Imaging lens

Abstract

To reduce axial chromatic aberration, to secure high resolution, and to satisfactorily reduce spherical aberration and coma aberration.SOLUTION: An aperture diaphragm STO is arranged between a first lens group G1 and a second lens group G2, and the first lens group G1 is provided with a cemented lens L15 obtained by cementing a first positive lens L16 and a first negative lens J1 and having negative power. The cemented lens J1 is configured to satisfy "ndp> 1.85" (conditional formula 1), "Δ θ gF> 0.02" (conditional formula 2), "ndp> ndn" (conditional formula 3), and "ν dp <ν dn" (conditional formula 4), where ndp is an index of refraction of the first positive lens L15, ndn is an index of refraction of the first negative lens L16, Δ θ gF is anomalous dispersion, ν dp is an Abbe number of the first positive lens L15, and ν dn is an Abbe number of the first negative lens L16.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an imaging lens having an entire optical system that is composed of, in order from the object side to the image side, a first lens group, a second lens group, and a third lens group. [Background technology]

[0002] 2. Description of the Related Art Conventionally, the imaging lens described in Patent Document 1 is known as a large-diameter, compact medium-telephoto interchangeable lens.

[0003] The imaging lens described in the document 1 is an imaging lens used as an interchangeable lens for a digital still camera or the like, and is intended to achieve overall cost reduction, compactness, and weight reduction while ensuring sufficient optical performance. Specifically, the front lens group 101 is configured by arranging, in order from the object OBJ side to the image IMG side, three convex lenses L1, L2, and L3 and two concave lenses L4 and L5, with at least the two convex lenses L2 and L3 satisfying "nd<1.54" and "νd>76" (where nd is the refractive index at the d-line and νd is the Abbe number), and the entire front lens group 101 is set to have positive refractive power, while the rear lens group 102 is configured by arranging, in order from the object OBJ side to the image IMG side, a convex lens L6 and a concave lens L7, with the rear lens group 102 being configured to satisfy "1.5<[f1 / f]<3.5" where f1 is the focal length of the front lens group 101 and f is the focal length of the entire system 100. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-49919 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the imaging lens disclosed in the above-mentioned Patent Document 1 has the following problems to be solved.

[0006] That is, when attempting to increase the aperture in the medium telephoto range, there is a problem in that the degradation of resolution due to axial chromatic aberration becomes significant. Normally, to improve axial chromatic aberration, low-dispersion glass materials are often used for the positive lens elements, but with low-dispersion glass materials, the occurrence of coma becomes significant and the Petzval sum increases, which is a drawback.

[0007] On the other hand, there are cases where high refractive index glass is used to reduce coma and spherical aberration, and further to reduce the Petzval sum, but on the other hand, this increases dispersion, increases axial chromatic aberration, and causes problems such as reduced resolution.

[0008] Ultimately, when constructing a large-aperture, compact medium-telephoto imaging lens, it is not easy to ensure high resolution by reducing axial chromatic aberration, while also reducing spherical aberration and coma aberration. Therefore, there has been a demand for the practical application of a new imaging lens that improves overall optical performance by reducing these various aberrations, and that also achieves compactness, size reduction, and weight reduction of the entire lens.

[0009] An object of the present invention is to provide an imaging lens that solves the problems present in the background art. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the present invention provides an imaging lens 1 including an entire optical system 100 which is composed of, in order from the object OBJ side to the image IMG side, a first lens group G1 having positive power, a second lens group G2 including a positive lens L21, and a third lens group G3 having negative power, wherein an aperture stop STO is disposed between the first lens group G1 and the second lens group G2, and the first lens group G1 is provided with a cemented lens J1 having negative power, in which a first positive lens L15 and a first negative lens L16 are cemented together, and the cemented lens J1 is constructed so that the following (Conditional Formula 1) to (Conditional Formula 4) are satisfied, where ndp is the refractive index of the first positive lens L15, ndn is the refractive index of the first negative lens L16, ΔθgF is the anomalous dispersion, νdp is the Abbe number of the first positive lens L15, and νdn is the Abbe number of the first negative lens L16. ndp>1.85 … (conditional expression 1) ΔθgF>0.02 … (conditional expression 2) ndp>ndn … (conditional expression 3) νdp<νdn … (Conditional formula 4)

[0011] In this case, according to a preferred aspect of the invention, the first lens group G1 has at least two positive lenses L11, L12, ... in that order from the object OBJ side to the image IMG side, and when the length of the first lens group G1 is Lf and the distance on the optical axis Dc from the lens surface (i=1) of the first lens group G1 closest to the object OBJ to the lens surface (i=9...) of the cemented lens J1 closest to the object OBJ is LB, it is desirable to satisfy the following conditional expression 5: 0.6<[LB / Lf] … (conditional expression 5)

[0012] In the first lens group G1, the object OBJ-side lens surfaces (i=1, 3...) of all the positive lenses L11, L12... can be formed as convex surfaces. Furthermore, in the second lens group G2, the object OBJ-side lens surfaces (i=13...) of the lenses L21... positioned closest to the object OBJ can be formed as convex surfaces, and in the third lens group G3, the image IMG-side lens surfaces (i=16...) of the lenses L31... positioned closest to the image IMG can be formed as convex surfaces.

[0013] It is desirable that the entire optical system 100 satisfy the following conditional expression 6, where the combined focal length of the second lens group G2 and the third lens group G3 when focused at infinity is f23 and the focal length of the entire optical system 100 is f. 0.75<[f23 / f]<1.3 … (conditional expression 6)

[0014] On the other hand, the entire optical system 100 can be moved toward the object OBJ during focus adjustment to a short distance. In this case, the entire optical system 100 can also have the third lens group G3 fixed and the first lens group G1 and the second lens group G2 moved toward the object OBJ during focus adjustment to a short distance. On the other hand, all of the lenses L11... in the entire optical system 100 can be configured with spherical lenses. [Effects of the Invention]

[0015] The imaging lens 1 according to the present invention having such a configuration provides the following significant effects.

[0016] (1) The cemented lens J1, which has a negative power close to that of the aperture stop STO, is positioned and configured to satisfy the above-mentioned conditional expressions 1 to 4, thereby reducing axial chromatic aberration, thereby ensuring high resolution and effectively reducing spherical aberration and coma. In addition, the overall lens is made more compact, smaller, and lighter, and the overall optical performance is improved, thereby providing an imaging lens 1 that is large-aperture, compact, and optimal for use as a medium-telephoto lens.

[0017] (2) In a preferred embodiment, when the first lens group G1 is configured to include at least two positive lenses L11, L12, etc., in order from the object OBJ side to the image IMG side, so as to satisfy the above-mentioned conditional formula 5, the positive lenses L11, L12, etc. are rationally positioned, which makes it possible to reduce the overall optical length and, particularly when the aperture is increased, to reduce the outer diameter of the entire product. Note that if [LB / Lf] in conditional formula 5 is less than 0.6, the shape of spherical aberration differs for each wavelength, resulting in deterioration of axial chromatic aberration.

[0018] (3) In a preferred embodiment, when configuring the first lens group G1, the object-side lens surfaces (i=1, 3, etc.) of all the positive lenses L11, L12, etc. are formed as convex surfaces, thereby shortening the overall length of the product. However, if they are formed as concave surfaces, coma at close distances can be reduced, but the overall length of the product will be longer.

[0019] (4) In a preferred embodiment, when configuring the second lens group G2, if the lens surface (i=13...) of the lens L21... located closest to the object OBJ is formed as a convex surface on the object OBJ side, the back focus can be shortened, and coma in the sagittal direction and coma in the meridional direction at intermediate image heights can be reduced. However, if the lens surface is formed as a concave surface, the back focus will be long and the overall length of the product will be long.

[0020] (5) In a preferred embodiment, when configuring the third lens group G3, if the lens surface (i=16...) of the lens L31... located closest to the image IMG is formed as a convex surface on the image IMG side, a sufficient amount of peripheral light can be ensured, thereby suppressing degradation of optical performance during lens manufacturing.

[0021] (6) In a preferred embodiment, when configuring the entire optical system 100, if the composite focal length of the second lens group G2 and the third lens group G3 when focused at infinity is f23 and the focal length of the entire optical system 100 is f, and the configuration satisfies the above-mentioned conditional expression 6, the overall length of the product can be shortened and sufficient imaging performance can be ensured on the close-up side. Note that if [f23 / f] in conditional expression 6 exceeds 1.3, the overall length of the product tends to be long, while if it is less than 0.75, the overall length of the product can be shortened, but the distance variation of spherical aberration increases, resulting in degradation of imaging performance, particularly on the close-up side.

[0022] (7) In a preferred embodiment, when configuring the entire optical system 100, if the entire system is moved toward the object OBJ when adjusting the focus to a close distance, the entire optical system 100 can be moved, thereby reducing the size relative to the overall lens length (total product length).

[0023] (8) In a preferred embodiment, when configuring the entire optical system 100, the third lens group G3 is fixed and the first lens group G1 and the second lens group G2 are moved toward the object OBJ during focus adjustment to a close distance. This reduces the amount of movement compared to extending the entire optical system 100, thereby further improving functionality, particularly in the macro range.

[0024] (9) In a preferred embodiment, if all lenses L11... in the entire optical system 100 are constructed using spherical lenses, the use of aspherical lenses in the entire optical system 100 can be eliminated. This not only has the advantages of reducing costs and being less susceptible to manufacturing errors, but also makes it possible to further improve imaging performance and image flatness. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional view of the entire optical system of an imaging lens according to Example 1 of a preferred embodiment of the present invention; [Figure 2] A list of numerical values ​​corresponding to the conditional expressions and items of Examples 1 to 3; [Figure 3] FIG. 2 is a longitudinal aberration diagram of the entire optical system of the imaging lens according to Example 1 at infinity; [Figure 4] FIG. 10 is a cross-sectional view showing the overall optical system of the imaging lens according to Example 2; [Figure 5] FIG. 10 is a longitudinal aberration diagram of the entire optical system of the imaging lens according to Example 2 at infinity; [Figure 6] FIG. 10 is a cross-sectional view showing the overall optical system of the imaging lens according to Example 3; [Figure 7] FIG. 10 is a longitudinal aberration diagram of the entire optical system of the imaging lens according to Example 3 at infinity; DETAILED DESCRIPTION OF THE INVENTION

[0026] Next, preferred embodiments of the present invention will be described in detail with reference to the drawings.

[0027] First, the configuration of an imaging lens 1 according to this embodiment will be described with reference to FIG. 1 (and FIGS. 4 and 6).

[0028] Fig. 1 shows the entire optical system 100 of the imaging lens 1, and the basic main configuration will be described with reference to Fig. 1. The imaging lens 1 of Fig. 1 also serves as Example 1.

[0029] The entire optical system 100 is roughly divided into a first lens group G1, an aperture stop STO, a second lens group G2, and a third lens group G3, arranged in this order from the object (subject) OBJ side to the image (image sensor) IMG side. Therefore, the aperture stop STO is disposed between the first lens group G1 and the second lens group G2.

[0030] The first lens group G1 has positive power overall, and includes, as its basic main configuration, a cemented lens J1 having negative power formed by cementing a first positive lens L15 and a first negative lens L16 together, and this cemented lens J1 is configured to satisfy the following conditional formulas 1 to 4, where ndp is the refractive index of the first positive lens L15, ndn is the refractive index of the first negative lens L16, ΔθgF is the anomalous dispersion, νdp is the Abbe number of the first positive lens L15, and νdn is the Abbe number of the first negative lens L16. ndp>1.85 … (conditional expression 1) ΔθgF>0.02 … (conditional expression 2) ndp>ndn … (conditional expression 3) νdp<νdn … (Conditional formula 4)

[0031] The anomalous dispersion ΔθgF is a value calculated using [Equation 1].

[0032]

number

[0033] The first lens group G1 is equipped with at least two positive lenses L11, L12... in that order from the object OBJ side to the image IMG side, and is configured to satisfy the following conditional expression 5, where Lf is the length of the first lens group G1 and LB is the distance on the optical axis Dc from the lens surface (i=1) of the first lens group G1 closest to the object OBJ to the lens surface (i=9...) of the cemented lens J1 closest to the object OBJ. 0.6<[LB / Lf] … (conditional expression 5)

[0034] In this way, when configuring the first lens group G1, if it is configured to include at least two positive lenses L11, L12... in that order from the object OBJ side to the image IMG side and is configured so as to satisfy the above-mentioned conditional expression 5, the positive lenses L11, L12... will be rationally positioned, which will make it possible to reduce the overall optical length and, particularly when the aperture is increased, will make it possible to reduce the outer diameter of the entire product.

[0035] If [LB / Lf] in conditional expression 5 is less than 0.6, the shape of the spherical aberration will differ for each wavelength, resulting in an increase in axial chromatic aberration.

[0036] Furthermore, the lens surfaces (i=1, 3, ...) of all the positive lenses L11, L12, ... in the first lens group G1 facing the object OBJ are formed into convex surfaces, which makes it possible to shorten the overall length of the product.

[0037] If the lens surfaces (i=1, 3, . . . ) of the positive lenses L11, L12, . . . on the object OBJ side are formed as concave surfaces, the coma aberration at close distances can be reduced, but the overall length of the product increases.

[0038] On the other hand, the second lens group G2 basically includes positive lenses L21..., and the lens surface (i=13...) of the lens L21... located closest to the object OBJ is formed as a convex surface on the object OBJ side. Forming the lens in this manner shortens the back focal length, thereby reducing coma in the sagittal direction and coma in the meridional direction at intermediate image heights. If the lens were formed as a concave surface, the back focal length would be long, resulting in a longer overall product length.

[0039] Furthermore, the third lens group G3 has negative power as a whole, and as a basic main configuration, the lens surfaces (i=16...) of the lenses L31... located closest to the image IMG are formed as convex surfaces on the image IMG side. By forming them in this way, a sufficient amount of peripheral light can be secured, and deterioration of optical performance during lens manufacturing can be suppressed.

[0040] Furthermore, in the imaging lens 1, when the entire optical system 100 is configured, the following conditional expression 6 is satisfied, where the combined focal length of the second lens group G2 and the third lens group G3 when focused at infinity is f23 and the focal length of the entire optical system 100 is f. 0.75<[f23 / f]<1.3 … (conditional expression 6)

[0041] If the lens is configured to satisfy conditional expression 6, the overall product length can be shortened and sufficient imaging performance can be ensured on the close-up side. Note that if [f23 / f] in conditional expression 6 exceeds 1.3, the overall product length tends to be long, and if [f23 / f] in conditional expression 6 is less than 0.75, the overall product length can be shortened, but the distance variation of spherical aberration increases, resulting in degradation of imaging performance, particularly on the close-up side.

[0042] On the other hand, when configuring the entire optical system 100, the entire optical system 100 is configured to move toward the object OBJ during focus adjustment to a short distance. This allows the entire optical system 100 to move, thereby reducing the size relative to the overall lens length (total product length).

[0043] It should be noted that when configuring the entire optical system 100, it is also possible to fix the third lens group G3 and move the first lens group G1 and the second lens group G2 toward the object OBJ during focus adjustment to a close distance. In this case, the amount of movement can be reduced compared to extending the entire optical system 100, thereby further improving functionality, particularly in the macro range.

[0044] All of the lenses L11... in the entire optical system 100 are constructed of spherical lenses. This eliminates the need for aspherical lenses in the entire optical system 100, which not only has the advantages of reducing costs and being less susceptible to manufacturing errors, but also makes it possible to further improve imaging performance and image flatness.

[0045] The above configuration is the basic configuration of the imaging lens 1 according to this embodiment. In this manner, the basic configuration of the imaging lens 1 is to configure the imaging lens 1 with an entire optical system 100 that is configured, in order from the object OBJ side to the image IMG side, of a first lens group G1 having positive power, a second lens group G2 including a positive lens L21, and a third lens group G3 having negative power, in which an aperture stop STO is disposed between the first lens group G1 and the second lens group G2, and a cemented lens J1 having negative power in which a first positive lens L15 and a first negative lens L16 are cemented together is provided in the first lens group G1, and in configuring the cemented lens J1, ndp is the refractive index of the first positive lens L15, ndn is the refractive index of the first negative lens L16, When ΔθgF is the anomalous dispersion, νdp is the Abbe number of the first positive lens L15, and νdn is the Abbe number of the first negative lens L16, the following conditions (1) to (4) are satisfied: ndp > 1.85 (condition 1), ΔθgF > 0.02 (condition 2), ndp > ndn (condition 3), and νdp < νdn (condition 4). Specifically, the cemented lens J1 with negative power close to that of the aperture stop STO is disposed, and the above-mentioned conditions (1) to (4) are satisfied. This reduces axial chromatic aberration, ensuring high resolution. At the same time, spherical aberration and coma are effectively reduced. Furthermore, the overall lens can be made more compact, smaller, and lighter, while improving overall optical performance. This provides a large-aperture, compact imaging lens 1 that is optimal for medium-telephoto use.

[0046] Next, specific examples (Examples 1 to 3) of the imaging lens 1 according to this embodiment will be described with reference to FIGS. [Example]

[0047] First, an imaging lens 1 according to a first embodiment will be described with reference to FIGS. As shown in FIG. 2, the imaging lens 1 according to Example 1 has a focal length f of the entire optical system of 87.28 mm, an F-number of 2.05, a half angle of view ω of 13.71°, a total lens length LR of 60.38 mm, a back focus BF of 27.40 mm, and a total optical length LC of 87.78 mm.

[0048] As shown in FIG. 1, the imaging lens 1 includes, in order from the object OBJ side to the image IMG side, a first lens group G1, an aperture stop STO, a second lens group G2, and a third lens group G3.

[0049] The first lens group G1 is composed of, in order from the object OBJ side to the image IMG side, a positive meniscus lens (positive lens) L11 with a convex lens surface on the object OBJ side, a positive meniscus lens (positive lens) L12 with a convex lens surface on the object OBJ side, a positive meniscus lens (positive lens) L13 with a convex lens surface on the object OBJ side, a negative meniscus lens (negative lens) L14 with a convex lens surface on the object OBJ side, and a cemented lens J1 formed by cementing together a biconvex lens (positive lens) L15 and a biconcave lens (negative lens) L16. Each of the lenses L11-L14 is a single lens. The first lens group G1 as a whole has positive power.

[0050] In this case, the first lens group G1 is equipped with three positive lenses L11, L12, and L13, in that order from the object OBJ side to the image IMG side, and is configured to satisfy "0.6<[LB / Lf]" (conditional expression 5), where Lf is the length of the first lens group G1 and LB is the distance on the optical axis Dc from the lens surface (i=1) of the first lens group G1 closest to the object OBJ to the lens surface (i=9) of the cemented lens J1 closest to the object OBJ.

[0051] The lens surfaces (i=1, 3, 5) of all the positive lenses L11, L12, and L13 in the first lens group G1 facing the object OBJ are formed as convex surfaces.

[0052] Furthermore, the cemented lens J1 is configured to satisfy "ndp>1.85" (conditional formula 1), "ΔθgF>0.02" (conditional formula 2), "ndp>ndn" (conditional formula 3), and "νdp<νdn" (conditional formula 4), where ndp is the refractive index of the first positive lens L15, ndn is the refractive index of the first negative lens L16, ΔθgF is the anomalous dispersion, νdp is the Abbe number of the first positive lens L15, and νdn is the Abbe number of the first negative lens L16.

[0053] On the other hand, the second lens group G2 is composed of a biconvex lens (positive lens) L21 using one single lens. That is, the second lens group G2 includes the positive lens L21, and the lens surface (i=13) of the lens L21 located closest to the object OBJ, facing the object OBJ, is convex.

[0054] On the other hand, the third lens group G3 uses one single lens, and is configured as a negative meniscus lens (negative lens) L31 whose lens surface on the image IMG side is convex.

[0055] Furthermore, in the entire optical system 100, when the combined focal length of the second lens group G2 and the third lens group G3 at infinity focusing is f23 and the focal length of the entire optical system 100 is f, the optical system is configured to satisfy "0.75<[f23 / f]<1.3" (conditional formula 6).

[0056] Table 1 shows lens data for the entire optical system of the imaging lens 1 according to Example 1 shown in FIG.

[0057] [Table 1]

[0058] In Table 1, i indicates the surface number of the lens surface counted from the object (OBJ). This surface number corresponds to the symbol (number) shown in Figure 1. Correspondingly, the radius of curvature R(i), on-axis surface spacing D(i), lens refractive index nd(i), and lens Abbe number νd(i) are shown. nd(i) and νd(i) are values ​​relative to the d-line (587.56 nm). On-axis surface spacing D(i) indicates the lens thickness or air space between opposing surfaces. The units for radius of curvature R(i) and surface spacing D(i) are [mm]. OBJ in the surface number indicates the object, STO indicates the aperture stop, and IMG indicates the image position. Infinity in the radius of curvature R(i) indicates a flat surface. Furthermore, blank spaces for refractive index nd(i) and Abbe number νd(i) indicate air.

[0059] Figure 2 lists the numerical values ​​corresponding to each of conditional expressions 1-6. As shown in Figure 2, in Example 1, the refractive index ndp of the first positive lens L15 is "1.92286", which satisfies conditional expression 1 of "ndp>1.85". The anomalous dispersion ΔθgF is "0.0313", which satisfies conditional expression 2 of "ΔθgF>0.02". The refractive index ndn of the first negative lens L16 is "1.77047", which satisfies conditional expression 3 of "ndp>ndn". The Abbe number νdp of the first positive lens L15 is "20.88", and the Abbe number νdn of the first negative lens L16 is "29.74", which satisfies conditional expression 4 of "νdp<νdn".

[0060] Furthermore, the length Lf of the first lens group G1 is 29.6 mm, and the distance LB on the optical axis Dc from the lens surface of the first lens group G1 closest to the object OBJ to the lens surface of the cemented lens J1 closest to the object OBJ is 23.6 mm, so LB / Lf is 0.80, which satisfies conditional expression 5, 0.6<[LB / Lf]. Furthermore, the combined focal length f23 of the second lens group G2 and the third lens group G3 when focused at infinity is 78.89 mm, and the focal length f of the entire optical system 100 is 87.28 mm, so [f23 / f] is 0.90, which satisfies conditional expression 6, 0.75<[f23 / f]<1.3. In this way, the imaging lens 1 of Example 1 satisfies all of conditional expressions 1 to 6.

[0061] 3 shows longitudinal aberration diagrams for the entire optical system 100 of the imaging lens 1 according to Example 1. From the left, the longitudinal aberration diagrams show (a) spherical aberration diagram (656.27 nm, 587.56 nm, 435.83 nm), (b) astigmatism diagram (587.56 nm), and (c) distortion aberration diagram (587.56 nm). Note that the respective scales (1 division) are ±0.25 mm, ±0.25 mm, and ±2.5%. As can be seen, it can be confirmed that good aberrations are obtained in all cases.

[0062] The imaging lens 1 of Example 1 ensures sufficient optical performance (various aberrations) while realizing compactness and light weight of the entire lens, and can be configured as a large-aperture, compact imaging lens 1 for medium telephoto use. [Example]

[0063] Next, the imaging lens 1 according to the second embodiment will be specifically described with reference to FIGS. 4, 5 and 2. FIG.

[0064] As shown in FIG. 4, the imaging lens 1 according to Example 2 has a focal length f of the entire optical system of 97.41 mm, an F-number of 2.06, a half angle of view ω of 12.34°, a total lens length LR of 67.85 mm, a back focus BF of 29.99 mm, and a total optical length LC of 97.84 mm.

[0065] The imaging lens 1 according to Example 2 differs from the imaging lens 1 according to Example 1 in that, when configuring the negative lens L14 in the first lens group G1, Example 1 uses a negative meniscus lens L14 whose lens surface on the object OBJ side is convex, whereas Example 2 uses a biconcave lens L14 whose lens surface on the object OBJ side is concave.

[0066] The only fundamental difference between Example 2 and Example 1 is the negative lens L14, except for the detailed lens elements in Table 2 below and Table 1 described above. The other basic lens configurations of the entire optical system 100 in Example 2 and Example 1 are the same.

[0067] Table 2 shows lens data for the entire optical system of the imaging lens 1 according to Example 2 shown in FIG.

[0068] [Table 2]

[0069] As shown in FIG. 2 , in Example 2, the refractive index ndp of the first positive lens L15 is "1.92286", which satisfies conditional expression 1 of "ndp>1.85". The anomalous dispersion ΔθgF is "0.0313", which satisfies conditional expression 2 of "ΔθgF>0.02". The refractive index ndn of the first negative lens L16 is "1.77047", which satisfies conditional expression 3 of "ndp>ndn". The Abbe number νdp of the first positive lens L15 is "20.88", and the Abbe number νdn of the first negative lens L16 is "29.74", which satisfies conditional expression 4 of "νdp<νdn".

[0070] Furthermore, the length Lf of the first lens group G1 is 30.0 mm, and the distance LB on the optical axis Dc from the lens surface of the first lens group G1 closest to the object OBJ to the lens surface of the cemented lens J1 closest to the object OBJ is 24.4 mm, so LB / Lf is 0.81, which satisfies conditional expression 5, that is, 0.6<[LB / Lf]. Furthermore, the combined focal length f23 of the second lens group G2 and the third lens group G3 when focused at infinity is 104.16 mm, and the focal length f of the entire optical system 100 is 97.41 mm, so [f23 / f] is 1.07, which satisfies conditional expression 6, that is, 0.75<[f23 / f]<1.3.

[0071] In this way, the imaging lens 1 of Example 2 satisfies all of the conditions of Conditional Expressions 1 to 6.

[0072] 5 shows longitudinal aberration diagrams for the entire optical system 100 of the imaging lens 1 according to Example 2. It can be seen that good aberrations are obtained in all cases. Therefore, the imaging lens 1 of Example 2 also ensures sufficient optical performance (various aberrations), while realizing compactness and weight reduction of the entire lens, and it is possible to construct a large-aperture, compact imaging lens 1 for medium telephoto use. [Example]

[0073] Next, the imaging lens 1 according to Example 3 will be specifically described with reference to FIGS. 6, 7 and 2. FIG.

[0074] Example 3 differs in basic lens configuration from Example 1. As shown in Fig. 6, the imaging lens 1 according to Example 3 has a focal length f of the entire optical system of 36.08 mm, an F-number of 2.01, a half angle of view ω of 21.28°, a total lens length LR of 37.88 mm, a back focus BF of 12.49 mm, and a total optical length LC of 50.37 mm.

[0075] As shown in FIG. 6, the imaging lens 1 has a basic configuration including, in order from the object OBJ side to the image IMG side, a first lens group G1, an aperture stop STO, a second lens group G2, and a third lens group G3.

[0076] The first lens group G1 is composed of, in order from the object OBJ side to the image IMG side, a biconvex lens (positive lens) L11, a positive meniscus lens (positive lens) L12 whose lens surface on the object OBJ side is convex, and a cemented lens J1 formed by cementing together a biconcave lens (negative lens) L16 and a positive meniscus lens (positive lens) L15 whose lens surface on the object OBJ side is convex. Note that lenses L11 and L12 are single lenses. The first lens group G1 has positive power as a whole.

[0077] In this case, the first lens group G1 is equipped with two positive lenses L11 and L12, in that order from the object OBJ side to the image IMG side, and is configured to satisfy "0.6<[LB / Lf]" (conditional expression 5), where Lf is the length of the first lens group G1 and LB is the distance on the optical axis Dc from the lens surface (i=1) of the first lens group G1 closest to the object OBJ to the lens surface (i=5) of the cemented lens J1 closest to the object OBJ.

[0078] Furthermore, the lens surfaces (i=1, 3) of the all-positive lenses L11 and L12 in the first lens group G1 facing the object OBJ are formed as convex surfaces.

[0079] The cemented lens J1 is configured to satisfy "ndp>1.85" (conditional formula 1), "ΔθgF>0.02" (conditional formula 2), "ndp>ndn" (conditional formula 3), and "νdp<νdn" (conditional formula 4), where ndp is the refractive index of the first positive lens L15, ndn is the refractive index of the first negative lens L16, ΔθgF is the anomalous dispersion, νdp is the Abbe number of the first positive lens L15, and νdn is the Abbe number of the first negative lens L16.

[0080] On the other hand, the second lens group G2 is composed of, in order from the object OBJ side to the image IMG side, a cemented lens J2 formed by cementing together a negative meniscus lens (negative lens) L21, whose lens surface on the object OBJ side is convex, and a biconvex lens (positive lens) L22.

[0081] Furthermore, the third lens group G3 is composed of, in order from the object OBJ side to the image IMG side, a cemented lens J3 formed by cementing together a positive meniscus lens (positive lens) L31 having a convex surface facing the image IMG side and a negative meniscus lens (negative lens) L32 having a convex surface facing the image IMG side, and a negative meniscus lens (negative lens) L33 having a convex surface facing the image IMG side. The negative lens L33 is a single lens.

[0082] In the entire optical system 100, when the combined focal length of the second lens group G2 and the third lens group G3 at infinity focusing is f23 and the focal length of the entire optical system 100 is f, the optical system is configured to satisfy "0.75<[f23 / f]<1.3" (conditional formula 6).

[0083] Table 3 shows lens data for the entire optical system of the imaging lens 1 according to Example 3 shown in FIG.

[0084] [Table 3]

[0085] As shown in FIG. 2, in Example 3, the refractive index ndp of the first positive lens L15 is "1.92286", which satisfies conditional expression 1 of "ndp>1.85". The anomalous dispersion ΔθgF is "0.0313", which satisfies conditional expression 2 of "ΔθgF>0.02". The refractive index ndn of the first negative lens L16 is "1.77047", which satisfies conditional expression 3 of "ndp>ndn". The Abbe number νdp of the first positive lens L15 is "20.88", and the Abbe number νdn of the first negative lens L16 is "27.74", which satisfies conditional expression 4 of "νdp<νdn".

[0086] Furthermore, the length Lf of the first lens group G1 is 10.7 mm, and the distance LB on the optical axis Dc from the lens surface of the first lens group G1 closest to the object OBJ to the lens surface of the cemented lens J1 closest to the object OBJ is 7.2 mm, so LB / Lf is 0.67, which satisfies conditional expression 5, that is, 0.6<[LB / Lf]. Furthermore, the combined focal length f23 of the second lens group G2 and the third lens group G3 when focused at infinity is 33.84 mm, and the focal length f of the entire optical system 100 is 36.08 mm, so [f23 / f] is 0.94, which satisfies conditional expression 6, that is, 0.75<[f23 / f]<1.3. In this way, the imaging lens 1 of Example 3 satisfies all of conditional expressions 1 to 6.

[0087] 7 shows longitudinal aberration diagrams for the entire system 100 of the imaging lens 1 according to Example 3. It can be seen that good aberrations are obtained in all cases. Therefore, the imaging lens 1 of Example 3 also ensures sufficient optical performance (various aberrations), while realizing compactness and weight reduction of the entire lens, and it is possible to construct a large-aperture, compact imaging lens 1 for medium telephoto use.

[0088] The above describes in detail preferred embodiments including Examples 1, 2, and 3, but the present invention is not limited to such embodiments, and the detailed configuration, shape, material, quantity, numerical values, etc. can be changed, added, or deleted as desired within the scope of the gist of the present invention.

[0089] For example, the first lens group G1 has, in order from the object OBJ side to the image IMG side, at least two positive lenses L11, L12..., and when the length of the first lens group G1 is Lf and the distance on the optical axis Dc from the lens surface of the first lens group G1 closest to the object OBJ to the lens surface of the cemented lens J1 closest to the object OBJ is LB, it is desirable to satisfy conditional expression 5 of "0.6<[LB / Lf]", and further when the combined focal length of the second lens group G2 and the third lens group G3 when focused at infinity is f23 and the focal length of the entire optical system 100 is f, it is desirable to satisfy conditional expression 6 of "0.75<[f23 / f]<1.3", but satisfying each conditional expression is not an essential constituent requirement. In addition, in the first lens group G1, the lens surfaces of all of the positive lenses L11, L12... located closest to the object OBJ are formed convexly, and in the second lens group G2, the lens surfaces of the lenses L21... located closest to the object OBJ are formed convexly, and in the third lens group G3, the lens surfaces of the lenses L31... located closest to the image IMG are formed convexly, and when adjusting for a close focus, the entire lens group may be moved toward the object OBJ, or when adjusting for a close focus, the third lens group G3 may be fixed and the first lens group G1 and the second lens group G2 may be moved toward the object OBJ, or other configurations may be used as needed. Meanwhile, it is desirable that all of the lenses L11... in the entire optical system 100 be spherical lenses, but this does not exclude the possibility of some aspherical lenses being included. [Industrial Applicability]

[0090] The imaging lens according to the present invention can be used as a dedicated lens or an interchangeable lens in various optical devices such as digital still cameras and video cameras. [Explanation of symbols]

[0091] 1: imaging lens, 100: entire optical system, OBJ: object, IMG: image, STO: aperture stop, G1: first lens group, G2: second lens group, G3: third lens group, L11: positive lens, L12...: positive lens, L15: first positive lens, L16: first negative lens, L21...: positive lens, L31...: negative lens, J1: cemented lens, Dc: optical axis

Claims

1. An imaging lens having an entire optical system composed of, in order from the object side to the image side, a first lens group having positive power, a second lens group including a positive lens, and a third lens group having negative power, wherein an aperture stop is disposed between the first lens group and the second lens group, and a cemented lens having negative power in which a first positive lens and a first negative lens are cemented together is provided in the first lens group, and wherein, in constructing the cemented lens, the imaging lens satisfies the following conditional formulas 1 to 4, where ndp is the refractive index of the first positive lens, ndn is the refractive index of the first negative lens, ΔθgF is the anomalous dispersion, νdp is the Abbe number of the first positive lens, and νdn is the Abbe number of the first negative lens. ndp>1.85... (conditional expression 1) ΔθgF>0.02... (Conditional expression 2) ndp>ndn... (conditional expression 3) νdp<νdn (Conditional Expression 4)

2. 2. The imaging lens according to claim 1, wherein the first lens group includes, in order from the object side to the image side, at least two positive lenses, and the following conditional expression 5 is satisfied, where Lf is a length of the first lens group and LB is a distance on the optical axis from a lens surface of the first lens group closest to the object side to a lens surface of the cemented lens closest to the object side: 0.6<[LB / Lf]... (Conditional expression 5)

3. 2. The imaging lens according to claim 1, wherein the first lens group has all positive lenses whose object-side lens surfaces are convex.

4. 2. The imaging lens according to claim 1, wherein the second lens group has a lens closest to the object side whose object-side lens surface is formed as a convex surface.

5. 2. The imaging lens according to claim 1, wherein the third lens group has a lens element positioned closest to the image side whose image-side lens surface is formed as a convex surface.

6. 2. The imaging lens according to claim 1, wherein the entire optical system satisfies the following conditional expression 6, where f is a composite focal length of the second lens group and the third lens group when focused at infinity, and f is a focal length of the entire optical system. 0.75<[f23 / f]<1.3... (Conditional expression 6)

7. 2. The imaging lens according to claim 1, wherein the entire optical system is moved toward the object side during focus adjustment to a short distance.

8. 2. The imaging lens according to claim 1, wherein, in the entire optical system, during focus adjustment to a close distance, the third lens group is fixed, and the first lens group and the second lens group are moved toward the object side.

9. 2. The imaging lens according to claim 1, wherein all lenses in the entire optical system are spherical lenses.

Images