Optical system and imaging device

Abstract

To provide an optical system which achieves both reduction in size and correction of field curvature, and has a wide view angle.SOLUTION: An optical system is composed of a first lens group, an aperture diaphragm, and a second lens group. The first lens group has a first lens and a second lens from an object side. The second lens group has a third lens which is the second from an image side and a fourth lens which is a side closest to the image side. The first lens has negative refractive power on the axis, and positive refractive power on an off-axis, and the second lens has positive refractive power on the axis, and negative refractive power on the off-axis. The third lens has positive refractive power on the axis, and negative refractive power on the off-axis, and the fourth lens has negative refractive power on the axis, and positive refractive power on the off-axis. When an on-axis focal distance of the optical system is represented by f, and an off-axis focal distance having largest negative refractive index of the second lens when the maximum image height in the second lens is divided into 20 is represented by fn2, and when an off-axis focal distance at a maximum view angle ω of the optical system is represented by fω, conditions of -2.4≤fn2 / f<0.0 and 0.1≤fω / f≤2.1 are satisfied.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical system suitable for various imaging devices. [Background technology]

[0002] In order to shorten the overall length of a wide-angle optical system, it is necessary to reduce the thickness of the lenses constituting the optical system and the distance between the lenses. It is also generally known that in a wide-angle optical system, the curvature of field and distortion can be corrected by arranging a lens group closer to the object side than the aperture stop. However, this configuration tends to make the optical system larger.

[0003] Patent Document 1 discloses an optical system having, arranged in order from the object side to the image side, a first lens group with negative refractive power, a second lens group with positive refractive power, an aperture stop, and a lens group with negative refractive power. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Publication No. 2021 / 0199933 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a first lens group with strong negative refractive power and a second lens group with weak positive refractive power are arranged in that order from the object side as in the optical system disclosed in Patent Document 1, the thickness of the first lens group tends to increase, making it difficult to simultaneously achieve compactness of the optical system and correction of field curvature.

[0006] The present invention provides a wide-angle optical system that is capable of achieving both compactness and correction of field curvature. [Means for solving the problem]

[0007] An optical system according to one aspect of the present invention is composed of a first lens group, an aperture stop, and a second lens group, which are arranged in this order from the object side to the image side. The first lens group has a first lens and a second lens arranged in this order from the object side to the image side, and the second lens group has a third lens that is second from the image side and a fourth lens that is closest to the image side. The first lens has negative refractive power on the axis and positive refractive power off the axis, and the second lens has positive refractive power on the axis and negative refractive power off the axis. The third lens has positive refractive power on the axis and negative refractive power off the axis, and the fourth lens has negative refractive power on the axis and positive refractive power off the axis. Let f be the axial focal length of the optical system, fn2 be the off-axial focal length at which the second lens has the largest negative refractive power when the maximum image height is divided by 20, and fω be the off-axial focal length at the maximum angle of view ω of the optical system. -2.4≦fn2 / f<0.0 0.1≦fω / f≦2.1 The optical system according to the present invention is characterized in that it satisfies the following conditions: An image pickup apparatus including the optical system described above also constitutes another aspect of the present invention. Effect of the Invention

[0008] According to the present invention, it is possible to achieve both size reduction and correction of field curvature in a wide-angle optical system. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a cross-sectional view of the optical system of the first embodiment. [Diagram 2] 4A to 4C are longitudinal aberration diagrams of the optical system of Numerical Example 1. [Diagram 3] FIG. 11 is a cross-sectional view of an optical system according to a second embodiment. [Figure 4] 6A to 6C are longitudinal aberration diagrams of the optical system of Numerical Example 2. [Diagram 5] FIG. 11 is a cross-sectional view of the optical system of the third embodiment. [Figure 6] 11A to 11C are longitudinal aberration diagrams of the optical system of Numerical Example 3. [Figure 7] FIG. 11 is a cross-sectional view of an optical system according to a fourth embodiment. [Figure 8] 11A to 11C are longitudinal aberration diagrams of the optical system of Numerical Example 4. [Figure 9] FIG. 13 is a cross-sectional view of the optical system of the fifth embodiment. [Figure 10] 11A to 11C are longitudinal aberration diagrams of the optical system of Numerical Example 5. [Figure 11] FIG. 13 is a cross-sectional view of the optical system of the sixth embodiment. [Figure 12] FIG. 11 is a longitudinal aberration diagram of the optical system of Numerical Example 6. [Figure 13] FIG. 1 is a diagram showing hit points of off-axis rays on an optical surface. [Figure 14] FIG. 1 is a diagram showing an imaging device using the optical systems according to Examples 1 to 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0011] First, before describing specific Examples 1 to 6, matters common to each of the Examples will be described. Figures 1, 3, 5, 7, 9, and 11 respectively show cross sections of the optical systems of Examples 1, 2, 3, 4, 5, and 6 in a state where the optical systems are focused on an object at infinity (hereinafter, referred to as an infinity focused state). The optical systems of each of the Examples are used in imaging devices such as digital still cameras, digital video cameras, surveillance cameras, vehicle-mounted cameras, and smartphone cameras.

[0012] In each figure, the left side is the object side and the right side is the image side. The optical system of each embodiment has a first lens group L1, an aperture stop SP, and a second lens group L2 arranged in this order from the object side to the image side. Each lens group includes at least two lenses. The lens group may include an aspheric lens that has no refractive power on the optical axis (hereinafter also referred to as on-axis), that is, an aspheric lens with infinite curvature on the optical axis.

[0013] The second lens group L2 of the optical system in each embodiment includes at least one negative lens, of which the negative lens Gn is disposed closest to the object side. The negative lens Gn will be described later.

[0014] Furthermore, each lens group in the optical system of each embodiment includes at least one aspheric surface having an inflection point, which is a point where the sign of the refractive power of the lens changes from on-axis to off-axis.

[0015] In each diagram, FL is an optical block that corresponds to an optical filter, a low-pass filter, an infrared cut filter, etc. IP is an image plane. The image plane IP is where the imaging surface (light receiving surface) of an imaging element (photoelectric conversion element) such as a CCD sensor or a CMOS sensor, or the film surface (photosensitive surface) of a silver halide film is located.

[0016] Here, the "off-axis focal length" used in each embodiment will be described. The "off-axis focal length" is the focal length of an off-axis ray that is incident at an angle ω [°] with respect to the optical axis and passes through the center of the aperture stop SP to form an image on the image side, when the maximum half angle of view of an optical system with an axial focal length of f0 is ω [°].

[0017] Fig. 13 shows a method for calculating the off-axis focal length. In Fig. 13, Gi is an optical surface (lens surface). The optical axis direction on the optical surface is defined as the Z direction, and two directions that are perpendicular to the Z direction and perpendicular to each other are defined as the X direction and the Y direction. When the Z direction is parallel to the horizontal direction, the X direction is also parallel to the horizontal direction, and the Y direction is parallel to the vertical direction.

[0018] The intersection point between the axial ray and the optical surface Gi is defined as hp_0. The intersection point between the off-axis ray incident at the maximum angle of view ω [°] and the optical surface Gi is defined as the hit point hp_ω. When the curvature near the off-axis chief ray at the hit point hp_ω is calculated, the curvature differs depending on the azimuth angle. As described in Reference 1 below, the focal length for the off-axis chief ray can be obtained by calculating the curvature at the hit point.

[0019] (Reference 1) Keisuke Araki, “Development of non-coaxial optical systems for imaging systems”, Optics, Optical Society of Japan, June 2008, Vol. 37, No. 6, pp. 334-339 Next, the characteristic configuration of the optical system of each embodiment will be described. In the optical system of each embodiment, the first lens group L1 has a first lens and a second lens arranged in this order from the object side to the image side. The first lens has negative refractive power on the axis (center) and positive refractive power off the axis (periphery). This first lens is an aspheric lens having an inflection point where the refractive power changes from negative to positive from on the axis to off the axis. The second lens has positive refractive power on the axis and negative refractive power off the axis. This second lens has a refractive power that changes from positive to negative from on the axis to off the axis without having an inflection point.

[0020] The second lens group L2 has a third lens arranged second from the image side and a fourth lens arranged closest to the image side. The third lens has positive refractive power on the axis and negative refractive power off-axis. This third lens is an aspheric lens having an inflection point where the refractive power changes from positive to negative from on-axis to off-axis. The fourth lens has negative refractive power on the axis and positive refractive power off-axis. This fourth lens is an aspheric lens having an inflection point where the refractive power changes from negative to positive from on-axis to off-axis.

[0021] The optical system of each embodiment configured as described above satisfies the condition of the following formula (1), where f is the on-axis focal length of the optical system (the focal length at an angle of view of 0°) and fn2 is the minimum negative off-axis focal length of the second lens when the maximum image height of the second lens is divided by 20.

[0022] -2.4≦fn2 / f<0.0 (1) Condition (1) indicates a preferable range of the ratio between the on-axis focal length of the optical system and the off-axis focal length of the second lens. Since the optical system includes an aspheric surface, the focal length changes greatly at each image height. Therefore, fn2 is set to the off-axis focal length at which the negative refractive power is the largest when the maximum image height is divided into 20 equal parts. If fn2 / f is below the lower limit of formula (1), the refractive power of the second lens becomes small and the curvature of field increases in the negative direction, which is not preferable. If fn2 / f is above the upper limit of formula (1), positive curvature of field occurs, making it difficult to correct the curvature of field and reduce the size of the optical system (shorten the overall length), which is not preferable.

[0023] Furthermore, the optical system of each embodiment satisfies the condition of the following formula (2), where fω is the off-axis focal length at the maximum angle of view ω [°] of the optical system.

[0024] 0.1≦fω / f≦2.1 (2) The condition of formula (2) shows the preferable range of the ratio of the axial focal length to the off-axial focal length of the optical system. If fω / f is below the lower limit of formula (2), the off-axial refractive power becomes stronger relative to the axial refractive power of the optical system, which increases the overall length of the optical system and makes it difficult to reduce the size, which is not preferable. If fω / f is above the upper limit of formula (2), the off-axial refractive power becomes weaker relative to the axial refractive power of the optical system, which reduces distortion aberration but significantly reduces the amount of peripheral light, making it difficult to correct field curvature, which is not preferable.

[0025] As described above, by satisfying the conditions of expressions (1) and (2), it is possible to achieve both size reduction and correction of field curvature in a wide-angle optical system.

[0026] Moreover, it is preferable that the optical system of each embodiment satisfies at least one of the conditions of the following expressions (3) to (5).

[0027] The lens thickness of the first lens (the length on the optical axis from the lens surface on the object side to the lens surface on the image side) is t12, and the total length of the optical system (the length on the optical axis from the lens surface closest to the object side to the image surface) is TTL. In this case, it is preferable to satisfy the condition of the following formula (3).

[0028] 0.01≦t12 / TTL≦0.10 (3) The condition of formula (3) shows a preferable range of the ratio of the lens thickness of the first lens to the total length of the optical system. If t12 / TTL is below the lower limit of formula (3), the lens thickness of the first lens will be small, which is undesirable because it becomes difficult to achieve both compactness and astigmatism correction. If t12 / TTL is above the upper limit of formula (3), the lens thickness of the first lens will be large relative to the total length, which is undesirable because it becomes difficult to achieve compactness.

[0029] In addition, the composite focal length of the first lens and the second lens is f0, and the composite focal length of the third lens and the fourth lens is f i In this case, it is preferable to satisfy the following expressions (4) and (5).

[0030] -5.0≦fо / f<0.0 (4) -15.0≦fi / f<0.0 (5) The condition of formula (4) indicates a preferable range of the ratio between the axial focal length of the optical system and the composite focal length of the first and second lenses. When fо / f is below the lower limit of formula (4), the refractive power of the first and second lenses becomes weak, which is undesirable because it becomes difficult to correct spherical aberration and curvature of field. When fо / f is above the upper limit of formula (4), the composite refractive power of the first and second lenses becomes positive, which is undesirable because it becomes difficult to correct astigmatism.

[0031] The condition of formula (5) indicates a preferable range of the ratio between the axial focal length of the optical system and the composite focal length of the third and fourth lenses. When fi / f is below the lower limit of formula (5), the refractive power of the third and fourth lenses is weakened, and distortion is reduced, but the peripheral light amount is significantly reduced, making it difficult to correct astigmatism, which is not preferable. When fi / f is above the upper limit of formula (5), the composite refractive power of the third and fourth lenses becomes positive, making it difficult to correct astigmatism, which is not preferable.

[0032] Furthermore, when the maximum image height of the optical system is ImgH, it is preferable to satisfy the condition of the following expression (6).

[0033] 1.2≦TTL / ImgH≦2.1 (6) The condition of formula (6) indicates a preferable range of the ratio between the maximum image height of the optical system and the total length of the optical system. If TTL / ImgH is below the lower limit of formula (6), the total length relative to the maximum image height becomes short, which is undesirable as it becomes difficult to correct aberrations. If TTL / ImgH is above the upper limit of formula (6), the total length relative to the maximum image height becomes large, which is undesirable as it becomes difficult to miniaturize the optical system.

[0034] Furthermore, when the Abbe number of the negative lens Gn in the second lens group L2 with respect to the d-line is taken as νN_2, it is preferable that the condition of the following formula (7) be satisfied.

[0035] 15.0≦νN_2≦40.0 (7) The condition of formula (7) indicates a preferable range of the Abbe number of the negative lens Gn. In order to satisfactorily correct axial chromatic aberration, it is preferable that the material of the negative lens Gn is a high-dispersion material that satisfies formula (7).

[0036] It is more preferable that the numerical ranges of the formulas (1) to (8) are as follows:

[0037] -2.39≦fn2 / f≦-0.30 (1a) 0.300≦fω / f≦2.095 (2a) 0.03≦t12 / TTL≦0.09 (3a) -4.0≦fо / f≦-1.0 (4a) -14.9≦fi / f≦-2.0 (5a) 1.50≦TTL / ImgH≦2.00 (6a) 16.0≦νN_2≦30.0 (7a) Moreover, it is more preferable that the numerical ranges of the formulas (1) to (8) are as follows:

[0038] -2.385≦fn2 / f≦-0.450 (1b) 0.45≦fω / f≦2.09 (2b) 0.05≦t12 / TTL≦0.08 (3b) -3.10≦fо / f≦-2.00 (4b) -14.80≦fi / f≦-4.00 (5b) 1.85≦TTL / ImgH≦1.98 (6b) 17.0≦νN_2≦25.0 (7a) Examples 1 to 6 will be specifically described below. The optical systems of Examples 1 to 5 are configured with a first lens group L1 consisting of a first lens, a second lens, and a positive lens, and a second lens group L2 consisting of an aperture stop SP, a positive lens, a negative lens Gn, a third lens, and a fourth lens, arranged in order from the object side to the image side. That is, the optical systems of Examples 1 to 5 are configured with seven lenses.

[0039] The optical system of Example 6 is composed of a first lens group L1 consisting of a first lens, a second lens, and a positive lens, and a second lens group L2 consisting of an aperture stop SP, a positive lens, a positive lens, a negative lens Gn, a third lens, and a fourth lens, which are arranged in this order from the object side to the image side. That is, the optical system of Example 6 is composed of eight lenses.

[0040] Numerical Examples 1 to 6 corresponding to Examples 1 to 6, respectively, are shown below. In each numerical example, surface number i indicates the order of the surface when counted from the object side. r is the radius of curvature (mm) of the i-th surface from the object side, d is the lens thickness or air space (mm) between the i-th and (i+1)-th surfaces, and nd is the refractive index at the d-line of the optical material between the i-th and (i+1)-th surfaces. νd is the Abbe number based on the d-line of the optical material between the i-th and (i+1)-th surfaces. The Abbe number νd based on the d-line is given by Nd, NF, and NC, respectively, when the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines are Nd, NF, and NC, respectively. νd=(Nd-1) / (NF-NC) It is expressed as:

[0041] In each numerical example, the half angle of view (°) of the optical system is shown, and the maximum image height corresponding to the half angle of view is shown as the "image height". In addition, in each numerical example, the focal length of each lens group at the d line is shown as lens group data. Note that d, focal length [mm], F-number, and half angle of view [°] are values ​​in the infinity focus state. BF represents back focus (mm). "Back focus" is the distance on the optical axis from the final surface (the lens surface closest to the image) of the optical system to the paraxial image surface expressed as an air-equivalent length. "Lens total length" is the distance on the optical axis from the foreground (the lens surface closest to the object) of the optical system to the final surface plus the back focus, and is also called the optical total length.

[0042] An "*" next to a surface number means that the surface has an aspheric shape. An aspheric shape is expressed by the following formula, where X is the displacement from the apex of the surface in the optical axis direction, h is the height from the optical axis in a direction perpendicular to the optical axis, the light traveling direction is positive, R is the paraxial radius of curvature, k is the conic constant, and A4 to A12 are aspheric coefficients. The conic constant and the aspheric coefficients "e±x" are expressed as x10 ±x means.

[0043] X = (h 2 / R) / [1+{1-(1+k)(h / R) 2}] 1 / 2 +A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 +A12×h 12 2, 4, 6, 8, 10, and 12 show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) in the infinity focused state of Numerical Examples 1 to 6, respectively. In the spherical aberration diagrams, Fno shows the F-number, the solid line shows the spherical aberration for the d-line (wavelength 587.6 nm), and the two-dot chain line shows the spherical aberration for the g-line (wavelength 435.8 nm). The horizontal axis shows the defocus amount, which is -0.100 to +0.100 [mm]. In the astigmatism diagrams, the solid line S shows the astigmatism on the sagittal image plane, and the dashed line M shows the astigmatism on the meridional image plane. The horizontal axis is the same as the spherical aberration. The distortion diagrams show the distortion at the d-line, which is -75.000 to +75.000 [%]. The chromatic aberration diagram shows the lateral chromatic aberration at the g-line, with the horizontal axis ranging from -0.020 to +0.020 [mm], and ω being the half angle of view (°). [Numerical example 1] Unit: mm Surface Data Surface number rd nd νd 1* -2.153 0.55 1.53504 55.7 2* -5.896 0.67 3* 3.383 0.45 1.66080 20.4 4* 3.945 0.28 5* -13.408 0.53 1.53504 55.7 6* -3.174 0.04 7 (Aperture) ∞ 0.08 8* 9.311 1.28 1.53504 55.7 9* -1.844 0.06 10* -25.275 0.38 1.68040 18.1 11* 5.395 0.28 12* 1.767 0.52 1.53504 55.7 13* 1.863 0.63 14* 3.261 0.50 1.61550 25.9 15* 1.814 (variable) 16 ∞ 0.50 1.51633 64.1 17 ∞ (variable) Image plane ∞ Aspheric Data Front page K =-8.90442e+00 A 4= 2.98003e-02 A 6=-4.90809e-03 A 8= 5.36491e-04 A10=-3.06997e-05 A12= 7.38643e-07 2nd side K =-9.50000e+01 A 4= 5.71748e-02 A 6=-1.03599e-02 A 8= 7.85535e-03 A10=-3.38782e-03 A12= 6.10530e-04 3rd page K = 4.82390e+00 A 4=-7.86027e-02 A 6= 5.11485e-02 A 8=-5.29483e-02 A10=2.87664e-02 A12=-8.03727e-03 Side 4 K = 1.20816e+01 A 4=-7.15570e-02 A 6= 9.49841e-02 A 8=-1.70955e-01 A10= 1.61951e-01 A12=-8.04839e-02 5th page K = 5.21346e+01 A 4=-3.88140e-02 A 6= 3.41892e-02 A 8=-9.47289e-02 A10=6.92657e-02 A12=-7.78674e-03 Side 6 K =-1.29526e+01 A 4=-5.42383e-02 A 6=-7.02349e-02 A 8= 2.83156e-01 A10=-5.34995e-01 A12= 3.71188e-01 Side 8 K = 3.89083e+01 A 4=-4.54594e-03 A 6=-3.18995e-02 A 8=-2.55284e-02 A10= 2.75181e-02 A12=-3.77724e-02 Page 9 K = 9.72904e-01 A 4= 5.02580e-02 A 6=-2.03537e-01 A 8= 2.44742e-01 A10=-1.41332e-01 A12= 3.32532e-02 Page 10 K = 9.50000e+01 A 4= 4.89696e-02 A 6=-2.07394e-01 A 8= 1.93855e-01 A10=-9.98224e-02 A12= 2.07364e-02 Page 11 K =-9.50000e+01 A 4= 2.83432e-02 A 6=-4.58574e-02 A 8= 2.82740e-02 A10=-8.55942e-03 A12= 1.03899e-03 Page 12 K =-9.27257e-01 A 4=-1.56136e-01 A 6= 6.75315e-02 A 8=-2.56189e-02 A10= 5.59475e-03 A12=-5.51194e-04 Page 13 K =-4.50662e-01 A 4=-1.22825e-01 A 6= 3.53258e-02 A 8=-9.85598e-03 A10= 1.44614e-03 A12=-9.86466e-05 Page 14 K =-1.64799e+01 A 4=-1.38067e-01 A 6= 4.97288e-02 A 8=-1.21056e-02 A10= 1.83406e-03 A12=-1.18494e-04 Page 15 K =-7.92908e+00 A 4=-7.61939e-02 A 6= 2.25869e-02 A 8=-4.53542e-03 A10=5.01713e-04 A12=-2.22446e-05 Various data Focal length 2.96 F-number: 2.58 Half angle of view (°) 75.2 Image height 3.88 Lens length 7.51 BF 0.40 [Numerical example 2] Unit: mm Surface Data Surface number rd nd νd 1* -2.357 0.52 1.53500 56.0 2* -7.959 0.61 3* 3.492 0.41 1.66080 20.4 4* 4.064 0.24 5* -11.782 0.56 1.53500 56.0 6* -4.003 0.11 7(Aperture) ∞ 0.11 8* 5.578 1.27 1.53500 56.0 9* -1.806 0.10 10* -20.535 0.51 1.68040 18.1 11* 5.692 0.15 12* 1.840 0.51 1.53500 56.0 13* 1.910 0.73 14* 3.252 0.46 1.61550 25.8 15* 1.853 0.27 16 ∞ 0.50 1.51633 64.1 17 ∞ (variable) Image plane ∞ Aspheric Data Front page K =-7.96702e+00 A 4= 3.03406e-02 A 6=-4.90379e-03 A 8= 5.29082e-04 A10=-3.13899e-05 A12= 7.94213e-07 Page 2 K =-9.83074e+01 A 4= 5.70614e-02 A 6=-9.08607e-03 A 8= 8.53959e-03 A10=-3.43204e-03 A12= 6.10530e-04 Page 3 K = 5.08055e+00 A 4=-7.36188e-02 A 6= 5.14921e-02 A 8=-5.34628e-02 A10= 2.81850e-02 A12=-8.03727e-03 Page 4 K = 1.23489e+01 A 4=-7.20088e-02 A 6= 8.87441e-02 A 8=-1.79243e-01 A10= 1.61365e-01 A12=-8.04839e-02 Page 5 K = 1.00000e+02 A 4=-4.79570e-02 A 6= 2.94673e-02 A 8=-9.09536e-02 A10= 5.60623e-02 A12=-7.78674e-03 Page 6 K =-8.82987e+00 A 4=-6.35036e-02 A 6=-6.28448e-02 A 8= 2.86636e-01 A10=-5.53069e-01 A12= 3.71188e-01 Page 8 K =-2.60813e+01 A 4=-1.67778e-02 A 6=-3.84405e-02 A 8=-1.11453e-02 A10=-4.94887e-03 A12=-3.77724e-02 Page 9 K = 1.04302e+00 A 4= 5.44895e-02 A 6=-2.03001e-01 A 8= 2.43301e-01 A10=-1.42349e-01 A12= 3.32532e-02 Side 10 K =-7.08649e+01 A 4= 5.03916e-02 A 6=-2.06234e-01 A 8= 1.93042e-01 A10=-1.00795e-01 A12= 2.07364e-02 Page 11 K =-1.00000e+02 A 4= 2.81133e-02 A 6=-4.55919e-02 A 8= 2.85022e-02 A10=-8.69833e-03 A12= 1.03899e-03 Side 12 K =-8.66378e-01 A 4=-1.55058e-01 A 6= 6.90297e-02 A 8=-2.55475e-02 A10=5.55846e-03 A12=-5.51194e-04 Page 13 K =-4.50535e-01 A 4=-1.19172e-01 A 6= 3.56883e-02 A 8=-9.81491e-03 A10= 1.43858e-03 A12=-9.87518e-05 Side 14 K =-2.00348e+01 A 4=-1.37253e-01 A 6= 4.96636e-02 A 8=-1.21433e-02 A10= 1.82282e-03 A12=-1.18507e-04 Page 15 K =-8.07300e+00 A 4=-7.74289e-02 A 6= 2.27787e-02 A 8=-4.53597e-03 A10=5.01198e-04 A12=-2.22885e-05 Various data Focal length 2.96 F-number: 2.57 Half angle of view (°) 80.2 Image height 3.88 Lens length 7.50 BF 0.44 [Numerical example 3] Unit: mm Surface Data Surface number rd nd νd 1* -2.345 0.50 1.53500 56.0 2* -7.846 0.58 3* 3.452 0.46 1.66080 20.4 4* 4.006 0.22 5* -11.954 0.53 1.53500 56.0 6* -3.854 0.16 7(Aperture) ∞ 0.16 8* 5.774 1.23 1.53500 56.0 9* -1.815 0.16 10* -22.024 0.50 1.68040 18.1 11* 5.522 0.18 12* 1.819 0.51 1.53500 56.0 13* 1.882 0.73 14* 3.307 0.48 1.61550 25.8 15* 1.886 0.27 16 ∞ 0.50 1.51633 64.1 17 ∞ (variable) Image plane ∞ Aspheric Data Front page K =-7.94396e+00 A 4= 3.03080e-02 A 6=-4.90324e-03 A 8= 5.29299e-04 A10=-3.13671e-05 A12= 7.94627e-07 2nd side K =-8.20098e+01 A 4= 5.56072e-02 A 6=-9.70365e-03 A 8= 8.45081e-03 A10=-3.40433e-03 A12= 6.10530e-04 Page 3 K = 5.09035e+00 A 4=-7.27936e-02 A 6= 5.17309e-02 A 8=-5.33759e-02 A10= 2.82191e-02 A12=-8.03727e-03 Page 4 K = 1.23884e+01 A 4=-7.28271e-02 A 6= 8.85060e-02 A 8=-1.79261e-01 A10= 1.61501e-01 A12=-8.04839e-02 Page 5 K = 8.88208e+01 A 4=-4.62170e-02 A 6= 3.11314e-02 A 8=-8.85475e-02 A10= 5.96894e-02 A12=-7.78674e-03 Page 6 K =-9.66928e+00 A 4=-6.15636e-02 A 6=-5.55548e-02 A 8= 2.90862e-01 A10=-5.54060e-01 A12= 3.71188e-01 Page 8 K =-2.67675e+01 A 4=-1.73275e-02 A 6=-4.02189e-02 A 8=-1.34203e-02 A10=-1.37941e-02 A12=-3.77724e-02 Page 9 K = 1.06330e+00 A 4= 5.22714e-02 A 6=-2.03821e-01 A 8= 2.42645e-01 A10=-1.43054e-01 A12= 3.32532e-02 Page 10 K =-9.80302e+01 A 4= 5.24891e-02 A 6=-2.05551e-01 A 8= 1.93400e-01 A10=-1.00539e-01 A12= 2.07364e-02 Page 11 K =-1.00000e+02 A 4= 2.76744e-02 A 6=-4.56581e-02 A 8= 2.84860e-02 A10=-8.70344e-03 A12= 1.03899e-03 Side 12 K =-8.81510e-01 A 4=-1.55382e-01 A 6= 6.89104e-02 A 8=-2.55671e-02 A10=5.55946e-03 A12=-5.51194e-04 Page 13 K =-4.47522e-01 A 4=-1.18730e-01 A 6= 3.58008e-02 A 8=-9.80248e-03 A10= 1.43893e-03 A12=-9.87518e-05 Side 14 K =-1.77723e+01 A 4=-1.37079e-01 A 6= 4.96860e-02 A 8=-1.21403e-02 A10= 1.82305e-03 A12=-1.18507e-04 Page 15 K =-8.33851e+00 A 4=-7.74831e-02 A 6= 2.27765e-02 A 8=-4.53568e-03 A10=5.01314e-04 A12=-2.22794e-05 Various data Focal length 2.93 F-number: 2.57 Half angle of view (°) 80.0 Image height 3.88 Lens length 7.50 BF 0.32 [Numerical example 4] Unit: mm Surface Data Surface number rd nd νd 1* -2.122 0.59 1.53500 56.0 2* -5.698 0.70 3* 3.362 0.50 1.66080 20.4 4* 3.903 0.20 5* -15.264 0.50 1.53500 56.0 6* -3.492 0.20 7(Aperture) ∞ 0.20 8* 6.281 1.08 1.53500 56.0 9* -1.947 0.20 10* -28.354 0.50 1.68040 18.1 11* 4.628 0.22 12* 1.706 0.50 1.53500 56.0 13* 1.758 0.74 14* 3.351 0.50 1.61550 25.8 15* 1.952 0.20 16 ∞ 0.50 1.51633 64.1 17 ∞ (variable) Image plane ∞ Aspheric Data Front page K =-7.21560e+00 A 4= 2.96162e-02 A 6=-4.90109e-03 A 8= 5.40372e-04 A10=-3.06620e-05 A12= 7.43323e-07 2nd side K =-4.65652e+01 A 4= 5.53159e-02 A 6=-1.07442e-02 A 8= 7.88122e-03 A10=-3.35467e-03 A12= 6.01832e-04 3rd page K = 4.84021e+00 A 4=-7.72858e-02 A 6= 5.07713e-02 A 8=-5.29713e-02 A10= 2.88435e-02 A12=-7.97126e-03 Page 4 K = 1.20863e+01 A 4=-7.06743e-02 A 6= 9.84434e-02 A 8=-1.70975e-01 A10= 1.59397e-01 A12=-8.04839e-02 Page 5 K = 4.10317e+01 A 4=-3.83252e-02 A 6= 3.59522e-02 A 8=-8.79859e-02 A10= 7.21453e-02 A12=-2.71494e-02 Page 6 K =-1.15081e+01 A 4=-5.84345e-02 A 6=-6.34755e-02 A 8= 2.93967e-01 A10=-5.60155e-01 A12= 3.71188e-01 Page 8 K = 1.12709e+01 A 4=-1.41590e-02 A 6=-4.71187e-02 A 8= 2.86499e-03 A10= 2.01176e-02 A12=-3.77724e-02 Page 9 K = 1.07475e+00 A 4= 5.76015e-02 A 6=-2.00884e-01 A 8= 2.43575e-01 A10=-1.43964e-01 A12= 3.06522e-02 Page 10 K =-1.00000e+02 A 4= 5.49427e-02 A 6=-2.06483e-01 A 8= 1.94613e-01 A10=-9.91123e-02 A12= 2.04110e-02 Page 11 K =-6.80430e+01 A 4= 3.22885e-02 A 6=-4.57806e-02 A 8= 2.82254e-02 A10=-8.58985e-03 A12= 1.02785e-03 Side 12 K =-8.28971e-01 A 4=-1.53779e-01 A 6= 6.83415e-02 A 8=-2.57178e-02 A10=5.57893e-03 A12=-5.50760e-04 Page 13 K =-4.74215e-01 A 4=-1.22447e-01 A 6= 3.52257e-02 A 8=-9.76882e-03 A10= 1.44755e-03 A12=-1.04451e-04 Side 14 K =-1.66694e+01 A 4=-1.37950e-01 A 6= 4.97673e-02 A 8=-1.21026e-02 A10= 1.83281e-03 A12=-1.18500e-04 Page 15 K =-6.70194e+00 A 4=-7.72180e-02 A 6= 2.25573e-02 A 8=-4.53654e-03 A10=5.02925e-04 A12=-2.19819e-05 Various data Focal length 2.89 F-number: 2.57 Half angle of view (°) 66.7 Image height 3.88 Lens length 7.58 BF 0.25 [Numerical example 5] position mm Surface Data Surface number rd nd νd 1* -1.958 0.50 1.53500 56.0 2* -4.735 0.53 3* 3.467 0.50 1.66080 20.4 4* 3.968 0.20 5* -13.669 0.50 1.53500 56.0 6* -4.188 0.20 7(Aperture) ∞ 0.20 8* 5.295 1.02 1.53500 56.0 9* -1.814 0.20 10* -37.671 0.50 1.68040 18.1 11* 4.120 0.22 12* 1.661 0.50 1.53500 56.0 13* 1.797 0.80 14* 2.854 0.50 1.61550 25.8 15* 1.817 0.20 16 ∞ 0.50 1.51633 64.1 17 ∞ (variable) Image plane ∞ Aspheric Data Front page K =-6.32327e+00 A 4= 2.98233e-02 A 6=-4.89280e-03 A 8= 5.28430e-04 A10=-3.14439e-05 A12= 7.88495e-07 2nd side K =-2.66407e+01 A 4= 5.71424e-02 A 6=-1.07057e-02 A 8= 8.41392e-03 A10=-3.44907e-03 A12= 6.36941e-04 3rd page K = 5.10782e+00 A 4=-6.99134e-02 A 6= 5.14056e-02 A 8=-5.34902e-02 A10=2.83560e-02 A12=-7.84641e-03 Side 4 K = 1.21094e+01 A 4=-6.97918e-02 A 6= 9.28029e-02 A 8=-1.78301e-01 A10= 1.60126e-01 A12=-8.30285e-02 Page 5 K = 5.75143e+01 A 4=-4.21003e-02 A 6= 3.34269e-02 A 8=-8.12141e-02 A10= 6.49798e-02 A12=-2.22412e-02 Page 6 K =-1.75233e+01 A 4=-5.38032e-02 A 6=-7.85747e-02 A 8= 3.63531e-01 A10=-6.11446e-01 A12= 3.71188e-01 Page 8 K =-1.58958e+01 A 4=-1.29494e-02 A 6=-6.83635e-02 A 8= 5.35684e-02 A10=-5.17316e-02 A12=-3.77724e-02 Page 9 K = 1.20294e+00 A 4= 6.69581e-02 A 6=-1.97766e-01 A 8= 2.41467e-01 A10=-1.47528e-01 A12= 3.25591e-02 Page 10 K =-1.00000e+02 A 4= 6.44454e-02 A 6=-2.04555e-01 A 8= 1.93704e-01 A10=-1.00126e-01 A12= 2.08559e-02 Page 11 K =-4.35225e+01 A 4= 3.30428e-02 A 6=-4.51939e-02 A 8= 2.84559e-02 A10=-8.73708e-03 A12= 1.03881e-03 Page 12 K =-8.13180e-01 A 4=-1.53492e-01 A 6= 6.92297e-02 A 8=-2.61525e-02 A10=5.64017e-03 A12=-5.22070e-04 Page 13 K =-4.79617e-01 A 4=-1.17916e-01 A 6= 3.50747e-02 A 8=-9.78255e-03 A10= 1.45667e-03 A12=-9.78775e-05 Side 14 K =-6.66525e+00 A 4=-1.41251e-01 A 6= 4.88729e-02 A 8=-1.21835e-02 A10= 1.84394e-03 A12=-1.13953e-04 Page 15 K =-5.39760e+00 A 4=-7.57721e-02 A 6= 2.24461e-02 A 8=-4.60070e-03 A10=4.99848e-04 A12=-2.10938e-05 Various data Focal length 2.81 F-number 2.80 Half angle of view (°) 79.5 Image height 3.88 Lens length 7.28 BF 0.20 [Numerical example 6] Unit: mm Surface Data Surface number rd nd νd 1* -2.264 0.53 1.53500 56.0 2* -6.811 0.53 3* 3.398 0.50 1.66080 20.4 4* 3.890 0.24 5* -12.364 0.50 1.53500 56.0 6* -3.174 0.20 7(Aperture) ∞ 0.20 8* 9.821 0.50 1.53500 56.0 9* -2.544 0.20 10* -2.591 0.50 1.53500 56.0 11* -1.870 0.20 12* -22.511 0.50 1.68040 18.1 13* 5.236 0.21 14* 1.835 0.50 1.53500 56.0 15* 1.889 0.60 16* 3.443 0.51 1.61550 25.8 17* 2.053 0.28 18 ∞ 0.50 1.51633 64.1 19 ∞ (variable) Image plane ∞ Aspheric Data Front page K =-8.48514e+00 A 4= 2.98937e-02 A 6=-4.89630e-03 A 8= 5.40086e-04 A10=-3.03562e-05 A12= 7.69678e-07 2nd side K =-6.96032e+01 A 4= 5.51141e-02 A 6=-1.04265e-02 A 8= 7.98096e-03 A10=-3.33899e-03 A12= 6.10531e-04 3rd page K = 4.80851e+00 A 4=-7.77879e-02 A 6= 5.10667e-02 A 8=-5.29969e-02 A10=2.87467e-02 A12=-8.11052e-03 Side 4 K = 1.22869e+01 A 4=-7.23979e-02 A 6= 9.54751e-02 A 8=-1.71339e-01 A10= 1.61245e-01 A12=-8.04839e-02 Page 5 K = 8.29624e+00 A 4=-3.59199e-02 A 6= 3.81398e-02 A 8=-8.87521e-02 A10= 6.31704e-02 A12=-7.78674e-03 Page 6 K =-1.29526e+01 A 4=-5.42383e-02 A 6=-7.02349e-02 A 8= 2.83156e-01 A10=-5.34995e-01 A12= 3.71188e-01 Page 8 K = 3.39963e+01 A 4=-4.63083e-03 A 6=-4.30132e-02 A 8=-2.05610e-02 A10= 3.31711e-02 A12=-3.77724e-02 Page 9 K = 1.74358e-01 A 4= 9.25056e-04 A 6= 5.85249e-03 A 8=-5.51633e-03 A10=-1.05788e-02 A12= 4.89402e-09 Page 10 K =-1.73287e-01 A 4=-9.34234e-04 A 6=-2.71135e-03 A 8=-2.13601e-04 A10=-2.16700e-04 A12= 1.40173e-09 Page 11 K = 9.30718e-01 A 4= 4.86727e-02 A 6=-2.02884e-01 A 8= 2.44200e-01 A10=-1.42504e-01 A12= 3.32532e-02 Page 12 K = 9.50000e+01 A 4= 5.24093e-02 A 6=-2.07137e-01 A 8= 1.93848e-01 A10=-9.98295e-02 A12= 2.05156e-02 Page 13 K =-9.50000e+01 A 4= 2.63413e-02 A 6=-4.59628e-02 A 8= 2.82964e-02 A10=-8.55314e-03 A12= 1.03899e-03 Side 14 K =-9.39433e-01 A 4=-1.56205e-01 A 6= 6.72460e-02 A 8=-2.56683e-02 A10=5.58589e-03 A12=-5.51193e-04 Page 15 K =-4.80369e-01 A 4=-1.20535e-01 A 6= 3.55447e-02 A 8=-9.86631e-03 A10= 1.44048e-03 A12=-9.86463e-05 Page 16 K =-2.08344e+01 A 4=-1.37508e-01 A 6= 4.97409e-02 A 8=-1.21110e-02 A10= 1.83210e-03 A12=-1.18492e-04 Page 17 K =-7.59250e+00 A 4=-7.68722e-02 A 6= 2.25402e-02 A 8=-4.52758e-03 A10=5.02767e-04 A12=-2.20988e-05 Various data Focal length 3.06 F-number: 2.57 Half angle of view (°) 66.50 Image height 3.88 Lens length 7.50 BF 0.30 Table 1 shows the values ​​of the conditions of formulas (1) to (8) in each example.

[0044] [Table 1]

[0045] [Imaging device] 14 shows a digital still camera (imaging device) 10 that uses the optical system of Examples 1 to 6 as an imaging optical system. In the camera 10, 13 is a camera body, and 11 is an imaging optical system configured with any of the optical systems of Examples 1 to 6. 12 is an imaging element such as a CCD sensor or a CMOS sensor that is built into the camera body and receives a subject image formed by the imaging optical system 11.

[0046] The optical system of each embodiment can be applied to various imaging devices such as a digital video camera, a silver halide film camera, etc., in addition to the digital still camera shown in Fig. 14. The camera may be either an integrated lens type or an interchangeable lens type.

[0047] The above embodiment includes the following configurations.

[0048] (Configuration 1) An optical system including a first lens group, an aperture stop, and a second lens group, which are arranged in this order from an object side to an image side, the first lens group includes a first lens and a second lens arranged in this order from the object side to the image side, the second lens group includes a third lens that is second from the image side and a fourth lens that is closest to the image side, the first lens has a negative refractive power on the axis and a positive refractive power off the axis; the second lens has a positive refractive power on the axis and a negative refractive power off the axis, the third lens has a positive refractive power on the axis and a negative refractive power off the axis, the fourth lens has a negative refractive power on the axis and a positive refractive power off the axis, Let f be the axial focal length of the optical system, fn2 be the off-axial focal length at which the negative refractive power of the second lens is the largest when the maximum image height of the second lens is divided by 20, and fω be the off-axial focal length at the maximum angle of view of the optical system. -2.4≦fn2 / f<0.0 0.1≦fω / f≦2.1 An optical system characterized by satisfying the following conditions. (Configuration 2) Let t12 be the length on the optical axis from the lens surface on the object side of the first lens to the lens surface on the image side, and let TTL be the length on the optical axis from the lens surface closest to the object side of the optical system to the image plane. 0.01≦t12 / TTL≦0.10 2. The optical system according to claim 1, wherein the following condition is satisfied: (Configuration 3) When a composite focal length of the first lens and the second lens is fо and a composite focal length of the third lens and the fourth lens is fi, -5.0≦fо / f<0.0 -15.0≦fi / f<0.0 3. The optical system according to configuration 1 or 2, which satisfies the following condition: (Configuration 4) Let ImgH be the maximum image height of the optical system. 1.2≦TTL / ImgH≦2.1 4. The optical system according to any one of configurations 1 to 3, wherein the following condition is satisfied: (Configuration 5) Let νN_2 be the Abbe number of the negative lens included in the second lens group with respect to the d-line, 15.0≦νN_2≦40.0 5. The optical system according to any one of configurations 1 to 4, wherein the following condition is satisfied: (Configuration 6) the first lens group includes the first lens, the second lens, and a positive lens, arranged in this order from an object side to an image side, The optical system described in any one of configurations 1 to 5, wherein the second lens group consists of a positive lens, a negative lens, the third lens, and the fourth lens, arranged in order from the object side to the image side. (Configuration 7) the first lens group includes the first lens, the second lens, and a positive lens, arranged in this order from an object side to an image side, The optical system described in any one of configurations 1 to 5, wherein the second lens group consists of a positive lens, a positive lens, a negative lens, the third lens, and the fourth lens, arranged in that order from the object side to the image side. (Configuration 8) An optical system according to any one of configurations 1 to 7; and an image sensor for capturing an image of a subject through the optical system.

[0049] The preferred embodiments and examples of the present invention described above are representative examples, and various modifications and changes are possible for each example when implementing the present invention. [Explanation of symbols]

[0050] L1 First lens group SP aperture stop L2 Second lens group Gn negative lens

Claims

1. An optical system including a first lens group, an aperture stop, and a second lens group, which are arranged in this order from an object side to an image side, the first lens group includes a first lens and a second lens arranged in this order from the object side to the image side, the second lens group includes a third lens that is second from the image side and a fourth lens that is closest to the image side, the first lens has a negative refractive power on the axis and a positive refractive power off the axis; the second lens has a positive refractive power on the axis and a negative refractive power off the axis, the third lens has a positive refractive power on the axis and a negative refractive power off the axis, the fourth lens has a negative refractive power on the axis and a positive refractive power off the axis, Let f be the axial focal length of the optical system, fn2 be the off-axial focal length at which the negative refractive power of the second lens is the largest when the maximum image height of the second lens is divided by 20, and fω be the off-axial focal length at the maximum angle of view ω of the optical system. -2.4≦fn2 / f<0.0 0.1≦fω / f≦2.1 An optical system characterized by satisfying the following conditions.

2. Let t12 be the length on the optical axis from the object side lens surface of the first lens to the image side lens surface, and let TTL be the length on the optical axis from the lens surface closest to the object side of the optical system to the image surface. 0.01≦t12 / TTL≦0.10 2. The optical system according to claim 1, which satisfies the following condition:

3. When a composite focal length of the first lens and the second lens is f, and a composite focal length of the third lens and the fourth lens is fi, -5.0≦fо / f<0.0 -15.0≦fi / f<0.0 2. The optical system according to claim 1, which satisfies the following condition:

4. When the maximum image height of the optical system is ImgH, 1.2≦TTL / ImgH≦2.1 2. The optical system according to claim 1, which satisfies the following condition:

5. When the Abbe number of the negative lens included in the second lens group based on the d-line is νN_2, 15.0≦νN_2≦40.0 2. The optical system according to claim 1, which satisfies the following condition:

6. the first lens group includes the first lens, the second lens, and a positive lens, arranged in this order from an object side to an image side, 2. The optical system according to claim 1, wherein the second lens group comprises, in order from the object side to the image side, a positive lens, a negative lens, the third lens, and the fourth lens.

7. the first lens group includes the first lens, the second lens, and a positive lens, arranged in this order from an object side to an image side, 2. The optical system according to claim 1, wherein the second lens group comprises, in order from the object side to the image side, a positive lens, a positive lens, a negative lens, the third lens, and the fourth lens.

8. An optical system according to any one of claims 1 to 7; and an image sensor for capturing an image of a subject through the optical system.

Images