Optical system and imaging device

Abstract

To provide a compact optical system having a wide angle of view and high optical performance.SOLUTION: An optical system is comprised of a first lens group B1 having negative or positive refractive power, an aperture stop SP, and a second lens group B2 having positive refractive power which are arranged in order from the object side to the image side, and has a half view angle of 80° or more. The first lens group has a plurality of negative lenses, including a first negative lens and a second negative lens, and at least one positive lens arranged in order from the object side to the image side. The second lens group has at least one negative lens and a plurality of positive lenses. The optical system satisfies conditions expressed as: 2.00≤td / f≤6.50 and 0.80≤|fn / fp|≤2.00, where td represents an optical axial distance from the most object-side lens surface to the most image-side lens surface, f represents a focal length of the optical system, fn represents an average focal length of the a plurality of negative lenses of the first lens group, and fp represents an average focal length of the at least one positive lens of the first lens group.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system suitable for imaging devices such as digital still cameras, video cameras, surveillance cameras, and vehicle-mounted cameras. [Background technology]

[0002] Such optical systems include fisheye lenses and ultra-wide-angle lenses with a half angle of view of 80° or more, as disclosed in Patent Documents 1 and 2. There are also imaging devices that perform image processing, such as cutting out a portion of an image obtained by capturing an image or synthesizing multiple images to generate a bird's-eye view image or a stereoscopic image. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-72085 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-243711 Summary of the Invention [Problem to be solved by the invention]

[0004] The fisheye lenses and super wide-angle lenses used in such imaging devices are required to have high optical performance and be compact.

[0005] The present invention provides a compact optical system with a wide angle of view and high optical performance, and an imaging device using the same. [Means for solving the problem]

[0006] An optical system according to one aspect of the present invention is an optical system having a half angle of view of 80° or greater, which includes, arranged in order from the object side to the image side, a first lens group having negative or positive refractive power, an aperture stop, and a second lens group having positive refractive power. The first lens group has, arranged in order from the object side to the image side, a plurality of negative lenses including a first negative lens and a second negative lens, and at least one positive lens.The first lens group is composed of a negative meniscus lens convex on the object side as a first negative lens, a biconcave lens as a second negative lens, and a positive lens. The second lens group is composed of, arranged in order from the object side to the image side, a cemented lens formed by cementing a positive lens and a negative lens, a positive lens, and a positive lens. When the distance on the optical axis from the lens surface closest to the object side to the lens surface closest to the image side is td, the focal length of the optical system is f, the average focal length of the plurality of negative lenses in the first lens group is fn, and the average focal length of the at least one positive lens in the first lens group is fp, 2.00≦td / f≦6.50 0.96≦|fn / fp|≦1.35 The following conditions are satisfied: Note that an imaging device for imaging an object through the above optical system also constitutes another aspect of the present invention. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a compact optical system that has a wide angle of view, yet has excellent correction of aberrations and high optical performance. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view of an optical system according to a first embodiment. [Figure 2] FIG. 4 is a longitudinal aberration diagram of the optical system of Example 1. [Figure 3] FIG. 10 is a cross-sectional view of the optical system of the second embodiment. [Figure 4] 10A and 10B are longitudinal aberration diagrams of the optical system of Example 2. [Figure 5] FIG. 10 is a cross-sectional view of the optical system of the third embodiment. [Figure 6] 10A and 10B are longitudinal aberration diagrams of the optical system of Example 3. [Figure 7] FIG. 1 is a diagram showing an imaging device equipped with an optical system according to first to third embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, before describing the optical systems of specific embodiments (1 to 3), matters common to the optical systems of the embodiments will be described.

[0010] The optical system of each embodiment (hereinafter referred to as a wide-angle lens) is used as an imaging lens in various imaging devices such as a digital still camera, a video camera, a surveillance camera, and an in-vehicle camera.

[0011] 1, 3 and 5 show cross sections of the wide-angle lenses of Examples 1 to 3. The wide-angle lenses of each Example are single-focus lenses (optical systems with a fixed focal length).

[0012] In each diagram, the left side is the object side and the right side is the image side. If i is the number indicating the order of the lens group from the object side, then Bi indicates the ith lens group. SP is the aperture stop that determines (limits) the light beam at the maximum F-number. IP is the image plane. The image plane is where the imaging surface of a solid-state image sensor (photoelectric conversion element) such as a CCD sensor or CMOS sensor, or the film surface (image plane) of a silver halide film camera, is located. GB is a glass block with no refractive power, such as an optical filter.

[0013] The optical system of each embodiment (hereinafter referred to as the wide-angle lens) is composed of, arranged in order from the object side to the image side, a first lens group (B1) with negative or positive refractive power, an aperture stop (SP), and a second lens group (B2) with positive refractive power, and has a half angle of view of 80° or more and 110° or less. The half angle of view is determined by ray tracing. In addition, in each embodiment of the wide-angle lens, the entire wide-angle lens moves along the optical axis during focusing.

[0014] The first lens group has multiple negative lenses, including a first negative lens located closest to the object and a second negative lens located second from the object, and at least one positive lens. The second lens group has at least one negative lens and multiple positive lenses. The order in which the negative and positive lenses are arranged in the second lens group does not matter. By configuring the first and second lens groups in this way, it is possible to appropriately correct chromatic aberration of magnification, which tends to be particularly large in wide-angle lenses.

[0015] Furthermore, let td be the distance on the optical axis from the lens surface closest to the object to the lens surface closest to the image, f be the focal length of the entire wide-angle lens system, fn be the average focal length of the multiple negative lenses in the first lens group, and fp be the average focal length of at least one positive lens in the first lens group. In this case, the wide-angle lenses of each embodiment satisfy the conditions of the following expressions (1) and (2).

[0016] 2.00≦td / f≦6.50 (1) 0.80≦|fn / fp|≦2.00 (2) Equation (1) shows the condition for the preferable relationship between the thickness of the entire wide-angle lens system in the optical axis direction and the focal length of the entire system. If the thickness of the entire system becomes too large, such that td / f exceeds the upper limit of equation (1), it is easy to suppress aberrations, but this undesirably leads to an increase in the size of the entire system. On the other hand, if the thickness of the entire system becomes too small, such that td / f falls below the lower limit of equation (1), it is good for reducing the size of the entire system, but it is undesirable because it makes it difficult to correct field curvature.

[0017] Equation (2) indicates a condition regarding a preferable relationship between the average focal length of the multiple negative lenses in the first lens group and the average focal length of at least one positive lens. If the average focal length of the multiple negative lenses in the first lens group becomes too small, so that |fn / fp| exceeds the upper limit of equation (2), the first lens group can be made more compact, but this is undesirable because the curvature of field increases. On the other hand, if the average focal length of the multiple negative lenses in the first lens group becomes too small, so that |fn / fp| falls below the lower limit of equation (2), this is preferable for correcting various aberrations, but is undesirable because the first lens group becomes larger.

[0018] The wide-angle lens of each embodiment satisfies at least the conditions of expressions (1) and (2), and thus has a wide angle of view, is compact, and yet corrects aberrations well, resulting in high optical performance.

[0019] It is preferable that the wide-angle lens of each embodiment satisfies at least one of the conditions of the following expressions (3) to (13).

[0020] First, when the distance on the optical axis (total lens length) from the lens surface of the wide-angle lens closest to the object side to the image plane (paraxial image plane) is L and the back focus, which is the air-equivalent distance on the optical axis from the lens surface of the wide-angle lens closest to the image side to the image plane, is sk, it is preferable to satisfy the condition of the following equation (3):

[0021] 1.41≦L / sk≦7.08 (3) Equation (3) is a condition regarding the preferable relationship between the overall lens length and back focal length of a wide-angle lens. If the overall lens length becomes too long, such that L / sk exceeds the upper limit of equation (3), it is easy to suppress aberrations, but this leads to an increase in the size of the entire system, which is undesirable. If the overall lens length becomes too short, such that L / sk falls below the lower limit of equation (3), it is easy to make the entire system compact, but it is undesirable because it becomes difficult to correct mainly field curvature.

[0022] It is also preferable to satisfy the following formula (4):

[0023] 0.25≦f / sk≦0.93 (4) Equation (4) shows the condition for the preferable relationship between the focal length and back focus of the entire wide-angle lens system. Between the lens surface closest to the image side of the wide-angle lens and the image plane, glass for protecting the image sensor located on the image plane, an optical filter, a shutter unit, and the like are arranged. Therefore, it is preferable that f / sk be within the range of equation (4).

[0024] Furthermore, when the focal length of the first negative lens arranged closest to the object in the first lens group (wide-angle lens) is fg1 and the focal length of the entire wide-angle lens system is f, it is preferable to satisfy the condition of the following equation (5):

[0025] 0.73≦|fg1 / f|≦3.60 (5) Equation (5) shows the condition for a preferable relationship between the focal length of the first negative lens and the focal length of the entire system. If the focal length of the first negative lens becomes too small so that |fg1 / f| exceeds the upper limit of equation (5), it is easy to suppress aberrations, but this leads to an increase in size of the entire system, which is not preferable. If the focal length of the first negative lens becomes too large so that |fg1 / f| falls below the lower limit of equation (5), it is easy to reduce the size of the entire system, but it is difficult to correct mainly field curvature, which is not preferable.

[0026] It is also preferable to satisfy the condition of the following formula (6).

[0027] 0.36≦|fg1 / sk|≦2.09 (6) Equation (6) shows the condition for a preferable relationship between the focal length of the first negative lens and the back focus. If the focal length of the first negative lens becomes too small so that |fg1 / sk| exceeds the upper limit of equation (6), it is easy to suppress aberrations, but this leads to an increase in size of the entire system, which is not preferable. If the focal length of the first lens becomes too large so that |fg1 / sk| falls below the lower limit of equation (6), it is easy to reduce the size of the entire system, but it is not preferable because it becomes difficult to correct mainly field curvature.

[0028] Furthermore, when the focal length of the second negative lens arranged second from the object side in the first lens group is taken as fg2, it is preferable that the condition of the following expression (7) be satisfied.

[0029] 0.43≦fg1 / fg2≦1.54 (7) Equation (7) shows the condition for a preferable relationship between the focal length of the first negative lens and the focal length of the second negative lens. If the focal length of the first negative lens is too small, such that fg1 / fg2 exceeds the upper limit of equation (7), the curvature of the image-side lens surface of the first negative lens becomes too small, making it difficult to process, which is undesirable. If the focal length of the first negative lens is too large, such that fg1 / fg2 falls below the lower limit of equation (7), the image-side lens surface of the first negative lens will have a curvature that is easy to process, but it will be difficult to miniaturize the entire system, which is undesirable.

[0030] Furthermore, when the focal length of the positive lens arranged third from the object side in the first lens group (hereinafter referred to as the third positive lens) is fg3, it is preferable that the condition of the following expression (8) be satisfied.

[0031] 0.47≦|fg2 / fg3|≦2.02 (8) Equation (8) shows the condition for the preferable relationship between the focal length of the second negative lens and the focal length of the third positive lens. If the focal length of the third positive lens becomes too small so as to exceed the upper limit of equation (8), the curvature of the image-side lens surface of the second negative lens becomes too small, making it difficult to process, which is undesirable. On the other hand, if the focal length of the third positive lens becomes too large so that fg1 / fg2 falls below the lower limit of equation (8), the image-side lens surface of the second negative lens will have a curvature that is easy to process, but it is also difficult to miniaturize the entire system, which is undesirable.

[0032] Furthermore, when the focal length of the second lens group having positive refractive power is fb2, it is preferable that the condition of the following expression (9) be satisfied.

[0033] 0.91≦fb2 / f≦3.95 (9) Equation (9) shows the condition for the preferable relationship between the focal length of the second lens group and the focal length of the entire system. If the focal length of the second lens group becomes too long, so that fb2 / f exceeds the upper limit of equation (9), it is favorable for correcting various aberrations, but it becomes difficult to make the entire system compact, which is unfavorable. If the focal length of the second lens group becomes too short, so that fb2 / f falls below the lower limit of equation (9), it is favorable for making the entire system compact, but it becomes difficult to make the correction of coma aberration, which is unfavorable.

[0034] It is also preferable to satisfy the condition of the following formula (10).

[0035] 0.45≦fb2 / sk≦2.29 (10) Equation (10) shows the condition for the preferable relationship between the focal length of the second lens group and the back focal length. If the focal length of the second lens group becomes too long, so that fb2 / sk exceeds the upper limit of equation (10), it is favorable for correcting various aberrations, but it becomes difficult to make the entire system compact, which is unfavorable. On the other hand, if the focal length of the second lens group becomes too short, so that fb2 / sk falls below the lower limit of equation (10), it is favorable for making the entire system compact, but it becomes difficult to make the correction of coma aberration, which is unfavorable.

[0036] Furthermore, when the positive lens is disposed closest to the image side in the second lens group, and the focal length of the positive lens is taken as fgf, it is preferable that the following formula (11) be satisfied.

[0037] 1.35≦fgf / sk≦9.44 (11) Equation (11) shows the condition for the preferable relationship between the focal length of the positive lens closest to the image in the second lens group and the back focus. If the focal length of the lens closest to the image in the second lens group becomes too long so that fgf / sk exceeds the upper limit of equation (11), it is good for correcting various aberrations, but it becomes difficult to make the entire system compact, which is undesirable. If the focal length of the lens closest to the image in the second lens group becomes too long so that fgf / sk falls below the lower limit of equation (11), it is good for making the entire system compact, but it becomes difficult to make the correction of field curvature, which is undesirable.

[0038] It is also preferable to satisfy the condition of the following formula (12).

[0039] 0.40≦fgf / L≦2.00 (12) Equation (12) shows the condition for the preferable relationship between the focal length of the positive lens closest to the image in the second lens group and the overall lens length. If the focal length of the lens closest to the image in the second lens group becomes too long so that fgf / L exceeds the upper limit of equation (12), it is good for correcting various aberrations, but it becomes difficult to make the entire system compact, which is undesirable. If the focal length of the lens closest to the image in the second lens group becomes too short so that fgf / L falls below the lower limit of equation (12), it is good for making the entire system compact, but it becomes difficult to make the correction of field curvature, which is undesirable.

[0040] Furthermore, it is preferable that the second lens group includes a cemented lens formed by cementing a positive lens and a negative lens together in order to correct axial chromatic aberration, and when the focal lengths of the positive lens and the negative lens in the cemented lens are denoted by fgp and fgn, respectively, it satisfies the condition of the following expression (13):

[0041] 0.32≦|fgp / fgn|≦1.23 (13) Equation (13) indicates a condition for a preferable relationship between the focal length fgp of the positive lens in the cemented lens and the focal length fgn of the negative lens in the second lens group. If the focal length of the positive lens in the cemented lens becomes too large so that |fgp / fgn| exceeds the upper limit of equation (13), it is good for correcting longitudinal chromatic aberration, but it becomes difficult to make the entire system compact, which is undesirable. If the focal length of the positive lens in the cemented lens becomes too small so that |fgp / fgn| falls below the lower limit of equation (13), it is good for making the entire system compact, but it becomes difficult to make the longitudinal chromatic aberration, which is undesirable.

[0042] It is more preferable to set the numerical ranges of the formulas (1) to (13) as follows:

[0043] 2.70≦td / f≦6.45 (1a) 0.85≦|fn / fp|≦1.50 (2a) 1.97≦L / sk≦6.14 (3a) 0.35≦f / sk≦0.80 (4a) 1.03≦|fg1 / f|≦3.12 (5a) 0.51≦|fg1 / sk|≦1.81 (6a) 0.60≦fg1 / fg2≦1.34 (7a) 0.66≦|fg2 / fg3|≦1.75 (8a) 1.27≦fb2 / f≦3.42 (9a) 0.63≦fb2 / sk≦1.98 (10a) 1.89≦fgf / sk≦8.18 (11a) 0.56≦fgf / L≦1.73 (12a) 0.45≦|fgp / fgn|≦1.06 (13a) It is more preferable to set the numerical ranges of the formulas (1) to (13) as follows:

[0044] 3.30≦td / f≦6.40 (1b) 0.90≦|fn / fp|≦1.35 (2b) 2.53≦L / sk≦5.19 (3b) 0.45≦f / sk≦0.68 (4b) 1.32≦|fg1 / f|≦2.64 (5b) 0.65≦|fg1 / sk|≦1.53 (6b) 0.77≦fg1 / fg2≦1.13 (7b) 0.85≦|fg2 / fg3|≦1.48 (8b) 1.63≦fb2 / f≦2.90 (9b) 0.81≦fb2 / sk≦1.68 (10b) 2.43≦fgf / sk≦6.92 (11b) 0.72≦fgf / L≦1.47 (12b) 0.58≦|fgp / fgn|≦0.90 (13b) Additionally, it is preferable that the first lens group be composed of three lenses arranged in order from the object side to the image side: a negative meniscus lens convex toward the object side as the first negative lens, a biconcave negative lens as the second negative lens, and a third positive lens. This makes it possible to effectively correct both lateral chromatic aberration and field curvature with a small number of lenses.

[0045] For good correction of various aberrations, it is preferable that the second lens group be composed of a cemented lens formed by cementing a positive lens and a negative lens, a positive lens, and another positive lens, all arranged from the object side to the image side.

[0046] It is also preferable that all of the lenses included in the wide-angle lens are spherical lenses, which can be manufactured more cheaply than aspherical lenses.

[0047] Below, Examples 1 to 3 will be explained. After the explanation of Examples 1 to 3, Numerical Examples 1 to 3 corresponding to Examples 1 to 3, respectively, will be shown. [Example]

[0048] The wide-angle lens of Example 1 (Numerical Example 1) shown in FIG. 1 is a fixed focal length lens with a focal length of 8.20 mm, a maximum aperture F-number of 4.12, and a half angle of view of 86.4°.

[0049] The wide-angle lens of this embodiment is composed of, arranged in order from the object side to the image side, a first lens unit B1 with negative refractive power, an aperture stop SP, and a second lens unit B2 with positive refractive power. When focusing from infinity to a close distance, the entire wide-angle lens moves toward the object side.

[0050] The first lens group B1 is composed of, arranged in order from the object side to the image side, a negative meniscus lens convex toward the object side as a first negative lens, a biconcave negative lens as a second negative lens, and a third positive lens. The second lens group B2 is composed of, arranged in order from the object side to the image side, a cemented lens formed by cementing a positive lens and a negative lens, a positive meniscus lens convex toward the image side, and a biconvex positive lens.

[0051] FIG. 2 shows the longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the wide-angle lens of this embodiment when focused at infinity. In the spherical aberration diagram, Fno indicates the maximum F-number, the solid line indicates the spherical aberration for the d-line (wavelength 587.6 nm), and the two-dot chain line indicates the spherical aberration for the g-line (wavelength 435.8 nm). In the astigmatism diagram, ω is the half angle of view (°), the solid line S indicates the sagittal image plane, and the dashed line M indicates the meridional image plane. The distortion diagram shows distortion for the d-line. The chromatic aberration diagram shows lateral chromatic aberration for the g-line. Note that while the wide-angle lens of this embodiment employs the equisolid angle projection method (Y=2fsin(θ / 2)), distortion is shown in the same manner as in general aberration diagrams. However, the projection method is not limited to the equisolid angle projection method, and other projection methods may also be used. The explanations for these aberration diagrams are the same for other embodiments described later.

[0052] The values ​​corresponding to the formulas (1) to (13) in this example are summarized in Table 1. As can be seen from Table 1, the wide-angle lens of this example satisfies the formulas (1) to (13). [Example]

[0053] The wide-angle lens of Example 2 (Numerical Example 2) shown in FIG. 3 is a fixed focal length lens with a focal length of 7.09 mm, a maximum aperture F-number of 4.12, and a half angle of view of 105.0°.

[0054] The wide-angle lens of this embodiment is composed of, arranged in order from the object side to the image side, a first lens unit B1 with positive refractive power, an aperture stop SP, and a second lens unit B2 with positive refractive power. When focusing from infinity to a close distance, the entire wide-angle lens moves toward the object side.

[0055] The first lens group B1 is composed of, arranged in order from the object side to the image side, a negative meniscus lens convex toward the object side as a first negative lens, a biconcave negative lens as a second negative lens, and a third positive lens. The second lens group B2 is composed of, arranged in order from the object side to the image side, a cemented lens formed by cementing a positive lens and a negative lens, a positive meniscus lens convex toward the image side, and a positive meniscus lens convex toward the object side.

[0056] FIG. 4 shows the longitudinal aberration of the wide-angle lens of this embodiment when focused at infinity.

[0057] The values ​​corresponding to the formulas (1) to (13) in this example are summarized in Table 1. As can be seen from Table 1, the wide-angle lens of this example satisfies the formulas (1) to (13). [Example]

[0058] The wide-angle lens of Example 3 (Numerical Example 3) shown in FIG. 5 is a fixed focal length lens with a focal length of 8.20 mm, a maximum aperture F-number of 4.12, and a half angle of view of 86.2°.

[0059] The wide-angle lens of this embodiment is composed of, arranged in order from the object side to the image side, a first lens unit B1 with positive refractive power, an aperture stop SP, and a second lens unit B2 with positive refractive power. When focusing from infinity to a close distance, the entire wide-angle lens moves toward the object side.

[0060] The first lens group B1 is composed of, arranged in order from the object side to the image side, a negative meniscus lens convex toward the object side as a first negative lens, a biconcave negative lens as a second negative lens, and a third positive lens. The second lens group B2 is composed of, arranged in order from the object side to the image side, a cemented lens formed by cementing a positive lens and a negative lens, and a positive meniscus lens convex toward the image side.

[0061] FIG. 6 shows the longitudinal aberration of the wide-angle lens of this embodiment when focused at infinity.

[0062] The values ​​corresponding to the formulas (1) to (13) in this example are summarized in Table 1. As can be seen from Table 1, the wide-angle lens of this example satisfies the formulas (1) to (13).

[0063] In the above examples, the first lens group B1 includes only one positive lens and the second lens group B2 includes only one negative lens, but the first lens group B1 may include multiple positive lenses and the second lens group B2 may include multiple negative lenses. However, from the perspective of compactness, it is preferable that the total number of lenses in the wide-angle lens be eight or less.

[0064] Numerical Examples 1 to 3 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 ith surface from the object side, d is the lens thickness or air gap (mm) between the ith and (i+1)th surfaces, and nd is the refractive index at the d-line of the optical material between the ith and (i+1)th surfaces. νd is the Abbe number based on the d-line of the optical material between the ith and (i+1)th surfaces. The Abbe number νd is expressed as νd = (Nd-1) / (NF-NC), where Nd, NF, and NC are the refractive indices at the Fraunhofer d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm).

[0065] sk is the back focal length (mm), which, as mentioned above, is the air-equivalent distance on the optical axis from the lens surface of a wide-angle lens closest to the image plane. The total lens length is the distance on the optical axis from the lens surface of a wide-angle lens closest to the object to the lens surface closest to the image plane, plus the back focal length.

[0066] (Numerical example 1) Unit: mm Surface Data Surface number rd nd νd 1 24.739 1.00 1.90366 31.3 2 7.409 6.20 3 -26.697 1.00 1.49700 81.5 4 9.521 1.17 5 12.154 3.10 2.00100 29.1 6 -63.879 3.88 7 (Aperture) ∞ 1.00 8 -44.003 5.73 1.59522 67.7 9 -5.028 1.00 1.78472 25.7 10 -12.591 0.10 11 -110.196 2.85 1.49700 81.5 12 -14.971 0.10 13 37.049 2.88 1.48749 70.2 14 -51.617 14.38 15 ∞ 1.50 1.51633 64.1 16∞0.58 Image plane ∞ Focal length 8.20 F-number 4.12 Angle of view (°): 86.4 (by ray tracing) Image height 11.15 Lens length 45.94 sk 15.94 Lens group data Group starting plane focal length 1 1 -63.08 2 7 14.87 3 15 ∞ (Numerical example 2) Unit: mm Surface Data Surface number rd nd νd 1 36.734 4.20 1.90366 31.3 2 8.980 9.10 3 -30.038 1.00 1.49700 81.5 4 11.011 3.22 5 16.020 3.23 2.00100 29.1 6 -53.904 4.60 7 (Aperture) ∞ 1.05 8 -89.022 5.50 1.59522 67.7 9 -4.632 1.00 1.78472 25.7 10 -12.747 0.47 11 -35.508 3.01 1.49700 81.5 12 -11.169 3.29 13 16.638 2.98 1.48749 70.2 14 29.711 9.52 15 ∞ 1.50 1.51633 64.1 16∞0.48 Image plane ∞ Focal length 7.09 F-number 4.12 Angle of view (°) 105.0 (by ray tracing) Image height 11.15 Lens length 53.63 sk 10.98 Lens group data Group starting plane focal length 1 1 121.08 2 7 16.24 3 15 ∞ (Numerical example 3) Unit: mm Surface Data Surface number rd nd νd 1 42.529 2.00 1.90366 31.3 2 12.269 8.34 3 -48.409 1.00 1.49700 81.5 4 11.929 9.30 5 22.640 2.94 2.00100 29.1 6 -182.933 9.58 7 (Aperture) ∞ 1.39 8 746.972 4.74 1.59522 67.7 9 -5.553 1.00 1.78472 25.7 10 -11.714 8.75 11 -44.629 2.95 1.49700 81.5 12 -16.884 11.87 13 ∞ 1.50 1.51633 64.1 14 ∞ 0.58 Image plane ∞ Focal length 8.20 F-number 4.12 Angle of view (°) 86.2 (by ray tracing) Image height 11.15 Lens length 65.44 BF 13.44 Lens group data Group starting plane focal length 1 1 223.89 2 7 21.60 3 13 ∞

[0067] [Table 1]

[0068] 7 shows a digital still camera as an imaging device that uses the wide-angle lens of each of the above-described embodiments as an imaging optical system. 10 denotes the camera body, and 11 denotes the wide-angle lens of any of the embodiments 1 to 3. 12 denotes a solid-state imaging element such as a CCD sensor or CMOS sensor that is built into the camera body 10 and captures the optical image (subject image) formed by the wide-angle lens 11.

[0069] By using the wide-angle lens of each embodiment, it is possible to obtain an imaging device that is small and has high optical performance. The imaging device may be a single-lens reflex camera with a quick-turn mirror, or a mirrorless camera without a quick-turn mirror.

[0070] The embodiments described above are merely representative examples, and various modifications and alterations are possible to each embodiment when implementing the present invention. [Explanation of symbols]

[0071] B1 First lens group SP aperture stop B2 Second lens group

Claims

1. An optical system having a half angle of view of 80° or more, the optical system comprising a first lens group having negative or positive refractive power, an aperture stop, and a second lens group having positive refractive power, which are arranged in this order from the object side to the image side, the first lens group has a plurality of negative lenses, including a first negative lens and a second negative lens, arranged in order from the object side to the image side, and at least one positive lens; the first lens group includes a negative meniscus lens convex toward the object side as a first negative lens, a biconcave lens as a second negative lens, and the positive lens; the second lens group includes, in order from the object side to the image side, a cemented lens formed by cementing a positive lens and a negative lens, and a positive lens; Let td be the distance on the optical axis from the lens surface closest to the object side to the lens surface closest to the image side in the optical system, f be the focal length of the optical system, fn be the average value of the focal lengths of the plurality of negative lenses in the first lens group, and fp be the average value of the focal length of the at least one positive lens in the first lens group. 2.00≦td / f≦6.50 0.96≦|fn / fp|≦1.35 An optical system characterized by satisfying the following conditions:

2. 2. The optical system according to claim 1, wherein the half angle of view is 110[deg.] or less.

3. Let L be the distance on the optical axis from the lens surface closest to the object side to the image plane, and sk be the air-equivalent distance on the optical axis from the lens surface closest to the image side to the image plane. 1.41≦L / sk≦7.08 3. The optical system according to claim 1, wherein the following condition is satisfied:

4. When the air-equivalent distance on the optical axis from the lens surface closest to the image side to the image plane is sk, 0.25≦f / sk≦0.93 4. The optical system according to claim 1, wherein the following condition is satisfied:

5. When the focal length of the first negative lens is fg1, 0.73≦|fg1 / f|≦3.60 5. The optical system according to claim 1, wherein the following condition is satisfied:

6. When the focal length of the first negative lens is fg1 and the air-equivalent distance on the optical axis from the lens surface closest to the image side to the image plane is sk, 0.36≦|fg1 / sk|≦2.09 6. The optical system according to claim 1, wherein the following condition is satisfied:

7. When the focal length of the first negative lens is fg1 and the focal length of the second negative lens is fg2, 0.43≦fg1 / fg2≦1.54 7. The optical system according to claim 1, wherein the following condition is satisfied:

8. When the focal length of the second negative lens is fg2 and the focal length of the positive lens that is disposed third from the object side in the first lens group is fg3, 0.47≦|fg2 / fg3|≦2.02 8. The optical system according to claim 1, wherein the following condition is satisfied:

9. When the focal length of the second lens group is fb2, 0.91≦fb2 / f≦3.95 9. The optical system according to claim 1, wherein the following condition is satisfied:

10. When the focal length of the second lens group is fb2 and the air-equivalent distance on the optical axis from the lens surface closest to the image side to the image plane is sk, 0.45≦fb2 / sk≦2.29 10. The optical system according to claim 1, wherein the following condition is satisfied:

11. When the focal length of the positive lens arranged closest to the image side in the second lens group is fgf and the air-equivalent distance on the optical axis from the lens surface closest to the image side to the image plane is sk, 1.35≦fgf / sk≦9.44 11. The optical system according to claim 1, wherein the following condition is satisfied:

12. When the focal length of the positive lens arranged closest to the image side in the second lens group is fgf and the distance on the optical axis from the lens surface closest to the object side to the image plane is L, 0.40≦fgf / L≦2.00 12. The optical system according to claim 1, wherein the following condition is satisfied:

13. When the focal lengths of the positive lens and the negative lens in the cemented lens are fgp and fgn, respectively, 0.32≦|fgp / fgn|≦1.23 13. The optical system according to claim 1, wherein the following condition is satisfied:

14. 14. The optical system according to claim 1, wherein all lens surfaces of the optical system are spherical.

15. An optical system according to any one of claims 1 to 14; and an imaging element for capturing an image of an object through the optical system.

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