JP7706745B2 - Imaging lens - Google Patents

Description

The present invention relates to an imaging lens suitable for use as an interchangeable lens in digital still cameras and the like.

In general, interchangeable lenses (imaging lenses) used in digital still cameras and the like are becoming more and more varied in lens type, while image sensors are becoming larger in size to the same size as photographic film and offering higher performance (higher definition) than photographic film. In addition, as camera bodies become smaller, interchangeable lenses are also required to become smaller, and users desire small, compact interchangeable lenses that have a certain degree of brightness and a wide angle of view while also ensuring high performance.

A retrofocus type imaging lens disclosed in Patent Document 1 is known as an imaging lens of this type. This imaging lens aims to provide a retrofocus type imaging lens that satisfactorily corrects various aberrations and has sufficient mechanical space so that the focus group does not interfere with the lens shutter mechanism even during rear focus focusing. Specifically, the lens includes, from the object side, a first lens group and a second lens group. The first lens group is composed of, from the object side, a front first lens group having negative refractive power overall and a rear first lens group having positive refractive power overall, and is configured to have positive refractive power overall, and the second lens group includes one or more positive lenses and negative lenses and is configured to have positive refractive power overall, and the front first lens group and the rear first lens group each include one or more positive lenses and two negative lenses.

JP 2003-29141 A

However, the conventional imaging lens (retrofocus imaging lens) disclosed in the above-mentioned Patent Document 1 had the following problems:

In general, when performing effective aberration correction with a reduced number of lenses in an imaging lens, a layout configuration in which the front lens group on the object side and the rear lens group on the image side, which are arranged on either side of the aperture stop, each have positive power is advantageous. However, on the other hand, since it is necessary to use multiple convex lenses, it is difficult to ensure peripheral light on the wide-angle side and a bright F-number. For this reason, in the retrofocus imaging lens in the above-mentioned Patent Document 1, the front lens group has negative power and the rear lens group has positive power to ensure peripheral light and a long back focus, but such a conventional retrofocus imaging lens becomes large overall due to the rear focus mechanism. For this reason, there are still areas that need to be improved from the perspective of realizing an imaging lens (interchangeable lens) that can meet user needs by ensuring sufficient brightness and a wide angle of view, as well as high performance and compact size.

The present invention aims to provide an imaging lens that solves the problems that exist in the background art.

In order to solve the above-mentioned problems, the present invention provides an imaging lens 1 in which the entire lens system 100 is composed of, in order from the object OBJ side, a first lens group Gf having negative power and a second lens group Gs having positive power, and the second lens group Gs is composed of, in order from the object OBJ side, a second A lens group Gsa having positive power and a second B lens group Gsb having positive power, and an aperture stop STO is disposed between the second A lens group Gsa and the second B lens group Gsb, the first lens group Gf is composed of at least one first lens L1 having negative power, the second A lens group Gsa is composed of one cemented lens J1 formed by cementing a concave lens Ln1 and a convex lens Lp2, and the second B lens group Gsb is composed of, in order from the object OBJ side, The lens system 100 is characterized in that it is configured to include a second lens L2 having a negative power with a concave surface on the object OBJ side, two third lenses L3 having positive power, and a final lens Le, and the total number of lenses in the entire lens system 100 is six or seven, and is configured to satisfy the following conditions [Condition 1] to [Condition 4], that is, νd1>52...[Condition 1], nd2>1.84...[Condition 2], νd2<31...[Condition 3], 0.4<(BF/LA)<0.6...[Condition 4], where LA is the distance from the lens surface of the first lens group Gf closest to the object OBJ to the image IMG surface, BF is the distance from the final lens surface of the second lens group Gs to the image IMG surface, νd1 is the Abbe number of the d-line of the first lens L1, nd2 is the refractive index of the d-line of the second lens L2, and νd2 is the Abbe number.

In this case, according to a preferred embodiment of the invention, the focus adjustment function Mf can be configured by a focus adjustment mechanism that moves the entire lens system 100 in the optical axis direction Fc. In addition, the first lens group Gf can be configured by arranging the first lens L1 as a negative meniscus lens with a convex surface on the object OBJ side, and a corrective convex lens Lp1 on the object OBJ side of the first lens L1. Instead of arranging the convex lens Lp1, the first lens L1 can be configured as an aspheric lens. Furthermore, the second B lens group Gsb can be configured to include at least a concave lens L2, two convex lenses L3 and Le in order from the object OBJ side (aperture stop STO), and the final lens Le located closest to the image side in this second B lens group Gsb can be configured as an aspheric lens using a positive meniscus lens with a convex surface on the image IMG side.

The imaging lens 1 according to the present invention, which has such a configuration, provides the following remarkable effects:

(1) The first lens group Gf is configured to include at least one first lens L1 having negative power, and the second B lens group Gsb is configured to include, in order from the object OBJ side, a second lens L2 having negative power with a concave surface on the object OBJ side, two third lenses L3 having positive power, and a final lens Le, and to satisfy the above-mentioned [Condition 1] to [Condition 4], so that a long back focus, sufficient brightness, a wide angle of view, high performance, and compact size can be ensured. This makes it possible to realize an imaging lens 1 (interchangeable lens) that can meet user needs. In addition, by ensuring the back focus length, it can be used not only with single-lens reflex cameras, but also with mirrorless cameras via a mount adapter, and can be provided as an imaging lens 1 with excellent versatility and expandability.

(2) The second A lens group Gsa is composed of a single cemented lens J1 formed by cementing a concave lens Ln1 and a convex lens Lp2 together, which contributes to substantially reducing the size and weight of the entire imaging lens 1 while effectively correcting chromatic aberration.

(3) In a preferred embodiment, if the focus adjustment function Mf is configured using a focus adjustment mechanism that moves the entire lens system 100 in the optical axis direction Fc, a rear focus mechanism as in the conventional art is not used, which can contribute to further increasing the aperture and reducing the size of the entire imaging lens 1.

(4) In a preferred embodiment, the first lens L1 in the first lens group Gf is a negative meniscus lens with a convex surface facing the object OBJ, thereby making the overall power of the first lens group Gf negative with a small number of lenses, thereby ensuring the amount of peripheral light and the back focus length.

(5) In a preferred embodiment, when constructing the first lens group Gf, a convex lens Lp1 is disposed on the object OBJ side of the first lens L1, so that even if distortion occurs due to the first lens L1, it can be easily corrected by adding a single convex lens.

(6) In a preferred embodiment, when constructing the first lens group Gf, if the first lens L1 is an aspheric lens, even if distortion occurs due to the first lens L1, it can be corrected by the aspheric lens, and therefore the first lens group Gf can be constructed with a minimum number of lenses.

(7) In a preferred embodiment, when the second B lens group Gsb is configured to include, in order from the object OBJ side, at least a concave lens L2 and two convex lenses L3 and Le, the first lens group Gf can ensure peripheral light amount and back focus length, while allowing the imaging lens 1 to be made smaller, and an optical system with good aberration correction can be constructed.

(8) In a preferred embodiment, when constructing the second B lens group Gsb, the final lens Le located closest to the image IMG side is constructed as an aspheric lens using a positive meniscus lens with a convex surface on the image IMG side. This allows the entrance and exit characteristics of the light passing through the final lens Le to be set by the aspheric surface, making it possible to perform good aberration correction in the final stage. In particular, since the exit pupil position is separated from the image plane by ensuring the back focus, deterioration of performance can be reduced even when a filter or the like is placed in front of the image plane.

FIG. 1 is a diagram showing the configuration of an imaging lens according to a first embodiment of the present invention; Each condition numerical value table of Examples 1-3 according to the preferred embodiment of the present invention, FIG. 2 is a longitudinal aberration diagram of the imaging lens of Example 1 at infinity; FIG. 2 is a diagram showing the configuration of an imaging lens according to a second embodiment of the present invention; FIG. 13 is a longitudinal aberration diagram of the imaging lens of Example 2 at infinity; FIG. 3 is a diagram showing the configuration of an imaging lens according to a third embodiment of the present invention; FIG. 13 is a longitudinal aberration diagram of the imaging lens of Example 3 at infinity;

Next, we will present examples 1-3, which are preferred embodiments of the present invention, and explain them in detail with reference to the drawings.

First, the imaging lens 1 of Example 1 according to this embodiment will be described in detail with reference to FIGS. 1 to 3.

First, an imaging lens 1 according to Example 1 (the present embodiment) will be described with reference to FIG. 1. It is possible to envision that this imaging lens 1 can be applied to an interchangeable lens for a digital still camera. In FIG. 1, OBJ indicates an object (subject), and IMG indicates an image (imaging element). Therefore, the object OBJ side is in the front in the direction of the optical axis Dc, and the image IMG side is in the rear in the direction of the optical axis Dc.

As shown in FIG. 1, the imaging lens 1 according to the first embodiment includes, in order from the object OBJ side, a first lens group Gf having a negative power overall and a second lens group Gs having a positive power overall, and the second lens group Gs is composed of, in order from the object OBJ side, a second A lens group Gsa having a positive power and a second B lens group Gsb having a positive power, and an aperture stop STO is disposed between the second A lens group Gsa and the second B lens group Gsb, thereby forming a basic lens system 100. If necessary, a wavelength cut filter, a parallel plane plate such as dustproof glass, or the like can be provided in front of the image IMG.

The first lens group Gf includes, from the object OBJ side, a convex lens Lp1 using a positive meniscus lens whose lens surface (i=1) on the object OBJ side is convex, and a first lens L1 with negative power arranged on the image IMG side of this convex lens Lp1. In Example 1, since the convex lens Lp1 is provided on the object OBJ side of the first lens L1 with negative power, even if distortion occurs due to the first lens L1, it can be easily corrected by adding one convex lens Lp1 according to the angle of view. Note that a similar correction can be made even if the first lens L1 is configured as an aspheric lens (as in Example 3) instead of adding the convex lens Lp1.

The first lens L1 is a negative meniscus lens with a convex surface on the object OBJ side. By configuring the first lens L1 with a negative meniscus lens with a convex surface on the object OBJ side in this way, the peripheral light amount and back focus length can be ensured by making the overall power of the first lens group Gf negative with a small number of lenses. In addition, when the Abbe number of the d-line of the first lens L1 is νd1, a lens that satisfies the following condition is used.
νd1 > 52 … [Condition 1]

In this [Condition 1], if νd1 is 52 or less, the lateral chromatic aberration deteriorates and color bleeding occurs.

On the other hand, the second lens group Gs is composed of, in order from the object OBJ side, a second A lens group Gsa having positive power and a second B lens group Gsb having positive power, with an aperture stop STO disposed between the second A lens group Gsa and the second B lens group Gsb. In this case, the second A lens group Gsa is composed of one cemented lens J1. That is, a cemented lens J1 is used, which is formed by cementing a biconcave lens Ln1 located on the object OBJ side with a biconvex lens Lp2 located on the image IMG side. In this way, by forming the second A lens group Gsa from one cemented lens J1, it is possible to contribute to substantially reducing the size and weight of the entire imaging lens 1.

The second lens group Gsb is configured to include, in order from the object OBJ side, a second lens L2 using a negative meniscus lens having a concave surface on the OBJ side, a third lens L3 using a positive meniscus lens having a convex surface on the image IMG side, and a final lens Le using a positive meniscus lens having a convex surface on the image IMG side. In this case, the second lens L2 uses a lens that satisfies the following condition when the refractive index for the d line is nd2 and the Abbe number is νd2.
nd2 > 1.84... [Condition 2]
νd2 < 31... [Condition 3]

In this [Condition 2], if nd2 is 1.84 or less, the second lens L2 requires a larger radius of curvature, which increases the occurrence of various aberrations, including coma and astigmatism. Also, in [Condition 3], if νd2 is 31 or more, the chromatic aberration of magnification deteriorates, and color bleeding occurs closer to the periphery of the lens.

Furthermore, when constructing the second B lens group Gsb, if the final lens Le located closest to the image side is constructed as an aspheric lens using a positive meniscus lens with a convex surface on the image IMG side, the entrance and exit characteristics of the light passing through the final lens Le can be set by the aspheric surface, allowing for good aberration correction in the final stage. In particular, since the exit pupil position is separated from the image plane by ensuring the back focus, deterioration of performance can be reduced even when a filter or the like is placed in front of the image plane.

On the other hand, when constructing the entire lens system 100, as shown in FIG. 1, assuming that the distance from the lens surface (i=1) of the first lens group Gf closest to the object OBJ to the image IMG surface is LA [mm], and the distance from the final lens surface (i=14A) of the second lens group Gs to the image IMG surface is BF [mm], the following condition is set to be satisfied.
0.4 < (BF/LA) < 0.6... [Condition 4]

In this [Condition 4], if (BF/LA) is 0.4 or less, it is advantageous for aberration correction and high performance can be expected, but it will lead to an increase in size of the entire lens system 100 and a loss of portability, and on the other hand, if (BF/LA) is 0.6 or more, the correction of spherical aberration will be insufficient and flare will occur, resulting in a blurred image. Therefore, when configuring the imaging lens 1, it is desirable to set it so that each of [Condition 1], [Condition 2], [Condition 3], and [Condition 4] is satisfied.

In addition, in FIG. 1, Mf indicates a focus adjustment function. This focus adjustment function Mf has the function of moving the entire lens system 100 in the optical axis direction Fc. By adopting such a focus adjustment function Mf, it is possible to contribute to further increasing the diameter and reducing the size of the entire imaging lens 1, since a rear focus mechanism as in the past is not used.

Table 1 shows the lens data of the entire lens system in the imaging lens 1 of Example 1. The entire lens system at the infinite object point has a focal length f of 29.4 mm, an F-number of F2.88, and an image height of 21.63 mm.

In the "Surface Data" of Table 1, the surface number of the lens surface counted from the object OBJ side is indicated by i, and this surface number i corresponds to the code (number) shown in Figure 1. Correspondingly, the radius of curvature R(i) of the lens surface, the axial surface distance D(i), the refractive index nd(i) of the lens, the Abbe number νd(i) of the lens, and the focal length FL(i) of the lens are indicated. nd(i) and νd(i) are values for the d-line (587.6 [nm]). The axial surface distance D(i) indicates the lens thickness or air space between the opposing surfaces. The radius of curvature R(i) and the surface distance D(i) are in units of [mm]. The surface number OBJ indicates the object, STO indicates the aperture stop, and IMG indicates the position of the image. The Infinity of the radius of curvature R(i) is a flat surface, and a surface with A after the surface number i indicates that the surface shape is aspherical. The blanks for the refractive index nd(i) and Abbe number νd(i) indicate air.

In addition, the "aspheric coefficients" in Table 1 are expressed by equation 1 in an orthogonal coordinate system (X, Y, Z) with the origin at the center of the surface and Z in the direction of the optical axis Dc, where ASP is the surface number of the aspheric surface. In equation 1, R is the central radius of curvature, A4, A6, A8, A10... are the aspheric coefficients of the 4th order, 6th order, 8th order, 10th order... respectively, and H is the distance from the origin on the optical axis. In Table 1, "E" means "x10".

As shown in the table in FIG. 2, in the case of Example 1, the result of [Condition 1] is 81.6, which satisfies [Condition 1] of "νd1>52". The result of [Condition 2] is 1.85, which satisfies [Condition 2] of "nd2>1.84". The result of [Condition 3] is 23.8, which satisfies [Condition 3] of "νd2<31". In addition, BF is 38.50 mm, LA is 69.15 mm, and BF/LA is 0.56. Therefore, [Condition 4] of "0.4<(BF/LA)<0.6" is satisfied. In the "Focus variable distance" in Table 1, ZD14 is the distance (optical axis length) from the final lens surface (i=14A) to the image IMG.

Figure 3 shows longitudinal aberration diagrams for the imaging lens 1 of Example 1 at infinity. From the left, each longitudinal aberration diagram shows spherical aberration (656.3 nm, 587.6 nm, 435.8 nm), astigmatism (587.6 nm), and distortion (587.6 nm). The scales are ±0.50 mm, ±0.50 mm, and ±3.0%. As is clear from the diagram, it can be confirmed that the imaging lens 1 of Example 1 has good aberration characteristics, i.e., imaging performance (optical performance), for all longitudinal aberrations.

Next, the imaging lens 1 of Example 2 according to this embodiment will be described with reference to Figures 4 and 5.

As shown in FIG. 4, the imaging lens 1 according to the second embodiment includes, in order from the object OBJ side, a first lens group Gf having a negative power overall and a second lens group Gs having a positive power overall, and the second lens group Gs is composed of, in order from the object OBJ side, a second A lens group Gsa having a positive power and a second B lens group Gsb having a positive power, and an aperture stop STO is disposed between the second A lens group Gsa and the second B lens group Gsb, thereby forming a basic lens system 100. As in the first embodiment, if necessary, a parallel plane plate such as a wavelength cut filter or dustproof glass can be provided in front of the image IMG.

The first lens group Gf includes a first lens L1 having negative power. This first lens L1 is composed of a negative meniscus lens with a convex surface facing the object OBJ. In this way, by using a negative meniscus lens with a convex surface facing the object OBJ, the overall power of the first lens group Gf can be made negative with a small number of lenses, thereby ensuring the amount of peripheral light and the back focus length. In addition, when the Abbe number of the d-line of the first lens L1 is νd1, the use of a lens that satisfies the above-mentioned [Condition 1] is the same as in Example 1.

In Example 2, a convex lens Lp1 is not provided on the object OBJ side with respect to the first lens L1 having negative power. That is, in Example 1, an example was shown in which one convex lens Lp1 was added to correct the distortion caused by the first lens L1, but in Example 2, the addition of the convex lens Lp1 is omitted. The addition of the convex lens Lp1 as in Example 1, or the correction by forming the first lens L1 from an aspheric lens, can be adopted depending on the angle of view and is not a required element. Therefore, the total number of lenses in Example 2 is one less than in Example 1, making the total number of lenses five (one cemented lens J1).

On the other hand, the second lens group Gs of Example 2 is composed of, in order from the object OBJ side, a second A lens group Gsa having a positive power and a second B lens group Gsb having a positive power, and an aperture stop STO is arranged between the second A lens group Gsa and the second B lens group Gsb. In this case, the second A lens group Gsa is composed of one cemented lens J1. That is, a cemented lens J1 is used in which a biconvex lens Lp2 located on the object OBJ side is cemented with a biconcave lens Ln1 located on the image IMG side. Therefore, although Example 1 and Example 2 are the same in that they use a cemented lens J1, the front-to-back positional relationship between the biconvex lens Lp2 and the biconcave lens Ln1 is reversed. In this way, if the second A lens group Gsa is composed of one cemented lens J1, it can contribute to substantially reducing the size and weight of the entire imaging lens 1.

The second B lens group Gsb is configured to include, in order from the object OBJ side, a second lens L2 using a negative meniscus lens with a concave surface on the OBJ side, a third lens L3 using a biconvex lens, and a final lens Le using a positive meniscus lens with a convex surface on the image IMG side. In this case, when the refractive index of the d-line of the second lens L2 is nd2 and the Abbe number is νd2, the lens that satisfies the above-mentioned [Condition 2] and [Condition 3] is used, as in Example 1.

In addition, in Example 2 (similar to Examples 1 and 3), when constructing the second B lens group Gsb, it is configured to include, in order from the object OBJ side, at least a concave lens L2 and two convex lenses, that is, a convex lens L3 and a convex lens Le. With this configuration, it is possible to construct an optical system in which the imaging lens 1 is small and has good aberration correction while ensuring peripheral light amount and back focus length with the first lens group Gf.

Furthermore, when constructing the second B lens group Gsb, if the final lens Le located closest to the image side is constructed as an aspheric lens using a positive meniscus lens with a convex surface on the image IMG side, the entrance and exit characteristics of the light passing through the final lens can be set by the aspheric surface, so that aberration correction in the final stage can be performed well. In particular, since the exit pupil position is separated from the image surface by ensuring the back focus, deterioration of performance can be reduced even when a filter or the like is placed in front of the image surface. In addition, when constructing the entire lens system 100, as shown in FIG. 4, the distance from the lens surface (i=1) of the first lens group Gf closest to the object OBJ to the image IMG surface is LA [mm], and the distance from the final lens surface (i=12A) of the second lens group Gs to the image IMG surface is BF [mm], and the above-mentioned [Condition 4] is satisfied, as in Example 1. In addition, in FIG. 4, Mf indicates a focus adjustment function. Similar to the first embodiment, this focus adjustment function Mf has the function of moving the entire lens system 100 in the optical axis direction Fc.

Table 2 shows the lens data of the entire lens system in the imaging lens 1 of Example 2. The entire lens system at the infinite object point has a focal length f: 36.0 mm, F-number: F2.83, and image height: 21.63 mm.

As shown in the table in FIG. 2, in the case of Example 2, the result of [Condition 1] is 70.4, which satisfies [Condition 1] of "νd1>52". The result of [Condition 2] is 1.92, which satisfies [Condition 2] of "nd2>1.84". The result of [Condition 3] is 20.9, which satisfies [Condition 3] of "νd2<31". In addition, BF is 38.49 mm, LA is 67.14 mm, and BF/LA is 0.57. Therefore, [Condition 4] of "0.4<(BF/LA)<0.6" is satisfied. In the "Focus variable distance" in Table 2, ZD12 is the distance (optical axis length) from the final lens surface (i=12A) to the image IMG. FIG. 5 shows a longitudinal aberration diagram corresponding to infinity in the imaging lens 1 of Example 2. As is clear from the figure, it can be confirmed that the imaging lens 1 of Example 2 has good aberration characteristics, i.e., imaging performance (optical performance), for all longitudinal aberrations.

Next, the imaging lens 1 of Example 3 according to this embodiment will be described with reference to Figures 6 and 7.

As shown in FIG. 6, the imaging lens 1 according to the third embodiment includes, in order from the object OBJ side, a first lens group Gf having a negative power overall and a second lens group Gs having a positive power overall, and the second lens group Gs is composed of, in order from the object OBJ side, a second A lens group Gsa having a positive power and a second B lens group Gsb having a positive power, and an aperture stop STO is disposed between the second A lens group Gsa and the second B lens group Gsb, thereby forming a basic lens system 100. As in the first embodiment, if necessary, a parallel plane plate such as a wavelength cut filter or dustproof glass can be provided in front of the image IMG.

The first lens group Gf includes a first lens L1 having negative power. This first lens L1 is composed of a negative meniscus lens with a convex surface facing the object OBJ. In this way, by using a negative meniscus lens with a convex surface facing the object OBJ, the overall power of the first lens group Gf can be made negative with a small number of lenses, thereby ensuring the amount of peripheral light and the back focus length. In addition, when the Abbe number of the d-line of the first lens L1 is νd1, the use of a lens that satisfies the above-mentioned [Condition 1] is the same as in Example 1.

Note that, in Example 3, like Example 2, no convex lens Lp1 is provided on the object OBJ side with respect to the first lens L1 having negative power. That is, in Example 1, an example was shown in which a single convex lens Lp1 was added to correct the distortion caused by the first lens L1, but in Example 3, instead of adding a convex lens Lp1, the first lens L1 is configured as an aspheric lens. By configuring the first lens L1 as an aspheric lens, it is possible to correct the distortion caused by the first lens L1, and it is possible to obtain the same correction effect as when a convex lens Lp1 is added.

On the other hand, the second lens group Gs in Example 3 is composed of, in order from the object OBJ side, a second A lens group Gsa having positive power and a second B lens group Gsb having positive power, with an aperture stop STO arranged between the second A lens group Gsa and the second B lens group Gsb. In this case, the second A lens group Gsa is composed of three single lenses, that is, in order from the object OBJ side, a positive meniscus lens Lp2 having a convex surface on the object OBJ side, a negative meniscus lens Ln2 having a convex surface on the image IMG side, and a biconvex lens Lp3. In the case of Example 1, it is composed of one cemented lens J1, which is different from Example 1.

The second B lens group Gsb is configured to include, in order from the object OBJ side, a second lens L2 using a negative meniscus lens with a concave surface on the OBJ side, a third lens L3 using a biconvex lens, and a final lens Le using a positive meniscus lens with a convex surface on the image IMG side. In this case, when the refractive index of the d-line of the second lens L2 is nd2 and the Abbe number is νd2, the lens that satisfies the above-mentioned [Condition 2] and [Condition 3] is used, as in Example 1.

In addition, when constructing the entire lens system 100, as shown in FIG. 6, the distance from the lens surface (i=1) of the first lens group Gf closest to the object OBJ to the image IMG surface is LA [mm], and the distance from the final lens surface (i=15A) of the second lens group Gs to the image IMG surface is BF [mm], and the settings are the same as in Example 1 in that they are set to satisfy the above-mentioned [Condition 4]. Additionally, in FIG. 6, Mf indicates a focus adjustment function. This focus adjustment function Mf has the function of moving the entire lens system 100 in the optical axis direction Fc, as in Example 1.

Table 3 shows the lens data of the entire lens system in the imaging lens 1 of Example 3. The entire lens system at the infinite object point has a focal length f of 25.0 mm, an F-number of F2.86, and an image height of 21.63 mm.

As shown in the table in FIG. 2, in the case of Example 3, the result of [Condition 1] is 61.2, which satisfies [Condition 1] of "νd1>52". The result of [Condition 2] is 1.92, which satisfies [Condition 2] of "nd2>1.84". The result of [Condition 3] is 20.9, which satisfies [Condition 3] of "νd2<31". In addition, BF is 38.30 mm, LA is 71.15 mm, and BF/LA is 0.54. Therefore, [Condition 4] of "0.4<(BF/LA)<0.6" is satisfied. In the "Focus variable distance" in Table 3, ZD15 is the distance (optical axis length) from the final lens surface (i=15A) to the image IMG. FIG. 7 shows a longitudinal aberration diagram corresponding to infinity in the imaging lens 1 of Example 3. As is clear from the figure, it can be confirmed that the imaging lens 1 of Example 3 has good aberration characteristics, i.e., imaging performance (optical performance), for all longitudinal aberrations.

In this way, in the basic form of any of the imaging lenses 1 according to Examples 1 to 3 described above, the entire lens system 100 is configured, in order from the object OBJ side, of a first lens group Gf having negative power and a second lens group Gs having positive power, and the second lens group Gs is configured, in order from the object OBJ side, of a second A lens group Gsa having positive power and a second B lens group Gsb having positive power, and an aperture stop STO is disposed between the second A lens group Gsa and the second B lens group Gsb, and in particular, the first lens group Gf is configured to include at least one first lens L1 having negative power, and the second B lens group Gsb is configured, in order from the object OBJ side, of a second lens having negative power and having a concave surface on the object OBJ side. The imaging lens 1 includes a first lens L2, two third lenses L3 having positive power, and a final lens Le, and is configured to satisfy the following conditions [Condition 1] to [Condition 4], i.e., vd1>52...[Condition 1], nd2>1.84...[Condition 2], vd2<31...[Condition 3], and 0.4<(BF/LA)<0.6...[Condition 4], where LA is the distance from the lens surface of the first lens group Gf closest to the object OBJ to the image IMG surface, BF is the distance from the final lens surface of the second lens group Gs to the image IMG surface, vd1 is the Abbe number for the d-line of the first lens L1, and vd2 is the refractive index for the d-line of the second lens L2. This ensures a long back focus, sufficient brightness, a wide angle of view, and high performance and compact size. This makes it possible to realize an imaging lens 1 (interchangeable lens) that can meet user needs. In addition, by ensuring a long back focal length, it can be used not only with single-lens reflex cameras, but also with mirrorless cameras via a mount adapter, making it possible to provide an imaging lens 1 with excellent versatility and expandability.

The above provides a detailed explanation of preferred embodiments including Examples 1-3, 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 without departing from the spirit of the present invention.

For example, it is desirable to configure the focus adjustment function Mf by a function of moving the entire lens system 100 in the optical axis direction Fc, but this does not exclude focus adjustment functions other than the rear focus adjustment function, such as when moving some lenses of the entire lens system 100, or when moving each of a plurality of lenses at different speeds. In addition, the first lens group Gf may be configured by adding a convex lens Lp1 to the object OBJ side of the first lens L1 as necessary, or the first lens L1 may be configured as an aspheric lens. Furthermore, it is desirable to configure the second B lens group Gsb to include at least a concave lens L2, two convex lenses L3 and Le in order from the object OBJ side (aperture stop STO), but this is not necessarily a required component. In this case, the convex lens Le is desirably an aspheric lens using a positive meniscus lens, but the type of lens is not limited. Therefore, it is also possible to provide an additional convex lens on the image IMG side of the concave lens L2.

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.

1: Imaging lens, 100: Whole lens system, Gf: First lens group, Gs: Second lens group, Gsa: Second A lens group, Gsb: Second B lens group, OBJ: Object, STO: Aperture stop, IMG: Image, L1: First lens, L2: Second lens, L3: Third lens, Le: Last lens, Lp1: Convex lens, Ln1: Concave lens, Lp2: Convex lens, Le: Last lens, LA: Distance from the lens surface of the first lens group closest to the object to the image surface, BF: Distance from the last lens surface of the second lens group to the image surface, Mf: Focus adjustment function, Fc: Optical axis direction, J1: Cemented lens

Claims (7)

An imaging lens in which an entire lens system is composed of, in order from the object side, a first lens group having negative power and a second lens group having positive power, and the second lens group is composed of, in order from the object side, a secondA lens group having positive power and a secondB lens group having positive power, and an aperture stop is disposed between the secondA lens group and the secondB lens group, and the first lens group is composed of at least one first lens having negative power, and the secondA lens group is composed of a cemented lens formed by cementing a concave lens and a convex lens, and the 2B lens group includes, in order from the object side, a second lens having a concave surface facing the object side and having negative power, two third lenses having positive power, and a final lens, the total number of lenses in the entire lens system being six or seven, and the imaging lens is characterized in that, when a distance from a lens surface of the first lens group closest to the object side to an image plane is LA, a distance from a final lens surface of the second lens group to the image plane is BF, an Abbe number for the d-line of the first lens is vd1, a refractive index for the d-line of the second lens is nd2, and an Abbe number for the d-line of the second lens is vd2, the imaging lens satisfies the following [Condition 1] to [Condition 4]:
νd1>52... [Condition 1]
nd2>1.84... [Condition 2]
νd2<31... [Condition 3]
0.4<(BF/LA)<0.6... [Condition 4] The imaging lens according to claim 1, characterized in that it has a focus adjustment function in which the entire lens system moves in the optical axis direction. The imaging lens according to claim 1, characterized in that the first lens group includes the first lens which is a negative meniscus lens whose object side is convex. The imaging lens according to claim 1 or 3, characterized in that the first lens group includes a corrective convex lens arranged on the object side relative to the first lens. The imaging lens according to claim 1 or 3, characterized in that the first lens group includes the first lens that is an aspheric lens. The imaging lens according to claim 1, characterized in that the second B lens group is composed of, in order from the object side, at least a concave lens and two convex lenses. The imaging lens according to claim 1 or 6, characterized in that the final lens of the second B lens group, which is located closest to the image side, is composed of an aspheric lens using a positive meniscus lens whose image side is convex.

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