Optical system and imaging device having the same

Abstract

To provide an optical system which suppresses oblique incidence to the periphery of an image sensor, has a short total lens length, and offers superior optical performance.SOLUTION: An optical system L0 provided herein consists of a first lens L1, an aperture stop SP, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5 with positive refractive power, and a sixth lens L6 with positive refractive power arranged in order from the object side to the image side, has two or more negative lenses, and satisfies the following conditional expressions: 0.5<SPIP / TTL<1.0, 1.64<NNdave<1.84, where SPIP represents an optical axial distance from the aperture stop to the image plane using a back focus as an air-equivalent length, TTL represents an optical axial distance from an object-side lens surface of the first lens to the image plane using the back focus as an air-equivalent length, and NNdave represents an average refractive index of materials of all negative lenses included in the optical system for the d-line.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical system and is suitable for digital video cameras, digital still cameras, broadcast cameras, silver halide film cameras, surveillance cameras, and the like.

Background Art

[0002] In recent years, imaging devices have been miniaturized, and optical systems (imaging optical systems) used in imaging devices are required to have a short overall lens length and high optical performance.

[0003] As an optical system that meets these requirements, Patent Document 1 discloses an optical system composed of six lenses.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Generally, when attempting to shorten the overall lens length of an optical system, various aberrations such as spherical aberration and field curvature increase, and the optical performance tends to deteriorate. Further, when the position of the aperture stop approaches the imaging surface due to the shortening of the overall lens length, in an imaging device using an image sensor such as a digital still camera, shading is likely to occur due to the oblique incidence of the light beam (oblique incidence) in the peripheral portion of the image sensor. Although the occurrence of shading can be suppressed by arranging the aperture stop on the object side from the center of the optical system, since the lens configuration (refractive power arrangement) becomes asymmetric with respect to the aperture stop of the optical system, it is difficult to correct various aberrations well, and the number of lenses tends to increase. In the optical system described in Patent Document 1, the compatibility between shortening the overall lens length and high optical performance was insufficient.

[0006] In order to suppress the oblique incidence to the peripheral part of the imaging element and achieve both shortening of the overall lens length and high performance, it is important to appropriately set the lens configuration (material, number of lenses, shape, etc.) of the optical system in addition to the number of lenses and the sign of the refractive power of each lens.

[0007] An object of the present invention is to provide an optical system that suppresses oblique incidence to the peripheral part of the imaging element, has a short overall lens length, and has excellent optical performance.

Means for Solving the Problems

[0008] An optical system according to one aspect of the present invention is an optical system including, in order from the object side to the image side, a first lens, an aperture stop, a second lens, a third lens, a fourth lens, a fifth lens having a positive refractive power, and a sixth lens having a positive refractive power, wherein the optical system has two or more negative lenses, The first lens has a positive refractive power, the sixth lens has aspherical surfaces formed on both sides, the lens surface on the object side of the sixth lens includes a region that is convex near the optical axis, and the lens surface on the image side of the sixth lens includes a region that is concave near the optical axis. when the distance on the optical axis from the aperture stop with the back focus as the air-equivalent length to the image plane is SPIP, the distance on the optical axis from the lens surface on the object side of the first lens with the back focus as the air-equivalent length to the image plane is TTL, and the average refractive index of the material of all the negative lenses included in the optical system at the d-line is NNdave, 0.5 < SPIP / TTL < 1.0 1.6 8 < NNdave < 1.84 and is characterized by satisfying the conditional expression.

[0009] Other objects and features of the present invention will be described in the following embodiments.

Effects of the Invention

[0010] According to the present invention, it is possible to realize an optical system that suppresses oblique incidence to the peripheral part of the imaging element, has a short overall lens length, and has excellent optical performance.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the optical system of the present invention and an imaging device having the same will be described based on the accompanying drawings.

[0013] FIG. 1, FIG. 3, FIG. 5, and FIG. 7 are lens cross-sectional views at infinity focus of the optical systems of Examples 1 to 4, respectively. The optical system L0 of each example is an optical system used in an imaging device such as a digital video camera, a digital still camera, a broadcast camera, a silver halide film camera, or a surveillance camera.

[0014] In each lens cross-sectional view, the left side is the object side (enlarged side), and the right side is the image side (reduced side). The optical system L0 of each example is configured to include a plurality of lenses.

[0015] The optical system L0 of each example includes a first lens L1, an aperture stop SP, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5 having a positive refractive power, and a sixth lens L6 having a positive refractive power, which are arranged in order from the object side to the image side.

[0016] In each lens cross-sectional view, "Li" (where i is a natural number) represents the "ith lens" when the lenses constituting the optical system L0 are numbered in order from the object side to the image side. SP is the aperture stop that determines (limits) the light beam of the open F-number (Fno). IP is the image plane. When the optical system L0 of each embodiment is used as the imaging optical system of a digital video camera or a digital still camera, the imaging plane of a solid-state imaging device (photoelectric conversion element) such as a CCD sensor or a CMOS sensor is arranged. When the optical system L0 of each embodiment is used as the imaging optical system of a camera for silver halide film, a photosensitive surface corresponding to the film surface is placed on the image plane IP.

[0017] Also, by moving the entire optical system L0 along the optical axis, focusing is performed from an infinite object point to a closest object point.

[0018] In addition, in the optical system L0 of each embodiment, one or more lenses may be decentered so as to include a component perpendicular to the optical axis during image blur correction, so that it has a function as an anti-vibration optical system. Also, a parallel plate having substantially no refractive power, such as a low-pass filter or an infrared cut filter, may be arranged between the lens arranged closest to the image side and the imaging plane.

[0019] Figures 2, 4, 6, and 8 are aberration diagrams of the optical system L0 of Examples 1 to 4 at infinity focus, respectively.

[0020] In the spherical aberration diagram, Fno is the F-number, and it shows the amount of spherical aberration with respect to the d-line (wavelength 587.6 nm) and the g-line (wavelength 435.8 nm). In the astigmatism diagram, dS shows the amount of astigmatism on the sagittal image plane, and dM shows the amount of astigmatism on the meridional image plane. The distortion aberration diagram shows the amount of distortion aberration with respect to the d-line. The chromatic aberration diagram shows the amount of chromatic aberration at the g-line. ω is the imaging semi-field angle (°), which is the field angle by paraxial calculation.

[0021] Next, the characteristic configurations in the optical system L0 of each embodiment will be described.

[0022] The optical system L0 of each embodiment is composed of a first lens L1, an aperture stop SP, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5 with positive refractive power, and a sixth lens L6 with positive refractive power, which are arranged in order from the object side to the image side.

[0023] Also, the optical system L0 has two or more negative lenses. This facilitates the correction of field curvature and the correction of axial chromatic aberration and lateral chromatic aberration.

[0024] Furthermore, the optical system L0 of each embodiment satisfies the following conditional expressions (1) and (2).

[0025] 0.5 < SPIP / TTL < 1.0 ···(1) 1.64 < NNdave < 1.84 ···(2) Here, SPIP is the distance on the optical axis from the aperture stop SP with the back focus as the air-equivalent length to the image plane IP. TTL is the overall lens length (the distance on the optical axis from the lens surface on the object side of the first lens L1 with the back focus as the air-equivalent length to the image plane IP). NNdave is the average refractive index at the d-line of the materials of all the negative lenses included in the optical system L0.

[0026] Conditional expression (1) relates to the ratio of the distance on the optical axis from the aperture stop SP with the back focus as the air-equivalent length to the image plane IP to the overall lens length. If it is below the lower limit value of conditional expression (1), since the position of the aperture stop SP approaches the image plane IP, the incident angle of off-axis rays to the image plane IP increases. Then, shading occurs in the peripheral part of the imaging device, which is not preferable. Exceeding the upper limit value of conditional expression (1) means that the position of the aperture stop SP is on the object side of the first lens L1. In this case, since the lens configuration with respect to the aperture stop SP of the optical system becomes asymmetric, it becomes difficult to correct various aberrations well. Also, the diameter of the lens close to the image plane IP tends to increase, making it difficult to miniaturize the lens.

[0027] Conditional expression (2) relates to the average refractive index of the d-line of the materials of all the negative lenses included in the optical system L0. Generally, to correct the field curvature well, it is sufficient to make the Petzval sum approach zero. For this reason, it is effective to increase the refractive index of the positive lens and decrease the refractive index of the negative lens. The dispersion of a material with a high refractive index tends to be larger than that of a material with a low refractive index, and a material with a large dispersion also tends to have a higher refractive index than a material with a small dispersion. Here, considering the compatibility of correcting the field curvature and the axial chromatic aberration, when a material with a high refractive index is used for the positive lens, in order to correct the axial chromatic aberration, it is preferable to use a material with a sufficiently larger dispersion for the negative lens than for the positive lens. For this reason, the refractive index of the negative lens tends to be high, making it difficult to correct the field curvature. On the other hand, when a material with a low refractive index is used for the negative lens and a material with a high refractive index is used for the positive lens to correct the field curvature, it becomes difficult to make the difference in dispersion between the negative lens and the positive lens sufficiently large, making it difficult to correct the axial chromatic aberration. When the average refractive index of the material of the negative lens becomes smaller than the lower limit value of conditional expression (2), the Petzval sum becomes smaller and the correction of the field curvature becomes easy, but it becomes difficult to make the difference in dispersion between the negative lens and the positive lens sufficiently large, making it difficult to correct the axial chromatic aberration. On the other hand, when exceeding the upper limit value of conditional expression (2), the Petzval sum tends to become large, making it difficult to correct the field curvature.

[0028] By having the above configuration, the optical system L0 of each embodiment suppresses the oblique incidence to the peripheral part of the imaging element while having a short overall lens length and realizes excellent optical performance.

[0029] Furthermore, it is more preferable that the numerical ranges of conditional expressions (1) and (2) are within the ranges of the following conditional expressions (1a) and (2a).

[0030] 0.6 < SPIP / TTL < 0.9 ···(1a) 1.64 < NNdave < 1.81 ···(2a) Moreover, it is even more preferable that the numerical ranges of conditional expressions (1) and (2) are within the ranges of the following conditional expressions (1b) and (2b).

[0031] 0.7 < SPIP / TTL < 0.9 ···(1b) 1.68 < NNdave < 1.81 ···(2b) Next, the preferable conditions that the optical system L0 of each embodiment satisfies will be described.

[0032] The optical system L0 of each embodiment preferably satisfies one or more of the following conditional expressions (3) to (7).

[0033] 0.20 < BF / TTL < 0.40 ···(3) 15 < Nνdave < 30 ···(4) 0.90 < TTL / f < 1.40 ···(5) 1.5 < (L1R2 + L1R1) / (L1R2 - L1R1) < 4.0 ···(6) 0.60 < f1 / f < 2.00 ···(7) Here, BF is the back focus of the optical system L0 and is the air-equivalent length of the distance on the optical axis from the lens surface on the image side of the sixth lens L6 to the image plane IP. Nνdave is the average Abbe number at the d-line of the materials of all the negative lenses included in the optical system L0. f is the focal length of the optical system L0. L1R2 is the radius of curvature of the lens surface on the image side of the first lens L1. L1R1 is the radius of curvature of the lens surface on the object side of the first lens L1. f1 is the focal length of the first lens L1.

[0034] The conditional expression (3) relates to the ratio of the back focus of the optical system L0 to the total lens length. If it is below the lower limit value of the conditional expression (3), the back focus becomes too short, the incident angle of the off-axis ray to the image plane IP becomes large, and shading occurs, which is not preferable. Or, in order to suppress the occurrence of shading, the lens diameter becomes large, making it difficult to miniaturize the lens. On the other hand, if it exceeds the upper limit value of the conditional expression (3), the back focus becomes too long and the total lens length increases, which is not preferable.

[0035] Conditional expression (4) relates to the average Abbe number of the materials of all the negative lenses included in the optical system L0. If it is below the lower limit value of conditional expression (4), the dispersion becomes large and it becomes difficult to correct the axial chromatic aberration and the magnification chromatic aberration, which is not preferable. Furthermore, generally, the refractive index of a material with a large dispersion tends to be higher than that of a material with a small dispersion. Therefore, the average refractive index of the negative lens becomes high, the Petzval sum tends to become large, and it becomes difficult to correct coma and the like, which is not preferable. On the other hand, if it exceeds the upper limit value of conditional expression (4), the dispersion becomes too small and it becomes difficult to correct the axial chromatic aberration and the magnification chromatic aberration, which is not preferable.

[0036] Conditional expression (5) relates to the ratio of the focal length of the optical system L0 to the overall lens length. If it is below the lower limit value of conditional expression (5), the overall lens length becomes too short and it becomes difficult to sufficiently correct various aberrations including spherical aberration. On the other hand, if it exceeds the upper limit value of conditional expression (5), the overall lens length becomes too long.

[0037] Conditional expression (6) relates to the shape of the first lens L1. Satisfying conditional expression (6) means that since the first lens L1 has a positive refractive power, the first lens L1 has a convex meniscus shape on the object side. By satisfying conditional expression (6), various aberrations including spherical aberration can be corrected well.

[0038] Conditional expression (7) relates to the ratio of the focal length of the first lens L1 to the focal length of the optical system L0. If it is below the lower limit value of conditional expression (7), the refractive power of the first lens L1 becomes strong and it becomes difficult to sufficiently correct various aberrations including spherical aberration. On the other hand, if it exceeds the upper limit value of conditional expression (7), the refractive power of the first lens L1 becomes weak and it becomes difficult to sufficiently shorten the overall lens length.

[0039] Note that the numerical ranges of conditional expressions (3) to (7) are more preferably the ranges of the following conditional expressions (3a) to (7a).

[0040] 0.25 < BF / TTL < 0.38 ···(3a) 15 < Nνdave < 25 ···(4a) 1.00 < TTL / f < 1.30 ···(5a) 1.5 < (L1R2 + L1R1) / (L1R2 - L1R1) < 3.5 ···(6a) 0.60 < f1 / f < 1.80 ···(7a) Also, it is more preferable that the numerical ranges of conditional expressions (3) to (7) are within the ranges of the following conditional expressions (3b) to (7b).

[0041] 0.28 < BF / TTL < 0.35 ···(3b) 18 < Nνdave < 25 ···(4b) 1.10 < TTL / f < 1.30 ···(5b) 2.0 < (L1R2 + L1R1) / (L1R2 - L1R1) < 3.5 ···(6b) 0.70 < f1 / f < 1.60 ···(7b) Next, in the optical system L0 of each example, a configuration that is preferably satisfied will be described.

[0042] The first lens L1 preferably has a positive refractive power. This facilitates shortening the overall lens length.

[0043] Also, the fourth lens L4 preferably has a region with a negative refractive power near the optical axis. Also, the fourth lens L4 has aspherical surfaces on both sides, and preferably includes a region where the lens surface on the image side is convex near the optical axis. This facilitates correction of field curvature. Note that in order to correct field curvature better, it is more preferable that the region where the lens surface on the object side of the fourth lens L4 is concave is included near the optical axis.

[0044] Also, the fifth lens L5 preferably has a region with a positive refractive power near the optical axis. Also, the fifth lens L5 has aspherical surfaces on both sides, and preferably includes a region where the lens surface on the object side is concave near the optical axis and a region where the lens surface on the image side is convex near the optical axis. This facilitates correction of field curvature.

[0045] Further, the sixth lens L6 preferably has a region with a positive refractive power in the vicinity of the optical axis. Also, the sixth lens L6 has aspherical surfaces on both sides, and preferably includes in the vicinity of the optical axis a region where the lens surface on the object side is convex and a region where the lens surface on the image side is concave. This facilitates correction of field curvature.

[0046] Note that the vicinity of the optical axis refers to the paraxial region. In the case of an aspherical lens, the concave and convex surfaces in the vicinity of the optical axis are defined by the sign of the paraxial curvature radius. Similarly, the positive and negative refractive powers are also calculated from the paraxial curvature radius.

[0047] Next, the optical system L0 of each embodiment will be described in detail.

[0048] The optical system L0 of Example 1 is composed of a first lens L1, an aperture stop SP, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5, and a sixth lens L6, which are arranged in order from the object side to the image side. The first lens L1 has a positive refractive power. The second lens L2 has a negative refractive power. The third lens L3 has a positive refractive power. The fourth lens L4 has a negative refractive power in the vicinity of the optical axis. The fifth lens L5 has a positive refractive power in the vicinity of the optical axis. The sixth lens L6 has a positive refractive power in the vicinity of the optical axis.

[0049] The optical systems L0 of Examples 2 to 4 are composed of a first lens L1, an aperture stop SP, a second lens L2, a third lens L3, a fourth lens, a fifth lens L5, and a sixth lens L6, which are arranged in order from the object side to the image side. The first lens L1 has a positive refractive power. The second lens L2 has a positive refractive power. The third lens L3 has a negative refractive power. The fourth lens has a negative refractive power in the vicinity of the optical axis. The fifth lens L5 has a positive refractive power in the vicinity of the optical axis. The sixth lens L6 has a positive refractive power in the vicinity of the optical axis.

[0050] In the optical systems L0 of Examples 1 to 4, the fourth lens L4 has aspherical surfaces formed on both sides. The surface on the object side faces concave toward the object side near the optical axis, and the surface on the image side faces convex toward the image side near the optical axis. The fifth lens L5 has aspherical surfaces formed on both sides. The surface on the object side faces concave toward the object side near the optical axis, and the surface on the image side faces convex toward the image side near the optical axis. The sixth lens L6 has aspherical surfaces formed on both sides. The surface on the object side faces convex toward the object side near the optical axis, and the surface on the image side faces concave toward the image side near the optical axis.

[0051] Numerical Examples 1 to 4 corresponding to Examples 1 to 4 are shown below.

[0052] In the surface data of each numerical example, r represents the radius of curvature of each optical surface, and d (mm) represents the on-axis distance (distance on the optical axis) between the m-th surface and the (m + 1)-th surface. Here, m is the surface number counted from the light incident side. Also, nd represents the refractive index with respect to the d-line of each optical member, and νd represents the Abbe number of the optical member. The Abbe number νd of a certain material is calculated as follows when the refractive indices at the Fraunhofer lines d (587.6 nm), F (486.1 nm), and C (656.3 nm) are Nd, NF, and NC: νd = (Nd - 1) / (NF - NC) and is represented by this formula.

[0053] In each numerical example, d, focal length (mm), F-number, and half angle of view (°) are all values when the optical system of each example is focused on an infinitely distant object. "Back focus (BF)" is the distance on the optical axis from the final lens surface (the lens surface closest to the image side) to the paraxial image plane, expressed in air-equivalent length. "Overall lens length" is the length obtained by adding the back focus to the distance on the optical axis from the lens surface farthest from the object side to the final surface.

[0054] The entrance pupil position is the distance from the lens surface closest to the object side (the first surface) to the entrance pupil. The exit pupil position is the distance from the lens surface closest to the image side (the final lens surface) to the exit pupil. The front principal point position is the distance from the first lens surface to the front principal point. The rear principal point position is the distance from the final lens surface to the rear principal point. Each of these numerical values is a paraxial quantity, and the sign is positive in the direction from the object side to the image side.

[0055] Also, when the optical surface is an aspherical surface, an asterisk (*) is attached to the right of the surface number. The aspherical shape is expressed as follows when X is the displacement amount from the vertex of the surface in the optical axis direction, h is the height from the optical axis in the direction perpendicular to the optical axis, R is the paraxial curvature radius, k is the conic constant, and A4, A6, A8, A10, A12, A14 are the aspherical coefficients of each order: 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 + A14 × h 14 where "e±XX" in each aspherical coefficient means "×10± XX ".

[0056] [Numerical Example 1] Unit: mm Surface Data Surface Number r d nd νd 1 5.233 0.90 1.77250 49.6 2 13.979 0.92 3 (Aperture Stop) ∞ 0.75 4 10.272 0.40 1.80810 22.8 5 4.237 0.10 6 5.247 0.76 1.95375 32.3 7 8.700 2.03 8* -3.252 0.80 1.67070 19.3 9* -8.850 0.10 10* 69.841 1.75 1.63560 23.9 11* -6.564 0.10 12* 7.123 1.75 1.53110 56.0 13* 7.659 BF Image plane ∞ Aspherical data The 8th surface K = 0.00000e+000 A 4= 2.44799e-005 A 6= 7.63377e-004 A 8=-1.01294e-004 A10= 1.24031e-005 A12=-5.55655e-007 The 9th surface K = 0.00000e+000 A 4=-7.17224e-003 A 6= 9.11444e-004 A 8=-8.25918e-005 A10= 6.46982e-006 A12=-2.27294e-007 The 10th surface K = 0.00000e+000 A 4=-1.01735e-003 A 6= 1.01290e-004 A 8=-1.54530e-005 A10= 8.92297e-007 A12=-1.63566e-008 The 11th surface K = 0.00000e+000 A 4= 2.81770e-003 A 6=-1.77867e-005 A 8=-7.77725e-006 A10= 3.42963e-007 A12=-3.52656e-009 The 12th surface K = 0.00000e+000 A 4=-6.67185e-003 A 6= 2.79572e-004 A 8=-6.98686e-006 A10= 1.03579e-007 A12=-8.12230e-010 Page 13 K = 0.00000e+000 A 4=-6.11521e-003 A 6= 2.12808e-004 A 8=-6.40220e-006 A10= 1.30967e-007 A12=-1.18938e-009 Focal length 13.26 F-number 2.88 Half field angle (°) 30.75 Image height 7.89 Overall lens length 15.50 BF 5.14 Entrance pupil position 1.74 Exit pupil position -7.96 Front principal point position 1.57 Rear principal point position -8.12 Single lens data Lens Starting surface Focal length 1 1 10.36 2 4 -9.20 3 6 12.51 4 8 -8.13 5 10 9.53 6 12 89.85 [Numerical Example 2] Unit: mm Surface data Surface number r d nd νd 1 6.172 0.75 1.78800 47.4 2 12.606 0.86 3 (Aperture stop) ∞ 0.42 4 6.266 0.90 1.88300 40.8 5 -115.141 0.40 1.72825 28.5 6 4.413 1.89 7* -11.615 0.80 1.67070 19.3 8* -68.826 0.46 9* -6.873 1.25 1.61550 25.8 10* -4.882 0.61 11* 7.239 1.90 1.53500 56.0 12* 8.458 BF Image plane ∞ Aspherical data The 7th surface K = 0.00000e+000 A 4=-1.17343e-002 A 6= 6.62086e-004 A 8= 7.11821e-005 A10=-1.57795e-005 A12= 1.24628e-007 The 8th surface K = 0.00000e+000 A 4=-9.33591e-003 A 6= 9.10480e-004 A 8=-5.56359e-005 A10= 1.86493e-006 A12=-2.51762e-008 The 9th surface K = 0.00000e+000 A 4= 8.82669e-003 A 6=-7.18264e-004 A 8= 3.71234e-005 A10=-8.01992e-007 A12= 8.77474e-010 The 10th surface K = 0.00000e+000 A 4= 3.92160e-003 A 6= 9.31874e-005 A 8=-8.10394e-006 A10= 2.91020e-007 A12=-2.38551e-009 The 11th surface K = 0.00000e+000 A 4=-7.49451e-003 A 6= 3.07261e-004 A 8=-5.64465e-006 A10= 9.78774e-010 A12= 1.44863e-009 A14=-1.61988e-011 Page 12 K = 0.00000e+000 A 4=-6.85204e-003 A 6= 2.82220e-004 A 8=-1.01045e-005 A10= 2.47381e-007 A12=-3.28540e-009 A14= 1.78261e-011 Focal length 13.00 F number 2.88 Half field angle (°) 31.25 Image height 7.89 Overall lens length 15.35 BF 5.11 Entrance pupil position 1.47 Exit pupil position -7.51 Front principal point position 1.07 Rear principal point position -7.89 Single lens data Lens start surface Focal length 1 1 14.60 2 4 6.75 3 (Aperture stop) ∞ 0.52 4 7 -20.95 5 9 22.10 6 11 60.82 [Numerical Example 3] Unit: mm Surface data Surface number r d nd νd 1 8.823 1.15 1.88300 40.8 2 16.892 2.16 3 (Aperture stop) ∞ 0.52 4 6.147 0.91 2.00100 29.1 5 -25.011 0.10 6 -14.102 0.40 1.92286 20.9 7 5.154 1.50 8* -45.358 0.80 1.67070 19.3 9* -357.393 0.57 10* -255.281 1.25 1.63910 23.5 11* -11.726 0.15 12* 6.200 1.90 1.54390 56.0 13* 6.564 BF Image plane ∞ Aspherical data The 8th surface K = 0.00000e+000 A 4=-4.26820e-003 A 6= 1.77770e-003 A 8=-2.26805e-004 A10= 1.42590e-005 A12=-3.72229e-007 The 9th surface K = 0.00000e+000 A 4=-1.10581e-002 A 6= 2.37005e-003 A 8=-2.13564e-004 A10= 9.92956e-006 A12=-1.98013e-007 The 10th surface K = 0.00000e+000 A 4=-2.65169e-003 A 6= 3.87732e-004 A 8=-3.29307e-005 A10= 1.51180e-006 A12=-2.70444e-008 The 11th surface K = 0.00000e+000 A 4= 1.51347e-003 A 6=-3.58082e-004 A 8= 3.04816e-005 A10=-1.03923e-006 A12= 1.28063e-008 The 12th surface K = 0.00000e+000 A 4=-7.34033e-003 A 6= 2.53991e-005 A 8= 1.97107e-005 A10=-1.02262e-006 A12= 2.13649e-008 A14=-1.76667e-010 Page 13 K = 0.00000e+000 A 4=-6.88663e-003 A 6= 1.95188e-004 A 8=-4.24138e-006 A10= 3.40242e-008 A12= 7.17160e-010 A14=-1.74573e-011 Focal length 12.61 F number 2.88 Half field angle (°) 32.03 Image height 7.89 Overall lens length 16.23 BF 4.82 Entrance pupil position 3.43 Exit pupil position -6.16 Front principal point position 1.55 Rear principal point position -7.79 Single lens data Lens Starting surface Focal length 1 1 19.61 2 4 5.00 3 (Aperture) ∞ 0.55 4 8 -77.54 5 10 19.19 6 12 72.36 [Numerical Example 4] Unit: mm Surface data Surface number r d nd νd 1 4.861 1.00 1.72916 54.7 2 8.926 1.09 3 (Aperture stop) ∞ 0.55 4 7.814 0.81 2.00100 29.1 5 -255.281 0.10 6 -61.921 0.40 1.92286 20.9 7 6.728 1.52 8* -10.405 0.80 1.68040 18.1 9* -18.578 0.69 10* -7.505 1.28 1.63560 23.9 11* -5.889 0.12 12* 7.492 1.90 1.54390 56.0 13* 7.255 BF Image plane ∞ Aspherical data The 8th surface K = 0.00000e+000 A 4=-1.16579e-002 A 6= 9.23400e-004 A 8=-1.96407e-004 A10= 3.26667e-005 A12=-5.02605e-006 The 9th surface K = 0.00000e+000 A 4=-1.19318e-002 A 6= 7.01526e-004 A 8= 7.06491e-005 A10=-1.24604e-005 A12= 5.15017e-007 The 10th surface K = 0.00000e+000 A 4=-2.01834e-004 A 6=-1.54371e-004 A 8= 3.25581e-005 A10=-1.09711e-006 A12=-2.61832e-009 The 11th surface K = 0.00000e+000 A 4= 1.50168e-003 A 6=-3.40868e-005 A 8=-1.74033e-005 A10= 1.74869e-006 A12=-4.27577e-008 The 12th surface K = 0.00000e+000 A 4=-6.28654e-003 A 6= 7.04829e-005 A 8= 1.06047e-005 A10 = -5.31978e-007 A12 = 9.95925e-009 A14 = -6.85704e-011 The 13th surface K = 0.00000e+000 A4 = -6.57786e-003 A6 = 2.21104e-004 A8 = -7.14307e-006 A10 = 1.69631e-007 A12 = -2.34045e-009 A14 = 1.30153e-011 Focal length 12.61 F-number 2.88 Half field angle (°) 32.03 Image height 7.89 Overall lens length 14.45 BF 4.19 Entrance pupil position 2.07 Exit pupil position -6.28 Front principal point position -0.50 Rear principal point position -8.42 Single lens data Lens Starting surface Focal length 1 1 13.26 2 4 7.59 3 6 -6.56 4 8 -36.20 5 10 32.91 6 12 231.34 The various values in each numerical example are summarized in Table 1 below.

[0057]

Table 1

[0058] [Imaging device] Next, an embodiment of a digital still camera (imaging device) 10 using the optical system L0 of each embodiment as an imaging optical system will be described with reference to FIG. 9. In FIG. 9, reference numeral 13 denotes a camera body, and reference numeral 11 denotes a photographing optical system constituted by any one of the optical systems L0 described in Embodiments 1 to 4. Reference numeral 12 denotes a solid-state imaging device (photoelectric conversion element) such as a CCD sensor or a CMOS sensor that is built in the camera body 13 and receives and photoelectrically converts an optical image formed by the photographing optical system 11. The camera body 13 may be a so-called single-lens reflex camera having a quick-return mirror, or a so-called mirrorless camera not having a quick-return mirror.

[0059] By applying the optical system of the present invention to an imaging device such as a digital still camera in this way, it is possible to obtain an imaging device that suppresses oblique incidence to the periphery of the imaging element, has a short overall lens length, and has excellent optical performance.

[0060] Note that the optical systems of the above-described embodiments are not limited to imaging devices such as digital still cameras, and can be applied to various optical devices such as telescopes.

[0061] Although the preferred embodiments and examples of the present invention have been described above, the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes are possible within the scope of the gist thereof.

Description of Reference Numerals

[0062] L0 Optical system L1 First lens L2 Second lens L3 Third lens L4 Fourth lens L5 Fifth lens L6 Sixth lens SP Aperture stop

Claims

1. An optical system including, in order from an object side to an image side, a first lens, an aperture stop, a second lens, a third lens, a fourth lens, a fifth lens having a positive refractive power, and a sixth lens having a positive refractive power, The optical system has two or more negative lenses, the first lens has a positive refractive power; the sixth lens has aspheric surfaces formed on both sides, the object side lens surface of the sixth lens includes a convex area near the optical axis, and the image side lens surface of the sixth lens includes a concave area near the optical axis, Let SPIP be the distance on the optical axis from the aperture stop to the image plane, with the back focus being the air-equivalent length, TTL be the distance on the optical axis from the lens surface on the object side of the first lens to the image plane, with the back focus being the air-equivalent length, and NNdave be the average refractive index at the d-line of the materials of all the negative lenses included in the optical system. 0.5<SPIP / TTL<1.0 1.68<NNdave<1.84 An optical system characterized in that the following condition is satisfied:

2. When the air-equivalent length of the distance on the optical axis from the image-side lens surface of the sixth lens to the image plane is BF, 0.20<BF / TTL<0.40 2. The optical system according to claim 1, wherein the following condition is satisfied:

3. When the average Abbe number at the d line of the materials of all the negative lenses included in the optical system is Nνdave, 15<Nvdave<30 3. The optical system according to claim 1, wherein the following condition is satisfied:

4. When the focal length of the optical system is f, 0.90<TTL / f<1.40 4. The optical system according to claim 1, wherein the following condition is satisfied:

5. When the radius of curvature of the lens surface on the object side of the first lens is L1R1 and the radius of curvature of the lens surface on the image side of the first lens is L1R2, 1.5<(L1R2+L1R1) / (L1R2-L1R1)<4.0 5. The optical system according to claim 1, wherein the following condition is satisfied:

6. When the focal length of the first lens is f1 and the focal length of the optical system is f, 0.60<f1 / f<2.00 6. The optical system according to claim 1, wherein the following condition is satisfied:

7. the fourth lens includes a region having negative refractive power in the vicinity of an optical axis, The fourth lens has aspheric surfaces on both sides thereof, 7. The optical system according to claim 1, wherein a lens surface of the fourth lens on the image side includes a convex area in the vicinity of the optical axis.

8. 8. The optical system according to claim 7, wherein the lens surface of the fourth lens on the object side includes a concave area in the vicinity of the optical axis.

9. the fifth lens has aspheric surfaces formed on both sides; the object-side lens surface of the fifth lens includes a concave area in the vicinity of the optical axis, 9. The optical system according to claim 1, wherein a lens surface of the fifth lens on the image side includes a convex area in the vicinity of the optical axis.

10. the second lens has a negative refractive power; The optical system according to claim 1 , wherein the third lens has a positive refractive power.

11. the second lens has a positive refractive power; The optical system according to claim 1 , wherein the third lens has a negative refractive power.

12. 12. An imaging apparatus comprising: the optical system according to claim 1; and an imaging element that receives an image formed by the optical system.

Images