -1.33687E-08 2.27928E-10
14 0.00000E+00 1.60363E-04 -2.38349E-06 -6.96530E-08 5.26355E-10
15 0.00000E+00 1.94354E-04 2.08207E-06 -1.47505E-07 1.27820E-09
16 0.00000E+00 -1.09702E-04 4.61856E-06 -8.14767E-08 4.40973E-10
17 0.00000E+00 1.07871E-04 -5.40057E-07 7.25856E-09 0.00000E+00
18 0.00000E+00 1.16402E-05 -5.68300E-07 0.00000E+00 0.00000E+00

(focal length of each lens group)
G1 42.848
G2 18.932
G3 -27.115

(Table 1)
Example 1 Example 2 Example 3 Example 4 Example 5
Formula (1) f1a/f1 -0.681 -0.800 -0.300 -0.900 -0.500
Formula (2) f1/f 1.863 1.791 2.600 1.357 2.600
Equation (3) (1-β2 ² )×β3 ² 1.669 1.549 1.948 3.201 3.201
Formula (4) Nd11 1.697 1.697 1.773 1.883 1.883
Formula (5) f11/f1a 1.154 0.952 1.400 1.082 0.980
Formula (6) f11/f1 -0.785 -0.762 -0.420 -0.974 -0.490
Formula (7) f3/f -2.726 -3.100 -2.706 -1.498 -1.645
f 16.482 15.811 16.160 16.481 16.482
f1 30.706 28.312 42.008 22.363 42.848
f1a -20.899 -22.652 -12.604 -20.127 -21.424
f11 -24.118 -21.563 -17.645 -21.773 -21.005
f3 -44.933 -49.017 -43.721 -24.683 -27.115
β2 0.384 0.409 0.266 0.381 0.210
β3 1.399 1.364 1.448 1.935 1.830

The optical system according to the present invention can be suitably applied as an optical system for imaging devices such as film cameras, digital still cameras, and digital video cameras.

S...aperture stop CG...cover glass IP...image surface G1...first lens group G2...second lens group G3...third lens group G1a...1a group G1b...1b group 1...imaging device 2...camera 3...lens 21...CCD sensor or CMOS sensor 22...cover glass or IR cut filter

Claims (5)

The lens comprises, in order from the object side to the image side, a first lens group having positive refractive power, a second lens group having positive refractive power, and a third lens group having negative refractive power,
During focusing, the first lens group and the third lens group are fixed in the optical axis direction with respect to an image plane, and the second lens group moves along the optical axis,
the first lens group is composed of, in order from the object side to the image side, a first a group having negative refractive power, an aperture stop, and a first b group having positive refractive power;
An optical system that satisfies the following equation:
-0.681 ≦ f1a/f1 ≦ -0.05 (1)
0.05 ≦ f1/f ≦ 2.60 (2)
1.669 ≦ (1-β2 ² )×β3 ² ≦ 5.00 (3)
1.60 ≦ Nd11 ≦ 2.15 (4)
however,
f1a: focal length of the 1a-th lens group f1: focal length of the 1st lens group
f: focal length of the optical system when focused at infinity
β2: lateral magnification of the second lens group when focused at infinity
β3: lateral magnification of the third lens group when focused at infinity
Nd11: the refractive index at the d line of the lens component arranged closest to the object in the first lens group 2. The optical system according to claim 1 , wherein the lens component arranged closest to the object side in the first lens group satisfies the following formula:
0.50 ≦ f11/f1a ≦ 1.70 (5)
however,
f11: focal length of the lens component located closest to the object in the first lens group 3. The optical system according to claim 1 , wherein the lens component arranged closest to the object side in the first lens group satisfies the following formula:
-1.25 ≦ f11/f1 ≦ -0.35 (6)
however,
f11: focal length of the lens component located closest to the object in the first lens group 4. The optical system according to claim 1, wherein the following formula is satisfied:
-3.45 ≦ f3/f ≦ -1.35 (7)
however,
f3: focal length of the third lens group f: focal length of the optical system when focused at infinity An imaging device comprising the optical system described in any one of claims 1 to 4 and an image sensor on the image side of the optical system that converts an optical image formed by the optical system into an electrical signal.