Image formation optical system, optical device, and method for manufacturing image formation optical system

Abstract

An image formation optical system (OL) comprises a front group (GF) including a plurality of lenses and having positive refractive power, and a rear group (GR) including a plurality of lenses, the front group and the rear group being disposed in order from the object side to the image side. The front group includes a negative lens disposed closest to the object side and having a concave surface facing the object side. The distance on the optical axis between a lens disposed closest to the image side in the front group and a lens disposed closest to the object side in the rear group is the maximum among the distances on the optical axis between the lenses adjacent to each other. The image formation optical system satisfies a predetermined conditional expression.

Description

Imaging optical system, optical device, and method for manufacturing imaging optical system

[0001] The present invention relates to an imaging optical system, an optical instrument using the same, and a method for manufacturing an imaging optical system.

[0002] Conventionally, imaging optical systems suitable for optical devices such as photo cameras, electronic still cameras, and video cameras have been proposed (see, for example, Patent Document 1). However, the imaging optical system described in Patent Document 1 has a large number of lenses and is not inexpensive.

[0003] Japanese Patent Application Laid-Open No. 2006-003569

[0004] An imaging optical system according to a first aspect has a front group including a plurality of lenses and having positive refractive power, arranged in order from the object side to the image side, and a rear group including a plurality of lenses, wherein the front group includes a negative lens arranged closest to the object side and having a concave surface facing the object side, and the axial distance between the lens arranged closest to the image side in the front group and the lens arranged closest to the object side in the rear group is the largest among the axial distances between adjacent lenses, and satisfies the following conditional expressions: 0.20 < FF / TT < 0.90 0.15 < Bf / TT < 0.60, where FF is the focal length of the front group, TT is the overall lens length of the imaging optical system, and Bf is the back focal length of the imaging optical system.

[0005] An optical apparatus according to a second aspect includes the imaging optical system described above.

[0006] A manufacturing method for an imaging optical system according to a third aspect is a manufacturing method for an imaging optical system having a front group including a plurality of lenses and having positive refractive power, and a rear group including a plurality of lenses, arranged in order from the object side to the image side, the manufacturing method comprising the steps of: arranging a negative lens with its concave surface facing the object side closest to the object side in the front group; making the axial distance between the lens arranged closest to the image side in the front group and the lens arranged closest to the object side in the rear group the largest among the axial distances between adjacent lenses; and arranging the lenses within a lens barrel so as to satisfy the following conditional expressions: 0.20 < FF / TT < 0.90 0.15 < Bf / TT < 0.60 where FF is the focal length of the front group, TT is the overall lens length of the imaging optical system, and Bf is the back focus of the imaging optical system.

[0007] FIG. 1 is a lens configuration diagram of an imaging optical system according to Example 1 in an infinity-focused state. FIG. 2 is a longitudinal aberration diagram showing various aberrations of the imaging optical system according to Example 1 in an infinity-focused state. FIG. 3 is a lateral aberration diagram of the imaging optical system according to Example 1 in an infinity-focused state. FIG. 4 is a lens configuration diagram of an imaging optical system according to Example 2 in an infinity-focused state. FIG. 5 is a longitudinal aberration diagram showing various aberrations of the imaging optical system according to Example 2 in an infinity-focused state. FIG. 6 is a lateral aberration diagram of the imaging optical system according to Example 2 in an infinity-focused state. FIG. 7 is a lens configuration diagram of an imaging optical system according to Example 3 in an infinity-focused state. FIG. 8 is a longitudinal aberration diagram showing various aberrations of the imaging optical system according to Example 3 in an infinity-focused state. FIG. 9 is a lateral aberration diagram of the imaging optical system according to Example 3 in an infinity-focused state. FIG. 10 is a lens configuration diagram of an imaging optical system according to Example 4 in an infinity-focused state. FIG. 11 is a longitudinal aberration diagram showing various aberrations in the infinity-focused state of the imaging optical system according to Example 4. FIG. 12 is a lateral aberration diagram showing the infinity-focused state of the imaging optical system according to Example 4. FIG. 13 is a lens configuration diagram of the infinity-focused state of the imaging optical system according to Example 5. FIG. 14 is a longitudinal aberration diagram showing various aberrations in the infinity-focused state of the imaging optical system according to Example 5. FIG. 15 is a lateral aberration diagram of the infinity-focused state of the imaging optical system according to Example 5. FIG. 16 is a lens configuration diagram of the infinity-focused state of the imaging optical system according to Example 6. FIG. 17 is a longitudinal aberration diagram showing various aberrations in the infinity-focused state of the imaging optical system according to Example 6. FIG. 18 is a lateral aberration diagram of the infinity-focused state of the imaging optical system according to Example 6. FIG. 19 is a lens configuration diagram of the infinity-focused state of the imaging optical system according to Example 7. Fig. 20 is a longitudinal aberration diagram showing various aberrations in the infinity-focused state of the imaging optical system according to Example 7. Fig. 21 is a lateral aberration diagram in the infinity-focused state of the imaging optical system according to Example 7. Fig. 22 is a lens configuration diagram in the infinity-focused state of the imaging optical system according to Example 8. Fig. 23 is a longitudinal aberration diagram showing various aberrations in the infinity-focused state of the imaging optical system according to Example 8. Fig. 24 is a lateral aberration diagram in the infinity-focused state of the imaging optical system according to Example 8.Fig. 25 is a diagram showing the configuration of a camera equipped with the imaging optical system according to this embodiment, and Fig. 26 is a flowchart showing a method for manufacturing the imaging optical system according to this embodiment.

[0008] A preferred embodiment of the present disclosure will now be described. First, a camera 1 as an optical device equipped with an imaging optical system OL according to this embodiment will be described with reference to FIG. 25 . As shown in FIG. 25 , this camera 1 is composed of a main body 2 and a photographic lens 3 attached to the main body 2. The main body 2 includes an image sensor 4, a main body control unit (not shown) that controls the operation of the digital camera, and an LCD screen 5. The photographic lens 3 includes an image sensor 4, an image sensor 5, a lens position control mechanism (not shown) that controls the position of each lens group. The lens position control mechanism includes a sensor that detects the position of the lens group, a motor that moves the lens group back and forth along the optical axis, and a control circuit that drives the motor. The image sensor 4 is a solid-state image sensor (photoelectric conversion element) such as a CCD sensor or CMOS sensor that receives and photoelectrically converts the optical image formed by the imaging optical system OL of the photographic lens 3.

[0009] Light from the subject is collected by the imaging optical system OL of the photographic lens 3 and reaches the image plane IP of the image sensor 4. The light from the subject that reaches the image plane IP is photoelectrically converted by the image sensor 4 and recorded as digital image data in a memory (not shown). The digital image data recorded in the memory can be displayed on the liquid crystal screen 5 in response to a user operation. Note that this camera may be a mirrorless camera or a single-lens reflex camera with a quick-return mirror.

[0010] Next, an imaging optical system OL according to this embodiment will be described. As shown in FIG. 1 , the imaging optical system OL(1), which is an example of the imaging optical system (photographing lens) OL according to this embodiment, has a front group GF with positive refractive power and including multiple lenses, and a rear group GR with multiple lenses, arranged in order from the object side to the image side. The front group GF includes a negative lens (first lens L1 in the example of FIG. 1 ) arranged closest to the object and with a concave surface facing the object side. The optical axial distance between the lens (third lens L3 in the example of FIG. 1 ) arranged closest to the image in the front group GF and the lens (fourth lens L4 in the example of FIG. 1 ) arranged closest to the object in the rear group GR is the largest of the optical axial distances between adjacent lenses. That is, the front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(1) between them.

[0011] Incidentally, optical members such as a cover glass and a filter may be disposed before and after the imaging optical system OL according to this embodiment.

[0012] With the above configuration, the imaging optical system OL according to this embodiment satisfies the following conditional expressions (1) and (2): 0.20<FF / TT<0.90 (1) 0.15<Bf / TT<0.60 (2) where FF is the focal length of the front group GF, TT is the overall lens length of the imaging optical system OL, and Bf is the back focus of the imaging optical system OL.

[0013] The total lens length of the imaging optical system OL is the distance on the optical axis from the object-side lens surface of the lens arranged closest to the object (the lens surface closest to the object) in the imaging optical system OL to the image plane IP.

[0014] According to this embodiment, it is possible to provide an imaging optical system OL that has a small number of lenses, is compact, and yet is capable of correcting aberrations in a well-balanced manner. The imaging optical system OL according to this embodiment may be the imaging optical system OL(2) shown in Fig. 4, the imaging optical system OL(3) shown in Fig. 7, the imaging optical system OL(4) shown in Fig. 10, the imaging optical system OL(5) shown in Fig. 13, the imaging optical system OL(6) shown in Fig. 16, the imaging optical system OL(7) shown in Fig. 19, or the imaging optical system OL(8) shown in Fig. 22.

[0015] Conditional expression (1) defines an appropriate relationship between the focal length of the front group GF and the overall lens length of the imaging optical system OL. By satisfying conditional expression (1), it is possible to provide an imaging optical system OL that is compact, uses a small number of lenses, and yet is capable of correcting aberrations in a well-balanced manner.

[0016] If the corresponding value of conditional expression (1) exceeds the upper limit, the focal length of the front group GF becomes large relative to the overall lens length of the imaging optical system OL. This results in a larger lens shape for the front group GF, and an increase in the size of the imaging optical system OL. By setting the upper limit of conditional expression (1) to 0.84, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (1) may be set to 0.78, 0.72, 0.66, or even 0.60.

[0017] If the corresponding value of conditional expression (1) falls below the lower limit, the focal length of the front group GF becomes short relative to the overall lens length of the imaging optical system OL. This increases the power of each lens element constituting the front group GF, resulting in higher-order spherical aberration, coma, field curvature, and distortion, making it difficult to correct these aberrations. Setting the lower limit of conditional expression (1) to 0.25 further ensures the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (1) may be set to 0.30, 0.35, 0.40, or even 0.45.

[0018] Conditional expression (2) defines an appropriate relationship between the back focus of the imaging optical system OL and the overall lens length of the imaging optical system OL. By satisfying conditional expression (2), it is possible to provide an imaging optical system OL that is small in size despite having a small number of lenses and is capable of correcting aberrations in a well-balanced manner.

[0019] If the corresponding value of conditional expression (2) exceeds the upper limit, the back focus of the imaging optical system OL becomes longer relative to the overall lens length of the imaging optical system OL, resulting in an increase in the size of the imaging optical system OL. By setting the upper limit of conditional expression (2) to 0.54, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (2) may be set to 0.48, 0.42, 0.36, or even 0.30.

[0020] If the corresponding value of conditional expression (2) falls below the lower limit, the power of each lens constituting the imaging optical system OL becomes strong, causing high-order spherical aberration and making it difficult to correct this aberration. By setting the lower limit of conditional expression (2) to 0.16, the effect of this embodiment can be more reliably achieved. To further ensure the effect of this embodiment, the lower limit of conditional expression (2) may be set to 0.17, 0.18, 0.19, or even 0.20.

[0021] The imaging optical system OL according to this embodiment is preferably made up of six or fewer lenses, because a small number of lenses, such as six or fewer, makes it possible to provide a compact imaging optical system OL.

[0022] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (3): 1.20<|RF| / TT<7.00 (3) where |RF|: absolute value of the focal length of the rear group GR, and TT: total lens length of the imaging optical system OL.

[0023] Conditional expression (3) defines an appropriate relationship between the absolute value of the focal length of the rear group GR and the overall lens length of the imaging optical system OL. Satisfying conditional expression (3) is desirable because it makes it possible to provide an imaging optical system OL that is compact and capable of correcting aberrations in a balanced manner despite having a small number of lens elements.

[0024] If the corresponding value of conditional expression (3) exceeds the upper limit, the absolute value of the focal length of the rear group GR becomes large relative to the overall lens length of the imaging optical system OL. This results in a larger lens shape for the rear group GR, which undesirably increases the size of the imaging optical system OL. By setting the upper limit of conditional expression (3) to 6.60, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (3) may be set to 6.20, 5.80, 5.40, or even 5.00.

[0025] If the corresponding value of conditional expression (3) falls below the lower limit, the absolute value of the focal length of the rear group GR becomes small relative to the overall lens length of the imaging optical system OL. This increases the power of each lens element constituting the rear group GR, resulting in high-order spherical aberration, coma, field curvature, and distortion, making it difficult to correct these aberrations, which is undesirable. Setting the lower limit of conditional expression (3) to 1.29 further ensures the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (3) may be set to 1.38, 1.47, 1.56, or even 1.65.

[0026] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (4): 0.050<FF / |RF|<0.700 (4) where FF is the focal length of the front group GF, and |RF| is the absolute value of the focal length of the rear group GR.

[0027] Conditional expression (4) defines an appropriate relationship between the focal length of the front group GF and the absolute value of the focal length of the rear group GR. Satisfying conditional expression (4) is desirable because it enables coma and distortion to be corrected in a well-balanced manner.

[0028] If the corresponding value of conditional expression (4) exceeds the upper limit, the focal length of the front group GF becomes large relative to the absolute value of the focal length of the rear group GR. This undesirably destroys the symmetry of the imaging optical system OL, making it difficult to correct coma and distortion in a balanced manner. Setting the upper limit of conditional expression (4) to 0.63 can further ensure the effects of this embodiment. To further ensure the effects of this embodiment, the upper limit of conditional expression (4) may be set to 0.56, 0.49, 0.42, or even 0.35.

[0029] If the corresponding value of conditional expression (4) falls below the lower limit, the focal length of the front group GF becomes small relative to the absolute value of the focal length of the rear group GR. This undesirably destroys the symmetry of the imaging optical system OL, making it difficult to correct coma and distortion in a balanced manner. Setting the lower limit of conditional expression (4) to 0.059 can further ensure the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (4) may be set to 0.068, 0.077, 0.086, or even 0.095.

[0030] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (5): 0.20<-G1f / TT<0.70 (5) where G1f is the focal length of the negative lens arranged closest to the object in the imaging optical system OL, and TT is the overall lens length of the imaging optical system OL.

[0031] Conditional expression (5) defines an appropriate relationship between the focal length of the first lens L1, which is the negative lens located closest to the object in the imaging optical system OL, and the overall lens length of the imaging optical system OL. Satisfying conditional expression (5) is desirable because it makes it possible to provide an imaging optical system OL that is capable of correcting field curvature despite having a small number of lenses.

[0032] If the corresponding value of conditional expression (5) exceeds the upper limit, the focal length of the first lens L1 becomes small relative to the overall lens length of the imaging optical system OL. This causes high-order spherical aberration and coma, making it difficult to correct these aberrations, which is undesirable. By setting the upper limit of conditional expression (5) to 0.69, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (5) may be set to 0.68, 0.67, 0.66, or even 0.65.

[0033] If the corresponding value of conditional expression (5) falls below the lower limit, the focal length of the first lens L1 becomes large relative to the overall lens length of the imaging optical system OL. This undesirably increases the Petzval sum of the imaging optical system OL, making it difficult to correct the curvature of field. By setting the lower limit of conditional expression (5) to 0.25, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the lower limit of conditional expression (5) may be set to 0.30, 0.35, 0.40, or even 0.45.

[0034] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (6): 0.70<-G1f / FF<2.00 (6) where G1f is the focal length of the negative lens located closest to the object in the imaging optical system OL, and FF is the focal length of the front group GF.

[0035] Conditional expression (6) defines an appropriate relationship between the focal length of the first lens L1, which is the negative lens located closest to the object in the imaging optical system OL, and the focal length of the front group GF. Satisfying conditional expression (6) is desirable because it enables well-balanced correction of spherical aberration and coma.

[0036] If the corresponding value of conditional expression (6) exceeds the upper limit, the focal length of the first lens L1 becomes smaller relative to the focal length of the front group GF. This causes high-order spherical aberration and coma, making it difficult to correct these aberrations, which is undesirable. By setting the upper limit of conditional expression (6) to 1.88, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (6) may be set to 1.76, 1.64, 1.52, or even 1.40.

[0037] If the corresponding value of conditional expression (6) falls below the lower limit, the focal length of the first lens L1 becomes large relative to the focal length of the front group GF. This results in a larger lens shape for the first lens, which undesirably increases the size of the imaging optical system OL. By setting the lower limit of conditional expression (6) to 0.74, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the lower limit of conditional expression (6) may be set to 0.78, 0.82, 0.86, or even 0.90.

[0038] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (7): 0.10<-G1f / |RF|<0.60 (7) where G1f is the focal length of the negative lens arranged closest to the object in the imaging optical system, and |RF| is the absolute value of the focal length of the rear group GR.

[0039] Conditional expression (7) defines an appropriate relationship between the focal length of the first lens L1, which is the negative lens located closest to the object in the imaging optical system OL, and the absolute value of the focal length of the rear group GR. Satisfying conditional expression (7) is desirable because it enables well-balanced correction of coma and curvature of field.

[0040] If the corresponding value of conditional expression (7) exceeds the upper limit, the focal length of the first lens L1 becomes small relative to the absolute value of the focal length of the rear group GR. This causes high-order coma and curvature of field, making it difficult to correct these aberrations, which is undesirable. By setting the upper limit of conditional expression (7) to 0.56, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (7) may be set to 0.52, 0.48, 0.44, or even 0.40.

[0041] If the corresponding value of conditional expression (7) falls below the lower limit, the focal length of the first lens L1 becomes large relative to the absolute value of the focal length of the rear group GR. This results in a larger lens shape for the first lens L1, which undesirably increases the size of the imaging optical system OL. Setting the lower limit of conditional expression (7) to 0.106 can further ensure the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (7) may be set to 0.112, 0.118, 0.124, or even 0.130.

[0042] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (8): 1.700<|RF| / F<9.500 (8) where |RF|: absolute value of the focal length of the rear group GR, and F: focal length of the imaging optical system OL.

[0043] Conditional expression (8) defines an appropriate relationship between the absolute value of the focal length of the rear group GR and the focal length of the imaging optical system OL. Satisfying conditional expression (8) is desirable because it enables balanced correction of coma and curvature of field. Furthermore, it is desirable because it enables provision of an imaging optical system OL that is compact and capable of balanced correction of aberrations despite the small number of lenses.

[0044] If the corresponding value of conditional expression (8) exceeds the upper limit, the absolute value of the focal length of the rear group GR becomes large relative to the focal length of the imaging optical system OL. This results in a larger lens shape for the rear group GR, which undesirably increases the size of the imaging optical system OL. By setting the upper limit of conditional expression (8) to 9.260, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (8) may be set to 9.020, 8.780, 8.540, or even 8.320.

[0045] If the corresponding value of conditional expression (8) falls below the lower limit, the absolute value of the focal length of the rear group GR becomes small relative to the focal length of the imaging optical system OL. This undesirably increases the power of each lens element constituting the rear group GR, resulting in high-order spherical aberration, coma, field curvature, and distortion, making these aberrations difficult to correct. Setting the lower limit of conditional expression (8) to 1.830 further ensures the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (8) may be set to 1.960, 2.090, 2.220, or even 2.350.

[0046] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (9): 0.50<G2f / FF<2.00 (9) where G2f is the focal length of the second lens element from the object side in the imaging optical system OL, and FF is the focal length of the front group GF.

[0047] Conditional expression (9) defines an appropriate relationship between the focal length of the second lens L2, which is the second lens from the object side in the imaging optical system OL, and the focal length of the front group GF. Satisfying conditional expression (9) is desirable because it enables well-balanced correction of spherical aberration and coma.

[0048] If the corresponding value of conditional expression (9) exceeds the upper limit, the focal length of the second lens L2 becomes large relative to the focal length of the front group GF. This results in a larger lens shape for the second lens L2, which undesirably increases the size of the imaging optical system OL. By setting the upper limit of conditional expression (9) to 1.93, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (9) may be set to 1.86, 1.79, 1.72, or even 1.65.

[0049] If the corresponding value of conditional expression (9) falls below the lower limit, the focal length of the second lens L2 becomes smaller relative to the focal length of the front group GF. This causes high-order spherical aberration and coma, making it difficult to correct these aberrations, which is undesirable. By setting the lower limit of conditional expression (9) to 0.59, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the lower limit of conditional expression (9) may be set to 0.68, 0.77, 0.86, or even 0.95.

[0050] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (10): 0.10 < G2f / |RF| < 0.80 (10), where G2f is the focal length of the second lens element from the object side in the imaging optical system OL, and |RF| is the absolute value of the focal length of the rear group GR.

[0051] Conditional expression (10) defines an appropriate relationship between the focal length of the second lens L2, which is the second lens from the object side in the imaging optical system OL, and the absolute value of the focal length of the rear group GR. Satisfying conditional expression (10) is desirable because it enables well-balanced correction of coma and curvature of field.

[0052] If the corresponding value of conditional expression (10) exceeds the upper limit, the focal length of the second lens L2 becomes large relative to the absolute value of the focal length of the rear group GR. This results in a larger lens shape for the second lens L2, which undesirably increases the size of the imaging optical system OL. Setting the upper limit of conditional expression (10) to 0.73 can further ensure the effects of this embodiment. To further ensure the effects of this embodiment, the upper limit of conditional expression (10) may be set to 0.66, 0.59, 0.52, or even 0.45.

[0053] If the corresponding value of conditional expression (10) falls below the lower limit, the focal length of the second lens L2 becomes small relative to the absolute value of the focal length of the rear group GR. This causes high-order coma and curvature of field, making it difficult to correct these aberrations, which is undesirable. By setting the lower limit of conditional expression (10) to 0.11, the effect of this embodiment can be more reliably achieved. To further ensure the effect of this embodiment, the lower limit of conditional expression (10) may be set to 0.12, 0.13, 0.14, or even 0.15.

[0054] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (11): 0.30<G2f / TT<1.00 (11) where G2f is the focal length of the second lens located from the object side in the imaging optical system OL, and TT is the total lens length of the imaging optical system OL.

[0055] Conditional expression (11) defines an appropriate relationship between the focal length of the second lens L2, which is the second lens from the object side in the imaging optical system OL, and the overall lens length of the imaging optical system OL. Satisfying conditional expression (11) is desirable because it makes it possible to provide an imaging optical system OL that is capable of correcting field curvature despite having a small number of lenses.

[0056] If the corresponding value of conditional expression (11) exceeds the upper limit, the focal length of the second lens L2 becomes large relative to the overall lens length of the imaging optical system OL. This makes it difficult to correct spherical aberration and coma, which is undesirable. By setting the upper limit of conditional expression (11) to 0.96, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (11) may be set to 0.92, 0.88, 0.84, or even 0.80.

[0057] If the corresponding value of conditional expression (11) falls below the lower limit, the focal length of the second lens L2 becomes small relative to the overall lens length of the imaging optical system OL. This increases the Petzval sum of the imaging optical system OL, making it difficult to correct the curvature of field, which is undesirable. By setting the lower limit of conditional expression (11) to 0.35, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the lower limit of conditional expression (11) may be set to 0.40, 0.45, 0.50, or even 0.55.

[0058] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (12): 1.5 < (AR1+AR2) / (AR2-AR1) < 5.0 (12) where AR1 is the radius of curvature of the object-side lens surface of the second lens from the image side in the imaging optical system OL, and AR2 is the radius of curvature of the image-side lens surface of the second lens from the image side in the imaging optical system OL.

[0059] Conditional expression (12) defines an appropriate range for the shape factor of the second lens element from the image side in the imaging optical system OL. Satisfying conditional expression (12) is desirable because it makes the second lens element from the image side more resistant to temperature changes.

[0060] If the corresponding value of conditional expression (12) exceeds the upper limit, a change in refractive index occurs due to a temperature change, making it difficult to correct aberrations caused by the change in refractive index, which is undesirable. By setting the upper limit of conditional expression (12) to 4.82, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (12) may be set to 4.64, 4.46, 4.28, or even 4.10.

[0061] If the corresponding value of conditional expression (12) falls below the lower limit, a change in refractive index occurs due to a temperature change, making it difficult to correct aberrations caused by the change in refractive index, which is undesirable. By setting the lower limit of conditional expression (12) to 1.54, the effect of this embodiment can be more reliably achieved. To further ensure the effect of this embodiment, the lower limit of conditional expression (12) may be set to 1.58, 1.62, 1.66, or even 1.70.

[0062] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (13): 0.6 < (BR1+BR2) / (BR2-BR1) < 5.5 (13) where BR1 is the radius of curvature of the object-side lens surface of the lens located closest to the image in the imaging optical system OL, and BR2 is the radius of curvature of the image-side lens surface of the lens located closest to the image in the imaging optical system OL.

[0063] Conditional expression (13) defines an appropriate range for the shape factor of the lens located closest to the image side in the imaging optical system OL. Satisfying conditional expression (13) is desirable because it makes the lens located closest to the image side more resistant to temperature changes.

[0064] If the corresponding value of conditional expression (13) exceeds the upper limit, a change in refractive index occurs due to a temperature change, making it difficult to correct aberrations caused by the change in refractive index, which is undesirable. By setting the upper limit of conditional expression (13) to 5.36, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (13) may be set to 5.22, 5.08, 4.94, or even 4.80.

[0065] If the corresponding value of conditional expression (13) falls below the lower limit, a change in refractive index occurs due to a temperature change, making it difficult to correct aberrations caused by the change in refractive index, which is undesirable. By setting the lower limit of conditional expression (13) to 0.62, the effect of this embodiment can be more reliably achieved. To further ensure the effect of this embodiment, the lower limit of conditional expression (13) may be set to 0.64, 0.66, 0.68, or even 0.70.

[0066] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (14): 0.50<FF / F<1.00 (14) where FF is the focal length of the front group GF, and F is the focal length of the imaging optical system OL.

[0067] Conditional expression (14) defines an appropriate relationship between the focal length of the front group GF and the focal length of the imaging optical system OL. Satisfying conditional expression (14) is desirable because it makes it possible to provide an imaging optical system OL that is small in size despite having a small number of lens elements and is capable of correcting aberrations in a balanced manner.

[0068] If the corresponding value of conditional expression (14) exceeds the upper limit, the focal length of the front group GF becomes larger than the focal length of the imaging optical system OL. This results in a larger lens shape for the front group GF, which undesirably increases the size of the imaging optical system OL. By setting the upper limit of conditional expression (14) to 0.965, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (14) may be set to 0.930, 0.895, 0.860, or even 0.825.

[0069] If the corresponding value of conditional expression (14) falls below the lower limit, the focal length of the front group GF becomes smaller than the focal length of the imaging optical system OL. This increases the power of each lens element constituting the front group GF, resulting in high-order spherical aberration, coma, curvature of field, and distortion, which are difficult to correct, which is undesirable. Setting the lower limit of conditional expression (14) to 0.545 further ensures the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (14) may be set to 0.590, 0.635, 0.680, or even 0.725.

[0070] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (15): 0.10<RDG / RD<0.67 (15) where RDG is the sum of the center thicknesses of the multiple lenses included in the rear group GR, and RD is the distance on the optical axis from the lens surface in the rear group GR closest to the object to the lens surface in the rear group GR closest to the image.

[0071] Conditional expression (15) defines the appropriate relationship between the sum of the center thicknesses of the multiple lenses included in the rear group GR and the distance on the optical axis from the lens surface in the rear group GR that is closest to the object to the lens surface in the rear group GR that is closest to the image. Satisfying conditional expression (15) is desirable because it makes it possible to provide an imaging optical system OL that is small in size and capable of correcting aberrations in a balanced manner, despite having a small number of lenses.

[0072] If the corresponding value of conditional expression (15) exceeds the upper limit, the lens thickness of the rear group GR increases, making the imaging optical system OL heavy, which is undesirable. By setting the upper limit of conditional expression (15) to 0.645, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (15) may be set to 0.620, 0.595, 0.570, or even 0.545.

[0073] If the corresponding value of conditional expression (15) falls below the lower limit, the length of the rear group GR increases, which undesirably increases the size of the imaging optical system OL. By setting the lower limit of conditional expression (15) to 0.159, the effect of this embodiment can be more reliably achieved. To further ensure the effect of this embodiment, the lower limit of conditional expression (15) may be set to 0.218, 0.277, 0.336, or even 0.395.

[0074] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (16): 0.50<GFrF / F<1.50 (16), where GFrF is the focal length of the lens in the front group GF that is located closest to the image side, and F is the focal length of the imaging optical system OL.

[0075] Conditional expression (16) defines an appropriate relationship between the focal length of the lens located closest to the image side in the front group GF and the focal length of the imaging optical system OL. Satisfying conditional expression (16) is desirable because it makes it possible to provide an imaging optical system OL that is capable of correcting field curvature with a small number of lenses.

[0076] If the corresponding value of conditional expression (16) exceeds the upper limit, the focal length of the lens positioned closest to the image in the front group GF becomes longer than the focal length of the imaging optical system OL. This makes it difficult to correct spherical aberration and coma, which is undesirable. By setting the upper limit of conditional expression (16) to 1.415, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (16) may be set to 1.330, 1.245, 1.160, or even 1.075.

[0077] If the corresponding value of conditional expression (16) falls below the lower limit, the focal length of the lens positioned closest to the image in the front group GF becomes shorter relative to the focal length of the imaging optical system OL. This increases the Petzval sum of the imaging optical system OL, making correction of field curvature undesirably difficult. Setting the lower limit of conditional expression (16) to 0.578 can further ensure the effects of this embodiment. To further ensure the effects of this embodiment, the lower limit of conditional expression (16) may be set to 0.656, 0.734, 0.812, or even 0.890.

[0078] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (17): 0.13 < DM / SD < 0.40 (17), where DM is the distance on the optical axis between the lens located closest to the image in the front group GF and the lens located closest to the object in the rear group GR, and SD is the distance on the optical axis from the lens surface located closest to the object to the lens surface located closest to the image in the imaging optical system OL.

[0079] Conditional expression (17) defines an appropriate relationship between the axial distance between the lens located closest to the image in the front group GF and the lens located closest to the object in the rear group GR, and the axial distance from the lens surface closest to the object to the lens surface closest to the image in the imaging optical system OL. Satisfying conditional expression (17) is desirable because it makes it possible to provide an imaging optical system OL that is small in size and capable of correcting aberrations in a balanced manner, despite the small number of lenses.

[0080] If the corresponding value of conditional expression (17) exceeds the upper limit, the distance on the optical axis between the lens in the front group GF located closest to the image and the lens in the rear group GR located closest to the object becomes large. This increases the overall lens length of the imaging optical system OL, undesirably increasing the size of the imaging optical system OL. By setting the upper limit of conditional expression (17) to 0.387, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (17) may be set to 0.374, 0.361, 0.348, or even 0.335.

[0081] If the corresponding value of conditional expression (17) falls below the lower limit, the distance on the optical axis between the lens element in the front group GF located closest to the image and the lens element in the rear group GR located closest to the object becomes small. This makes it difficult to correct spherical aberration and coma, which is undesirable. By setting the lower limit of conditional expression (17) to 0.137, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the lower limit of conditional expression (17) may be set to 0.144, 0.151, 0.158, or even 0.165.

[0082] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (18): ndR<1.7 (18) where ndR is the average value of the refractive indexes of the multiple lenses included in the rear group GR.

[0083] Furthermore, it is desirable that the imaging optical system OL according to this embodiment more satisfies the following conditional expression (18a), which sets a lower limit: 1.4<ndR<1.7 (18a)

[0084] Conditional expressions (18) and (18a) define an appropriate range for the average value of the refractive indexes of the multiple lenses included in the rear group GR. Satisfying conditional expression (18) or (18a) is desirable because it makes it possible to provide an inexpensive imaging optical system OL.

[0085] When the corresponding value of conditional expression (18) or conditional expression (18a) exceeds the upper limit, the refractive index of the multiple lenses included in the rear group GR becomes high. Lenses with high refractive indexes are expensive, which increases costs and is undesirable. By setting the upper limit of conditional expression (18) and conditional expression (18a) to 1.69, the effects of this embodiment can be further ensured. To further ensure the effects of this embodiment, the upper limit of conditional expression (18) and conditional expression (18a) may be set to 1.68, 1.66, 1.64, 1.62, or even 1.60.

[0086] If the corresponding value of conditional expression (18a) falls below the lower limit, the refractive index of the multiple lenses included in the rear group GR will be low. This makes aberration correction difficult, which is undesirable. By setting the lower limit of conditional expression (18a) to 1.42, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the lower limit of conditional expression (18a) may be set to 1.46, 1.50, 1.52, 1.54, or even 1.56.

[0087] In the imaging optical system OL according to this embodiment, the lenses included in the rear group GR are preferably made up of a plurality of resin lenses.

[0088] This is because resin lenses (plastic lenses) have a low specific gravity, which allows the weight of the imaging optical system OL to be reduced. Furthermore, the use of resin lenses makes it possible to mold more complex surface shapes than glass lenses, thereby reducing the number of lenses while maintaining optical performance. Furthermore, the multiple resin lenses arranged in the rear group GR can cancel out changes in optical performance caused by temperature changes, making the imaging optical system OL as a whole less susceptible to the effects of temperature changes.

[0089] In the imaging optical system OL according to this embodiment, it is preferable that the rear group GR includes a resin lens having a negative refractive power and a resin lens having a positive refractive power.

[0090] This is because by arranging a resin lens with negative refractive power and a resin lens with positive refractive power in the rear group GR, it is possible to reduce changes in the overall optical performance of the imaging optical system OL when there is a temperature change.

[0091] It is desirable that the imaging optical system OL according to this embodiment satisfy the following conditional expression (19): 18.4°<ω<33.2° (19) where ω: half angle of view (°) of the imaging optical system OL.

[0092] Conditional expression (19) defines an appropriate range for the angle of view of the imaging optical system OL. Satisfying conditional expression (19) is desirable because it makes it possible to provide an imaging optical system OL that is small in size despite having a small number of lenses and that is capable of correcting aberrations in a balanced manner.

[0093] If the corresponding value of conditional expression (19) exceeds the upper limit, the angle of view becomes wider, and the number of lenses required for aberration correction increases. This undesirably increases the size of the imaging optical system OL. By setting the upper limit of conditional expression (19) to 32.4, the effect of this embodiment can be further ensured. To further ensure the effect of this embodiment, the upper limit of conditional expression (19) may be set to 31.6°, 30.8°, 30.0°, or even 29.2°.

[0094] If the corresponding value of conditional expression (19) falls below the lower limit, the angle of view becomes narrow and chromatic aberration becomes noticeable, which is undesirable. By setting the lower limit of conditional expression (19) to 18.9°, the effect of this embodiment can be more reliably achieved. To further ensure the effect of this embodiment, the lower limit of conditional expression (19) may be set to 19.4°, 19.9°, 20.4°, or even 20.9°.

[0095] In the imaging optical system OL according to this embodiment, it is desirable that the lenses included in the front group GF include glass lenses. Furthermore, it is desirable that the negative lens closest to the object side be a glass lens. This is because the effects of temperature changes can be suppressed.

[0096] It is also desirable that the lens surface of the front group GF closest to the image side and the lens surface of the rear group GR closest to the object side have shapes that face each other with their concave surfaces facing each other, because this makes it possible to correct the Petzval sum of the imaging optical system OL.

[0097] Next, a manufacturing method for the imaging optical system OL according to this embodiment will be outlined with reference to FIG. In this manufacturing method, first, the front group GF and the rear group GR are arranged in order from the object side to the image side (Step ST1). Next, a negative lens with its concave surface facing the object side is arranged closest to the object side of the front group GF (Step ST2). Next, the axial distance between the lens closest to the image side in the front group and the lens closest to the object side in the rear group, which are adjacent to each other, is configured to be the longest axial distance between adjacent lenses (Step ST3). Finally, the lenses are arranged within the lens barrel so as to satisfy at least conditional expressions (1) and (2) above (Step ST4).

[0098] According to this manufacturing method, it is possible to manufacture an imaging optical system that has a small number of lenses, is compact, and yet is capable of correcting aberrations in a well-balanced manner.

[0099] The imaging optical systems OL according to each example of this embodiment will be described below with reference to the drawings. FIGS. 1, 4, 7, 10, 13, 16, 19, and 22 are cross-sectional views showing the configuration and refractive power distribution of the imaging optical systems OL {OL(1) to OL(8)} according to Examples 1 to 8, respectively. In the cross-sectional views of the imaging optical systems OL(1) to OL(8) according to Examples 1 to 8, the direction of movement along the optical axis of the focusing group when focusing from infinity to a close object is indicated by an arrow along with the word "focusing." In the imaging optical systems OL according to Examples 1 to 8, when focusing from infinity to a close object, the entire imaging optical system OL moves as a unit toward the object.

[0100] 1, 4, 7, 10, 13, 16, 19, and 22, the front and rear groups are represented by a combination of the symbol G, the symbol F, and the symbol R, and each lens is represented by a combination of the symbol L and a number. In this case, to prevent the symbols and numbers from becoming too numerous and complicated, lenses, etc. are represented by different combinations of symbols and numbers for each embodiment. Therefore, even if the same combinations of symbols and numbers are used between embodiments, this does not mean that the embodiments have the same configuration.

[0101] Tables 1 to 8 are shown below, with Table 1 showing data on the various specifications for Example 1, Table 2 for Example 2, Table 3 for Example 3, Table 4 for Example 4, Table 5 for Example 5, Table 6 for Example 6, Table 7 for Example 7, and Table 8 for Example 8. In each example, the d-line (wavelength λ=587.6 nm) was selected as the target for calculating aberration characteristics.

[0102] The table titled "Overall Specifications" lists the overall specifications of the imaging optical system OL, where F denotes the focal length of the entire imaging optical system OL, FNO denotes the F-number, ω denotes the half angle of view (unit: ° (degrees), where 2ω is the angle of view), and Y denotes the image height. These values ​​are those when the imaging optical system OL is focused on an object at infinity (when focused at infinity). TT denotes the distance from the frontmost lens surface to the last lens surface on the optical axis when focused at infinity plus BF, and BF denotes the distance from the last lens surface on the optical axis to the image plane IP when focused at infinity (back focus). Also, in the table titled "Overall Specifications," FF denotes the focal length of the front group GF when focused at infinity. RF denotes the focal length of the rear group GR when focused at infinity. AR1 denotes the radius of curvature of the object-side lens surface of the second lens element from the image side in the imaging optical system OL. AR2 indicates the radius of curvature of the image-side lens surface of the second lens from the image side in the imaging optical system OL. BR1 indicates the radius of curvature of the object-side lens surface of the lens located closest to the image side in the imaging optical system OL. BR2 indicates the radius of curvature of the image-side lens surface of the lens located closest to the image side in the imaging optical system OL. FfRfD indicates the axial distance from the lens surface located closest to the image side in the front group GF to the lens surface located closest to the image side in the rear group, and RDG indicates the sum of the central thicknesses of the multiple lenses included in the rear group GR. RD indicates the axial distance from the lens surface located closest to the object side in the rear group GR to the lens surface located closest to the image side in the rear group. GFrF indicates the focal length of the lens located closest to the image side in the front group GF. DM indicates the axial distance between the lens located closest to the image side in the front group GF and the lens located closest to the object side in the rear group. SD indicates the axial distance from the lens surface located closest to the object side to the lens surface located closest to the image side in the imaging optical system OL. ndR represents the average value of the refractive index of the multiple lenses included in the rear group GR, all of which are values ​​when the imaging optical system OL is focused at infinity.

[0103] In the "Lens Specifications" table, the surface number indicates the order of the optical surface from the object side along the direction of light ray travel, R is the radius of curvature of each optical surface (surfaces whose center of curvature is located on the image side have a positive value), D is the surface spacing, which is the distance on the optical axis from each optical surface to the next optical surface (or image plane), nd is the refractive index of the material of the optical element with respect to the d-line, and vd is the Abbe number of the material of the optical element with respect to the d-line. The "∞" next to the radius of curvature indicates a plane or an aperture, and (aperture SP) indicates the aperture stop SP. The refractive index of air, nd = 1.00000, is omitted. If the optical surface is aspherical, an * is added to the surface number, and the paraxial radius of curvature is shown in the "radius of curvature R" column.

[0104] In the table of [Aspherical Surface Data], the shape of the aspherical surface shown in [Lens Specifications] is expressed by the following formula (A). X(y) is the distance (amount of sag) along the optical axis from the tangent plane at the vertex of the aspherical surface to the position on the aspherical surface at height y, R is the radius of curvature of the reference spherical surface (paraxial radius of curvature), κ is the conic constant, and Ai is the ith aspherical coefficient. "E-n" is "×10 -n For example, 1.234E-05 = 1.234 x 10 -5 The second-order aspherical coefficient A2 is 0, and is therefore omitted.

[0105] X(y)=(Ry 2 ) / {1+(1-(1+κ)R 2 y 2 ) 1 / 2} + A4 × y 4 +A6×y 6 +A8×y 8 +A10×y 10 +A12×y 12 ...(A)

[0106] The [Lens Data] table shows the starting surface (the surface closest to the object), focal length, and lens construction length of each lens.

[0107] The table of [Values ​​Corresponding to Conditional Expressions] shows values ​​corresponding to the above conditional expressions (1) to (18).

[0108] In the following, for all specifications, the focal length f, radius of curvature R, surface spacing D, and other lengths are generally expressed in "mm" unless otherwise specified, but this is not limited to this, as the same optical performance can be obtained even when the optical system is proportionally enlarged or reduced.

[0109] The explanation of the tables up to this point is common to all the embodiments, and duplicate explanations will be omitted below.

[0110] First Example The first example will be described with reference to FIGS. 1 to 3 and Table 1. FIG. 1 is a diagram showing the lens configuration of an imaging optical system OL(1) according to the first example. The imaging optical system OL(1) according to the first example is composed of five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(1) between them.

[0111] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a biconcave lens with negative refractive power, a second lens L2 which is a biconvex lens with positive refractive power, and a third lens L3 which is a meniscus lens with positive refractive power and whose convex surface faces the object side.

[0112] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a fifth lens L5 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side and image-side surfaces.

[0113] An aperture stop SP is disposed between the second lens L2 and the third lens L3.

[0114] Table 1 below lists the values ​​of the specifications of the imaging optical system OL(1) according to the first example. (Table 1) [Overall specifications] F = 35.00 FNO = 1.80 ω (°) = 22.400 Y = 14.50 TT = 53.15 BF = 14.19 FF = 27.06 RF = -112.44 AR1 = -10.760 AR2 = -19.482 BR1=17.632 BR2=31.649 FfRfD=22.182 RDG=3.987 RD=9.487 GFrF=34.752 DM=12.694 SD=38.964 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -23.76 6.17 1.85 23.69 2 313.26 0.10 3 186.02 5.14 1.83 42.95 4 -28.66 0.10 5 ∞ 0.10 (Aperture) 6 18.26 5.17 1.83 42.95 7 42.92 12.69 8* -10.76 1.31 1.64 22.3 9* -19.48 5.50 10* 17.63 2.68 1.54 56.36 11* 31.65 14.19 [Aspheric surface data] Surface 8 κ=0.0000, A4=1.366E-04, A6=7.317E-06 A8=-6.554E-08, A10=-3.015E-10, A12=8.806E-12 A14=-1.822E-14, A16=-2.284E-16 9th side κ=0.0000, A4=-7.281E-06, A6=7.186E-06 A8=-4.959E-08, A10=-4.787E-10, A12=1.233E-11 A14=-8.876E-14, A16=2.338E-16 10th side κ=0.0000, A4=-2.823E-04, A6=-5.548E-07 A8=6.243E-08, A10=-1.358E-09, A12=1.516E-11 A14=-8.794E-14, A16=2.058E-16 Surface 11 κ=0.0000, A4=-1.462E-04, A6=-1.814E-06 A8=6.052E-08, A10=-1.019E-09, A12=9.574E-12 A14=-4.795E-14, A16=9.700E-17 [Lens Data] Lens Surface Focal Length L1 1 -25.869 L2 3 30.074 L3 6 34.752 L4 8 -39.760 L5 10 69.762

[0115] FIG. 2 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(1) according to Example 1 when focused at infinity. In the spherical aberration diagram, the amount of spherical aberration for the d-line (wavelength 587.56 nm) shown by the solid line is represented by the deviation (mm) in the optical axis direction from the paraxial image plane, and the vertical axis represents the value obtained by normalizing the incident height on the pupil by its maximum height (i.e., relative pupil height). In the astigmatism diagram, the dashed line T represents the tangential image plane for the d-line, and the solid line S represents the sagittal image plane for the d-line, each represented by the deviation (mm) in the optical axis direction from the paraxial image plane, and the vertical axis represents the image height (IMG HT, mm). In the distortion diagram, the horizontal axis represents the distortion (%) for the d-line, and the vertical axis represents the image height (IMG HT, mm). Note that the maximum value of the image height IMG HT corresponds to the maximum image height at the image plane IP.

[0116] 3 is a diagram showing lateral aberration of the imaging optical system OL(1) according to Example 1 when focused at infinity. In FIG. 3, the left column shows lateral aberration (mm) for tangential light beams, and the right column shows lateral aberration (mm) for sagittal light beams. Also shown is the lateral aberration at the image height ratio (half angle of view ω°) expressed in terms of relative field height, for the d-line (wavelength 587.56 nm) shown by the solid line. The image height ratio is the relative image height obtained by normalizing the image height IMG HT by the maximum image height.

[0117] In the aberration diagrams of the following examples, the same reference numerals as in the first example are used, and overlapping explanations will be omitted.

[0118] Second Example A second example will be described with reference to FIGS. 4 to 6 and Table 2. FIG. 4 is a diagram showing the lens configuration of an imaging optical system OL(2) according to the second example. The imaging optical system OL(2) according to the second example comprises five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(2) between them.

[0119] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a concave lens (meniscus lens) with negative refractive power and a concave surface facing the object side, a second lens L2 which is a meniscus lens with positive refractive power and a concave surface facing the object side, and a third lens L3 which is a meniscus lens with positive refractive power and a convex surface facing the object side.

[0120] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a fifth lens L5 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side and image-side surfaces.

[0121] An aperture stop SP is disposed between the third lens L3 and the fourth lens L4.

[0122] Table 2 below lists the values ​​of the specifications of the imaging optical system OL(2) according to the second example. (Table 2) [Overall specifications] F = 35.00 FNO = 1.80 ω (°) = 22.450 Y = 14.50 TT = 49.28 BF = 11.05 FF = 25.46 RF = -83.07 AR1 = -10.932 AR2 = -20.461 BR1=20.739 BR2=34.565 FfRfD=25.244 RDG=5.089 RD=12.556 GFrF=31.972 DM=12.689 SD=38.230 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -20.34 1.20 1.73 28.38 2 -304.31 3.02 3 -338.21 4.07 1.76 52.34 4 -24.49 0.10 5 17.18 4.59 1.76 52.34 6 52.75 2.71 7 ∞ 9.97 (Aperture) 8* -10.93 1.89 1.64 22.41 9* -20.46 7.47 10* 20.74 3.20 1.54 55.71 11* 34.57 11.05 [Aspheric surface data] Surface 8 κ=0.0000, A4=0.000E+00, A6=5.656E-05 A8=7.340E-06, A10=-7.965E-08, A12=3.795E-10 9th side κ=0.0000, A4=0.000E+00, A6=-2.012E-05 A8=6.199E-06, A10=-5.347E-08, A12=2.338E-10 10th side κ=0.0000, A4=0.000E+00, A6=-2.898E-04 A8=1.249E-06, A10=-5.660E-09, A12=1.306E-11 Page 11 κ=0.0000, A4=0.000E+00, A6=-2.172E-04 A8=6.208E-07, A10=-2.143E-09, A12=4.575E-13 [Lens data] Lens Initial surface Focal length L1 1 -29.982 L2 3 34.772 L3 5 31.972 L4 8 -39.634 L5 10 89.681.

[0123] FIG. 5 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(2) according to the second example when focused at infinity.

[0124] FIG. 6 is a diagram showing lateral aberration of the imaging optical system OL(2) according to the second example when focused at infinity.

[0125] Third Example A third example will be described with reference to FIGS. 7 to 9 and Table 3. FIG. 7 is a diagram showing the lens configuration of an imaging optical system OL(3) according to the third example. The imaging optical system OL(3) according to the third example is composed of five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having a positive refractive power overall and a rear group GR having a negative refractive power overall. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(3) between them.

[0126] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a biconcave lens with negative refractive power, a second lens L2 which is a biconvex lens with positive refractive power, and a third lens L3 which is a meniscus lens with positive refractive power and whose convex surface faces the object side.

[0127] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a fifth lens L5 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side and image-side surfaces.

[0128] An aperture stop SP is disposed between the second lens L2 and the third lens L3.

[0129] Table 3 below lists the values ​​of the specifications of the imaging optical system OL(3) according to the third example. (Table 3) [Overall specifications] F=50.17 FNO=1.80 ω(°)=22.600 Y=21.60 TT=70.13 BF=16.48 FF=38.74 RF=-163.41 AR1=-14.998 AR2=-28.838 BR1=26.334 BR2=55.710 FfRfD=33.768 RDG=7.879 RD=16.089 GFrF=48.026 DM=17.679 SD=53.655 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -35.06 4.00 1.81 23.72 2 203.28 1.00 3 185.08 6.58 1.86 41.96 4 -41.42 0.50 5 ∞ 0.50 (Aperture) 6 25.01 7.30 1.83 42.74 7 57.88 17.68 8* -15.00 2.12 1.64 22.41 9* -28.84 8.21 10* 26.33 5.76 1.54 55.71 11* 55.71 16.49 [Aspheric surface data] Surface 8 κ=0.0000, A4=5.015E-05, A6=1.359E-06 A8=-6.268E-09, A10=-1.506E-11, A12=2.171E-13 A14=-2.020E-16, A16=-6.848E-19 9th side κ=0.0000, A4=-4.237E-06, A6=1.329E-06 A8=-4.695E-09, A10=-2.271E-11, A12=3.098E-13 A14=-1.103E-15, A16=1.618E-18 10th side κ=0.0000, A4=-1.009E-04, A6=-1.092E-07 A8=5.919E-09, A10=-6.566E-11, A12=3.748E-13 A14=-1.105E-15, A16=1.336E-18 Surface 11 κ=0.0000, A4=-5.320E-05, A6=-3.364E-07 A8=5.745E-09, A10=-4.925E-11, A12=2.368E-13 A14=-6.027E-16, A16=6.279E-19 [Lens Data] Lens Surface Focal Length L1 1 -36.749 L2 3 40.111 L3 6 48.026 L4 8 -51.767 L5 10 87.375

[0130] FIG. 8 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(3) according to the third example when focused at infinity.

[0131] FIG. 9 is a diagram showing lateral aberration of the imaging optical system OL(3) according to the second example when focused at infinity.

[0132] Fourth Example A fourth example will be described using FIGS. 10 to 12 and Table 4. FIG. 10 is a diagram showing the lens configuration of an imaging optical system OL(4) according to the fourth example. The imaging optical system OL(4) according to the fourth example is composed of five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(4) between them.

[0133] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a concave lens (meniscus lens) with negative refractive power and a concave surface facing the object side, a second lens L2 which is a meniscus lens with positive refractive power and a concave surface facing the object side, and a third lens L3 which is a meniscus lens with positive refractive power and a convex surface facing the object side.

[0134] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a fifth lens L5 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side and image-side surfaces.

[0135] An aperture stop SP is disposed between the third lens L3 and the fourth lens L4.

[0136] Table 4 below lists the values ​​of the specifications of the imaging optical system OL(4) according to the fourth example. (Table 4) [Overall specifications] F = 51.44 FNO = 1.80 ω (°) = 21.237 Y = 20.05 TT = 73.15 BF = 14.73 FF = 38.00 RF = -127.41 AR1 = -16.278 AR2 = -30.216 BR1=27.173 BR2=41.576 FfRfD=38.987 RDG=7.946 RD=19.946 GFrF=47.825 DM=19.040 SD=58.422 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -30.47 1.80 1.73 28.38 2 -569.63 4.49 3 -703.05 6.13 1.76 52.34 4 -36.83 0.10 5 25.71 6.92 1.76 52.34 6 78.99 4.07 7 ∞ 14.97 (Aperture) 8* -16.28 2.65 1.64 22.41 9* -30.22 12.00 10* 27.17 5.29 1.54 55.71 11* 41.58 14.73 [Aspheric surface data] Surface 8 κ=0.0000, A4=1.155E-05, A6=1.042E-06 A8=-4.980E-09, A10=1.045E-11 9th side κ=0.0000, A4=-1.080E-05, A6=8.956E-07 A8=-3.626E-09, A10=7.453E-12 10th side κ=0.0000, A4=-8.188E-05, A6=1.364E-07 A8=-2.969E-10, A10=3.878E-13 11th side κ=0.0000, A4=-6.220E-05, A6=6.703E-08 A8=-1.264E-10, A10=1.187E-13 [Lens Data] Lens Initial Surface Focal Length L1 1 -44.270 L2 3 51.268 L3 5 47.825 L4 8 -59.375 L5 10 129.952

[0137] FIG. 11 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(4) according to the fourth example when focused at infinity.

[0138] FIG. 12 is a diagram showing lateral aberration of the imaging optical system OL(4) according to the fourth example when focused at infinity.

[0139] Fifth Example A fifth example will be described using FIGS. 13 to 15 and Table 5. FIG. 13 is a diagram showing the lens configuration of an imaging optical system OL(5) according to the fifth example. The imaging optical system OL(5) according to the fifth example is composed of five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(5) between them.

[0140] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a biconcave lens with negative refractive power, a second lens L2 which is a meniscus lens with positive refractive power and whose concave surface faces the object side, and a third lens L3 which is a meniscus lens with positive refractive power and whose convex surface faces the object side.

[0141] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a fifth lens L5 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side and image-side surfaces.

[0142] An aperture stop SP is disposed between the third lens L3 and the fourth lens L4.

[0143] Table 5 below lists the values ​​of the specifications of the imaging optical system OL(5) according to the fifth example. (Table 5) [Overall specifications] F = 28.84 FNO = 2.88 ω (°) = 26.170 Y = 14.50 TT = 46.80 BF = 12.93 FF = 21.98 RF = -106.16 AR1 = -11.871 AR2 = -30.891 BR1=21.532 BR2=95.984 FfRfD=18.910 RDG=4.835 RD=8.936 GFrF=28.448 DM=9.973 SD=33.871 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -20.69 1.20 1.73 28.38 2 2227.39 1.74 3 -524.83 7.53 1.76 52.34 4 -24.25 0.10 5 15.60 4.39 1.76 52.34 6 50.07 1.70 ∞ 8.28 (Aperture) 8* -11.87 1.59 1.64 22.41 9* -30.89 4.10 10* 21.53 3.24 1.54 55.71 11* 95.98 12.94 [Aspheric surface data] Surface 8 κ=0.0000, A4=1.691E-05, A6=1.129E-05 A8=-1.656E-07, A10=9.227E-10 9th side κ=0.0000, A4=-1.418E-04, A6=1.073E-05 A8=-1.128E-07, A10=5.632E-10 10th side κ=0.0000, A4=-2.947E-04, A6=8.428E-07 A8=-2.634E-09, A10=1.191E-11 11th side κ=0.0000, A4=-6.193E-05, A6=-1.323E-06 A8=1.131E-08, A10=-4.119E-11 [Lens Data] Lens Initial Surface Focal Length L1 1 -28.139 L2 3 33.464 L3 5 28.448 L4 8 -31.037 L5 10 51.107

[0144] FIG. 14 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(5) according to the fifth example when focused at infinity.

[0145] FIG. 15 is a diagram showing lateral aberration of the imaging optical system OL(5) according to the fifth example when focused at infinity.

[0146] Sixth Example The sixth example will be described with reference to FIGS. 16 to 18 and Table 6. FIG. 16 is a diagram showing the lens configuration of an imaging optical system OL(6) according to the sixth example. The imaging optical system OL(6) according to the sixth example is composed of five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(6) between them.

[0147] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a biconcave lens with negative refractive power, a second lens L2 which is a biconvex lens with positive refractive power, and a third lens L3 which is a meniscus lens with positive refractive power and whose convex surface faces the object side.

[0148] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power with its concave surface facing the object side on and near the optical axis, and a fifth lens L5 with positive refractive power and a biconvex lens on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces having inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side surface and the image-side surface.

[0149] An aperture stop SP is disposed between the third lens L3 and the fourth lens L4.

[0150] Table 6 below lists the values ​​of the specifications of the imaging optical system OL(6) according to the sixth example. (Table 6) [Overall specifications] F = 24.70 FNO = 2.88 ω (°) = 28.900 Y = 14.50 TT = 42.35 BF = 11.70 FF = 19.80 RF = -205.00 AR1 = -12.014 AR2 = -45.302 BR1=21.544 BR2=-130.752 FfRfD=16.585 RDG=4.201 RD=7.753 GFrF=26.177 DM=8.832 SD=30.645 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -20.93 1.20 1.73 28.38 2 316.37 3.52 3 8378.63 5.00 1.76 52.34 4 -24.22 0.10 5 14.55 4.23 1.76 52.34 6 48.21 2.61 7 ∞ 6.22 (Aperture) 8* -12.01 1.00 1.64 22.41 9* -45.30 3.55 10* 21.54 3.20 1.54 55.71 11* -130.75 11.71 [Aspheric data] Surface 8 κ=0.0000, A4=-6.618E-05, A6=1.895E-05 A8=-3.708E-07, A10=2.671E-09 9th side κ=0.0000, A4=-2.230E-04, A6=1.556E-05 A8=-1.966E-07, A10=9.769E-10 10th side κ=0.0000, A4=-2.044E-04, A6=-1.439E-06 A8=1.469E-08, A10=-3.754E-11 Page 11 κ=0.0000, A4=1.183E-04, A6=-3.819E-06 A8=2.799E-08, A10=-8.192E-11 [Lens Data] Lens Initial Surface Focal Length L1 1 -26.914 L2 3 31.992 L3 5 26.177 L4 8 -25.762 L5 10 34.825

[0151] FIG. 17 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(6) according to Example 6 when focused at infinity.

[0152] FIG. 18 is a diagram showing lateral aberration of the imaging optical system OL(6) according to the sixth example when focused at infinity.

[0153] Seventh Example The seventh example will be described using FIGS. 19 to 21 and Table 7. FIG. 19 is a diagram showing the lens configuration of an imaging optical system OL(7) according to the seventh example. The imaging optical system OL(7) according to the seventh example is composed of six lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the imaging optical system OL(6) between them.

[0154] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a biconcave lens with negative refractive power, a second lens L2 which is a biconvex lens with positive refractive power, a third lens L3 which is a meniscus lens with positive refractive power and whose concave surface faces the object side, and a fourth lens L4 which is a meniscus lens with positive refractive power and whose convex surface faces the object side.

[0155] The rear group GR includes, arranged in order from the object side to the image side, a fifth lens L5 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a sixth lens L6 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fifth lens L5 and the sixth lens L6 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fifth lens L5 and the sixth lens L6 are formed on both the object-side and image-side surfaces.

[0156] An aperture stop SP is disposed between the third lens L3 and the fourth lens L4.

[0157] Table 7 below lists the values ​​of the specifications of the imaging optical system OL(7) according to the seventh example. (Table 7) [Overall specifications] F = 51.29 FNO = 1.85 ω (°) = 21.700 Y = 21.60 TT = 70.10 BF = 17.40 FF = 40.30 RF = -176.00 AR1 = -15.003 AR2 = -28.123 BR1=27.823 BR2=64.871 FfRfD=31.816 RDG=7.879 RD=14.926 GFrF=45.868 DM=16.890 SD=52.700 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -57.84 2.00 1.75 24.55 2 53.88 1.00 3 55.30 5.78 1.88 40.57 4 -94.48 1.30 5 -51.27 2.36 1.86 41.64 6 -48.83 0.50 7 ∞ 0.50 (Aperture) 8 25.06 7.48 1.84 41.65 9 60.94 16.89 10* -15.00 2.12 1.64 22.41 11* -28.12 7.05 12* 27.82 5.76 1.54 55.71 13* 64.87 17.41 [Aspheric surface data] Surface 10 κ=0.0000, A4=5.135E-05, A6=1.362E-06 A8=-6.262E-09, A10=-1.493E-11, A12=2.189E-13 A14=-2.023E-16, A16=-1.293E-18 11th side κ=0.0000, A4=-3.651E-06, A6=1.345E-06 A8=-4.582E-09, A10=-2.239E-11, A12=3.080E-13 A14=-1.150E-15, A16=1.593E-18 12th side κ=0.0000, A4=-9.942E-05, A6=-9.814E-08 A8=5.901E-09, A10=-6.586E-11, A12=3.740E-13 A14=-1.105E-15, A16=1.359E-18 Surface 13 κ=0.0000, A4=-4.413E-05, A6=-3.447E-07 A8=5.701E-09, A10=-4.930E-11, A12=2.370E-13 A14=-6.020E-16, A16=6.282E-19 [Lens Data] Lens Surface Focal Length L1 1 -36.703 L2 3 40.135 L3 5 822.722 L4 8 45.868 L5 10 -52.928 L6 12 85.996.

[0158] FIG. 20 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(7) according to the seventh example when focused at infinity.

[0159] FIG. 21 is a diagram showing lateral aberration of the imaging optical system OL(7) according to the seventh example when focused at infinity.

[0160] Eighth Example Eighth Example will be described using FIGS. 22 to 24 and Table 8. FIG. 12 is a diagram showing the lens configuration of an imaging optical system OL(8) according to the eighth example. The imaging optical system OL(8) according to the eighth example is composed of five lenses, arranged in order from the object side to the image side along the optical axis, a front group GF having an overall positive refractive power and a rear group GR having an overall negative refractive power. The front group GF and the rear group GR are arranged with the largest air gap in the optical system OL(8) between them.

[0161] The front group GF includes, arranged in order from the object side to the image side, a first lens L1 which is a meniscus lens with negative refractive power and a concave surface facing the object side, a second lens L2 which is a meniscus lens with positive refractive power and a concave surface facing the object side, and a third lens L3 which is a meniscus lens with positive refractive power and a convex surface facing the object side. The first lens L1 and the second lens L2 are cemented together to form a single cemented lens (lens element).

[0162] The rear group GR includes, arranged in order from the object side to the image side, a fourth lens L4 with a meniscus shape and negative refractive power, with a concave surface facing the object side on and near the optical axis, and a fifth lens L5 with a meniscus shape and positive refractive power, with a convex surface facing the object side on and near the optical axis. Both the fourth lens L4 and the fifth lens L5 are aspherical lenses with aspherical surfaces that have inflection points in a cross section including the optical axis. The aspherical surfaces of the fourth lens L4 and the fifth lens L5 are formed on both the object-side and image-side surfaces.

[0163] An aperture stop SP is disposed between the second lens L2 and the third lens L3.

[0164] Table 8 below lists the values ​​of the specifications of the imaging optical system OL(8) according to the eighth example. (Table 8) [Overall specifications] F = 35.64 FNO = 2.88 ω (°) = 21.800 Y = 14.50 TT = 49.40 BF = 11.70 FF = 27.96 RF = -122.19 AR1 = -9.954 AR2 = -16.404 BR1=16.743 BR2=26.213 FfRfD=35.793 RDG=5.084 RD=11.758 GFrF=33.179 DM=12.297 SD=37.700 ndR=1.589 [Lens specifications] Surface number R D nd νd 1 -21.10 3.00 1.85 23.69 2 -285.18 4.50 1.83 42.95 3 -23.84 0.32 4 ∞ 0.10 (Aperture) 5 18.14 5.73 1.83 42.95 6 45.00 12.30 7* -9.95 1.57 1.64 22.41 8* -16.40 6.67 9* 16.74 3.51 1.54 55.71 10* 26.21 11.74 [Aspheric surface data] Surface 7 κ=0.0000, A4=-1.167E-05, A6=7.038E-06 A8=-4.539E-08, A10=-4.256E-10, A12=1.209E-11 A14=-1.028E-13, A16=3.675E-16 8th side κ=0.0000, A4=-1.167E-05, A6=7.038E-06 A8=-4.539E-08, A10=-4.256E-10, A12=1.209E-11 A14=-1.028E-13, A16=3.675E-16 9th side κ=0.0000, A4=-2.794E-04, A6=-7.120E-07 A8=6.261E-08, A10=-1.349E-09, A12=1.516E-11 A14=-8.831E-14, A16=2.095E-16 Surface 10 κ=0.0000, A4=-1.730E-04, A6=-1.653E-06 A8=6.032E-08, A10=-1.023E-09, A12=9.606E-12 A14=-4.759E-14, A16=9.640E-17 [Lens Data] Lens Surface Focal Length L1 1 -27.055 L2 2 30.916 L3 5 33.179 L4 7 -43.571 L5 9 76.701

[0165] FIG. 23 is a longitudinal aberration diagram showing various aberrations of the imaging optical system OL(8) according to Example 8 when focused at infinity.

[0166] FIG. 24 is a diagram showing lateral aberration of the imaging optical system OL(8) according to Example 8 when focused at infinity.

[0167] Next, the table of [Values ​​Corresponding to Conditional Expressions] is shown below, which summarizes the values ​​corresponding to each of the conditional expressions (1) to (18) for all the examples (Examples 1 to 8). Conditional expression (1) 0.20 < FF / TT < 0.90 Conditional expression (2) 0.15 < Bf / TT < 0.60 Conditional expression (3) 1.20 < |RF| / TT < 7.00 Conditional expression (4) 0.050 < FF / |RF| < 0.700 Conditional expression (5) 0.20 < -G1f / TT < 0.70 Conditional expression (6) 0.70 < -G1f / FF < 2.00 Conditional expression (7) 0.10 < -G1f / |RF| < 0.60 Conditional expression (8) 1.700 < |RF| / F < 9.500 Conditional expression (9) 0.50 < G2f / FF < 2.00 Conditional expression (10) 0.10 < G2f / |RF| <0.80 Conditional expression (11) 0.30 < G2f / TT <1.00 Conditional expression (12) 1.5 < (AR1+AR2) / (AR2-AR1) < 5.0 Conditional expression (13) 0.6 < (BR1+BR2) / (BR2-BR1) < 5.5 Conditional expression (14) 0.50 < FF / F < 1.00 Conditional expression (15) 0.10 < RDG / RD < 0.67 Conditional expression (16) 0.50 < GFrF / F < 1.50 Conditional expression (17) 0.13 < DM / SD < 0.40 Conditional expression (18) ndR < 1.7

[0168] [Values ​​corresponding to conditional expressions] (Examples 1 to 4) Conditional Expression Example 1 Example 2 Example 3 Example 4 (1) 0.509 0.517 0.552 0.519 (2) 0.267 0.224 0.235 0.201 (3) 2.115 1.686 2.330 1.742 (4) 0.241 0.306 0.237 0.298 (5) 0.487 0.608 0.524 0.605 (6) 0.956 1.178 0.949 1.165 (7) 0.230 0.361 0.225 0.347 (8) 3.212 2.374 3.258 2.477 (9) 1.111 1.366 1.035 1.349 (10) 0.267 0.419 0.245 0.402 (11) 0.566 0.706 0.572 0.701 (12) 3.467 3.295 3.167 3.336 (13) 3.516 4.000 2.793 4.773 (14) 0.773 0.727 0.772 0.739 (15) 0.420 0.405 0.490 0.398 (16) 0.993 0.914 0.957 0.930 (17) 0.326 0.332 0.329 0.326 (18) 1.589 1.589 1.589 1.589

[0169] [Values ​​corresponding to conditional expressions] (Fifth to Eighth Examples) Conditional Expression Fifth Example Sixth Example Seventh Example Eighth Example (1) 0.470 0.468 0.575 0.566 (2) 0.276 0.276 0.248 0.237 (3) 2.268 4.841 2.511 2.473 (4) 0.207 0.097 0.229 0.229 (5) 0.601 0.636 0.524 0.548 (6) 1.281 1.359 0.911 0.968 (7) 0.265 0.131 0.209 0.221 (8) 3.681 8.300 3.431 3.428 (9) 1.523 1.616 0.996 1.106 (10) 0.315 0.156 0.228 0.253 (11) 0.715 0.756 0.573 0.626 (12) 2.248 1.722 3.287 4.087 (13) 1.578 0.717 2.502 4.536 (14) 0.762 0.802 0.786 0.784 (15) 0.541 0.542 0.528 0.432 (16) 0.986 1.060 0.894 0.931 (17) 0.294 0.288 0.320 0.326 (18) 1.589 1.589 1.589 1.589

[0170] According to each of the above embodiments, it is possible to realize an imaging optical system that has a small number of lenses, is compact, and yet is capable of correcting aberrations in a well-balanced manner.

[0171] The above-described examples are merely illustrative examples of the present invention, and the present invention is not limited to these.

[0172] The following contents can be appropriately adopted within the scope that does not impair the optical performance of the imaging optical system of this embodiment.

[0173] Although examples of the imaging optical system of this embodiment have been shown to have a five-lens or six-lens configuration, the present application is not limited to this, and an optical system with other lens configurations (for example, a four-lens or seven-lens configuration, etc.) can also be configured. Specifically, a lens can be added to the imaging optical system of this embodiment closest to the object or closest to the image plane.

[0174] One or more lenses may be moved so as to include a component in a direction perpendicular to the optical axis, or rotated (oscillated) in a plane including the optical axis to serve as an anti-vibration lens that corrects image blur caused by camera shake.

[0175] The lens surface may be spherical, flat, or aspherical. Spherical or flat lens surfaces are preferred because they facilitate lens processing and assembly adjustment, and prevent degradation of optical performance due to errors in processing and assembly adjustment. Furthermore, they are preferred because they minimize degradation of imaging performance even when the image plane is misaligned.

[0176] When the lens surface is aspherical, the aspherical surface may be any of a ground aspherical surface, a glass-molded aspherical surface in which glass is molded into an aspherical shape, and a hybrid aspherical surface in which a resin is formed into an aspherical shape on the surface of glass. The lens surface may also be a diffractive surface, or the lens may be a gradient index lens (GRIN lens) or a plastic lens.

[0177] The aperture stop SP is preferably disposed adjacent to the image side of the second lens L2 or the third lens L3, but it is also possible to use the lens frame to fulfill the role of the aperture stop SP instead of providing a component for the aperture stop SP.

[0178] Each lens surface may be coated with an anti-reflection coating that has high transmittance over a wide wavelength range in order to reduce flare and ghosting and achieve high-contrast optical performance.

[0179] The present invention is not limited to the above. Other aspects conceivable within the scope of the technical idea of ​​the present invention are also included in the scope of the present invention. This embodiment may combine all or part of the above aspects.

[0180] OL Imaging optical system GF Front group GR Rear group L1 First lens L2 Second lens L3 Third lens L4 Fourth lens L5 Fifth lens L6 Sixth lens SP Aperture stop IP Image plane

Claims

1. An imaging optical system having a front group including multiple lenses and having positive refractive power, arranged in order from the object side to the image side, and a rear group including multiple lenses, wherein the front group includes a negative lens arranged closest to the object side with its concave surface facing the object side, and the axial distance between the lens arranged closest to the image side in the front group and the lens arranged closest to the object side in the rear group is the largest of the axial distances between adjacent lenses, and which satisfies the following conditional expressions: 0.20 < FF / TT < 0.90 0.15 < Bf / TT < 0.60 where FF: focal length of the front group, TT: overall lens length of the imaging optical system, and Bf: back focus of the imaging optical system.

2. The imaging optical system according to claim 1, wherein said imaging optical system is made up of six or fewer lenses.

3. The imaging optical system according to claim 1 or 2, which satisfies the following condition: 1.20<|RF| / TT<7.00, where |RF|: absolute value of the focal length of the rear group.

4. The imaging optical system according to any one of claims 1 to 3, which satisfies the following condition: 0.050 < FF / |RF| < 0.700, where |RF|: absolute value of the focal length of the rear group.

5. The imaging optical system according to any one of claims 1 to 4, which satisfies the following condition: 0.20 < -G1f / TT < 0.70, where G1f is the focal length of the negative lens arranged closest to the object in the imaging optical system.

6. The imaging optical system according to any one of claims 1 to 5, which satisfies the following condition: 0.70 < -G1f / FF < 2.00, where G1f is the focal length of the negative lens located closest to the object in the imaging optical system.

7. The imaging optical system according to any one of claims 1 to 6, which satisfies the following condition: 0.10 < -G1f / |RF| < 0.60, where G1f is the focal length of the negative lens arranged closest to the object in the imaging optical system, and |RF| is the absolute value of the focal length of the rear group.

8. The imaging optical system according to any one of claims 1 to 7, which satisfies the following condition: 1.700 < |RF| / F < 9.500, where |RF| is the absolute value of the focal length of the rear group, and F is the focal length of the imaging optical system.

9. The imaging optical system according to any one of claims 1 to 8, which satisfies the following condition: 0.50 < G2f / FF < 2.00, where G2f is the focal length of the second lens located from the object side in the imaging optical system.

10. An imaging optical system according to any one of claims 1 to 9, which satisfies the following condition: 0.10 < G2f / |RF| < 0.80, where G2f is the focal length of the second lens element from the object side in the imaging optical system, and RF is the absolute value of the focal length of the rear group.

11. The imaging optical system according to any one of claims 1 to 10, which satisfies the following condition: 0.30 < G2f / TT < 1.00, where G2f is the focal length of the second lens located from the object side in the imaging optical system.

12. The imaging optical system according to any one of claims 1 to 11, satisfying the following condition: 1.5 < (AR1 + AR2) / (AR2 - AR1) < 5.0, where AR1 is the radius of curvature of the object-side lens surface of the second lens from the image side in the imaging optical system, and AR2 is the radius of curvature of the image-side lens surface of the second lens from the image side in the imaging optical system.

13. The imaging optical system according to any one of claims 1 to 12, satisfying the following condition: 0.6<(BR1+BR2) / (BR2-BR1)<5.5, where BR1 is the radius of curvature of the object-side lens surface of the lens located closest to the image in the imaging optical system, and BR2 is the radius of curvature of the image-side lens surface of the lens located closest to the image in the imaging optical system.

14. The imaging optical system according to any one of claims 1 to 13, which satisfies the following condition: 0.50 < FF / F < 1.00, where F is the focal length of the imaging optical system.

15. An imaging optical system according to any one of claims 1 to 14, which satisfies the following condition: 0.10 < RDG / RD < 0.67, where RDG is the sum of the center thicknesses of the lenses included in the rear group, and RD is the distance on the optical axis from the lens surface in the rear group closest to the object to the lens surface in the rear group closest to the image.

16. An imaging optical system according to any one of claims 1 to 15, which satisfies the following condition: 0.50 < GFrF / F < 1.50, where GFrF is the focal length of the lens in the front group that is located closest to the image, and F is the focal length of the imaging optical system.

17. An imaging optical system according to any one of claims 1 to 16, satisfying the following condition: 0.13 < DM / SD < 0.40, where DM is the distance on the optical axis between the lens in the front group located closest to the image and the lens in the rear group located closest to the object, and SD is the distance on the optical axis from the lens surface closest to the object to the lens surface closest to the image in the imaging optical system.

18. An imaging optical system according to any one of claims 1 to 17, which satisfies the following condition: ndR<1.7, where ndR is the average value of the refractive indexes of the plurality of lenses included in the rear group.

19. An imaging optical system according to any one of claims 1 to 18, wherein the lenses included in the rear group are made of a plurality of resin lenses.

20. An optical instrument comprising an imaging optical system according to any one of claims 1 to 19.

21. A method for manufacturing an imaging optical system having a front group including multiple lenses and having positive refractive power, and a rear group including multiple lenses, arranged in order from the object side to the image side, comprising the steps of: arranging a negative lens with its concave surface facing the object side closest to the object in the front group; making the axial distance between the lens arranged closest to the image side in the front group and the lens arranged closest to the object side in the rear group the largest among the axial distances between adjacent lenses; and arranging the lenses in the lens barrel so as to satisfy the following conditional expressions: 0.20 < FF / TT < 0.90 0.15 < Bf / TT < 0.60 where FF is the focal length of the front group, TT is the overall lens length of the imaging optical system, and Bf is the back focus of the imaging optical system.

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