Imaging lens system and imaging device

Abstract

To provide an imaging lens system having relatively long back focus and capable of properly correcting curvature of field, and an imaging apparatus.SOLUTION: An imaging lens system 11 comprises a first lens group G1 including a first lens L1 having negative power and a second lens L2 having aspherical shape, and a second lens group G2 including at least two, a third lens L3 and a fourth lens L4. When entire optical length is denoted by L, back focus by BF, thickness in the outer edge of the second lens L2 and thickness on an optical axis respectively by ET2 and d2, distance from the first lens L1 to the third lens L3 by D24, and distances from the first lens L1 to the second lens L2 and from the second lens L2 to the third lens L3 respectively by df and dr, the following formulae (1) to (4) are satisfied. BF / L>0.1...(1). 0.9<ET2 / d2<1.1...(2). 0.3<D24 / L<0.5...(3). 0.05<df / dr<1.0...(4).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an imaging lens system and an imaging device, for example, an imaging lens system and an imaging device for vehicle use. [Background technology]

[0002] Conventionally, in order to correct field curvature, a lens having an inflection point on at least one of the object-side and image-side surfaces has been arranged in a position closest to the image side in an imaging lens system. For example, Patent Documents 1 and 2 describe imaging lens systems that include a first lens to a sixth lens arranged in order from the object side to the image side, and that use a lens having an inflection point on the object-side and image-side surfaces as the sixth lens. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-027256 [Patent Document 2] Japanese Patent Application Publication No. 2019-032462 Summary of the Invention [Problem to be solved by the invention]

[0004] However, Patent Documents 1 and 2 are based on the assumption that the imaging lens systems will be mounted on mobile phones and the like, and the back focal length of imaging lens systems mounted on mobile phones and the like is short. Therefore, the angle of incidence of chief rays incident on the imaging sensor is large, and the chief rays are separated, so that the field curvature can be suitably corrected by an aspherical lens arranged closest to the image side of the imaging lens system. However, the back focal length of an imaging lens system for vehicle use is relatively long, and it is difficult to suitably correct the field curvature even if an aspherical lens having an inflection point is arranged closest to the image side of the imaging lens system.

[0005] The present invention has been made in view of the above problems, and has as its object to provide an imaging lens system and an imaging device that can suitably correct field curvature in an imaging lens system with a relatively long back focus. [Means for solving the problem]

[0006] The imaging lens system of one embodiment comprises, in order from the object side to the image side, a first lens group, a diaphragm, and a second lens group, the first lens group comprises, in order from the object side to the image side, one or two lenses having negative power, and a correcting lens having an aspherical surface on the object side and the image side; the second lens group is made up of at least one lens, When L is an overall optical length that is the distance on the optical axis from the object-side surface of the lens in the first lens group that is located closest to the object to the image-side surface of the lens in the second lens group that is located closest to the image, and BF is a back focus that is the distance on the optical axis from the image-side surface of the lens in the second lens group that is located closest to the image to the object-side surface of the image sensor, the following formula (1) is satisfied: BF / L>0.1 (1) When the distance parallel to the optical axis between the intersection of a ray passing through the outermost diameter side of the object-side surface of the correction lens and the object-side surface of the correction lens and the intersection of a ray passing through the outermost diameter side of the object-side surface of the correction lens and the image-side surface is defined as ET2, and the thickness on the optical axis of the correction lens is defined as d2, the following formula (2) is satisfied: 0.9 <ET2 / d2<1.1 ···(2) When a distance on the optical axis from the image-side surface of the lens in the first lens group that is located closest to the object to the object-side surface of the lens in the second lens group that is located closest to the object is defined as D24, the following formula (3) is satisfied: 0.3 <D24 / L<0.5 ···(3) When the distance on the optical axis from the image-side surface of the lens adjacent to the object side of the correction lens to the object-side surface of the correction lens is df, and the distance on the optical axis from the image-side surface of the correction lens to the object-side surface of the lens adjacent to the image side of the correction lens is dr, the following equation (4) is satisfied. 0.05 <df / dr<1.0 ···(4) [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an imaging lens system and an imaging device that can appropriately correct field curvature using a correction lens arranged in the front group in an imaging lens system with a relatively long back focus. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view showing the configurations and light rays of an imaging lens system and an imaging device according to the first embodiment. [Figure 2] FIG. 10 is a diagram illustrating a distance ET2. [Figure 3] 1 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to Example 1. FIG. [Figure 4] 3A to 3C are diagrams showing spherical aberration (longitudinal aberration), curvature of field, and distortion in the imaging lens system of Example 1. [Figure 5] FIG. 10 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to a second embodiment. [Figure 6] 10A to 10C are diagrams showing spherical aberration (longitudinal aberration), curvature of field, and distortion in the imaging lens system of Example 2. [Figure 7] FIG. 10 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to Example 3. [Figure 8] 10A to 10C are diagrams showing spherical aberration (longitudinal aberration), curvature of field, and distortion in the imaging lens system of Example 3. [Figure 9] FIG. 10 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to a fourth embodiment. [Figure 10] 10A to 10C are diagrams showing spherical aberration (longitudinal aberration), curvature of field, and distortion in the imaging lens system of Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Embodiment 1 (Imaging lens system and imaging device) 1 is a cross-sectional view showing the configuration and light rays of an imaging lens system 11 and an imaging device 20 according to Embodiment 1. The imaging device 20 includes an imaging lens system 11 and an imaging element 21. The imaging lens system 11 and the imaging element 21 are housed in a housing (not shown). The imaging element 21 is an element that converts received light into an electrical signal, and is, for example, a CCD image sensor or a CMOS image sensor. The imaging element 21 is disposed at the imaging position of the imaging lens system 11. The imaging lens system 11 will now be described in detail.

[0010] As shown in FIG. 1, imaging lens system 11 of the first embodiment is composed of, in order from the object side to the image side, a first lens group G1, an aperture stop STOP, and a second lens group G2. The first lens group G1 is composed of, in order from the object side to the image side, one or two lenses with negative power and a corrector lens having an aspherical shape on the object side and the image side. The second lens group G2 is composed of at least two lenses, and the aperture stop STOP is located on the image side of the lens in second lens group G2 that is closest to the object side. Specifically, the first lens group G1 is composed of, from the object side to the image side, a first lens L1, which is a meniscus lens having negative power and a concave surface facing the object side, and a second lens L2, which has aspherical surfaces facing the object side and the image side and functions as a corrector lens. The second lens group G2 is composed of a third lens L3, which has positive power and a convex surface facing the object side, a fourth lens L4, which has negative power and a concave surface facing the image side, a fifth lens L5, which has positive power and a convex surface facing the object side and the image side, and a sixth lens L6, which has negative power and an aspherical surface facing the object side and the image side. Furthermore, as shown in FIG. 1, the imaging lens system 11 may include a wavelength filter glass 12 between the image-side lens surface S13 of the sixth lens L6 and an image plane IMG. The fourth lens L4 and the fifth lens L5 form a cemented lens. A cover glass 22 is provided on the imaging element 21, and the image plane of the imaging lens system 11 is indicated by IMG. The first lens L1 to the third lens L3 are preferably glass lenses, and the fourth lens L4 to the sixth lens L6 are preferably plastic lenses. The first lens L1 constituting the first group may be replaced with two lenses having negative power by splitting its negative power. The combined power of these two lenses is equal to the power of the first lens L1. In this case, the first lens group G1 is composed of a total of three lenses: the two lenses having the split negative power and the second lens L2.

[0011] The first lens L1 and second lens L2, which are located closer to the object than the aperture stop STOP, constitute the first lens group G1, while the third lens L3, which is located adjacent to the aperture stop STOP on the object side, and the fourth lens L4, fifth lens L5, and sixth lens L6, which are located adjacent to the aperture stop STOP on the image side, constitute the second lens group G2. The first lens group G1 has negative power, and the second lens group G2 has positive power. The aperture stop STOP is an aperture that determines the F-number (Fno) of the imaging lens system 11.

[0012] Furthermore, when the total optical length, which is the distance on the optical axis Z from the object-side surface S1 of the lens (first lens L1) located closest to the object in the first lens group G1 to the image-side surface S13 of the lens (sixth lens L6) located closest to the image in the second lens group G2, is defined as L, and the back focus, which is the distance on the optical axis Z from the image-side surface S13 of the lens (sixth lens L6) located closest to the image in the second lens group G2 to the object-side surface IMG of the image sensor 21, is defined as BF, the following equation (1) is satisfied. BF / L>0.1 (1) In other words, the imaging lens system 11 is an optical system with a relatively long back focus.

[0013] 2, when the distance parallel to the optical axis Z between an intersection P1 of a ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2, which is a corrective lens, and the object-side surface S3 of the second lens L2 and an intersection P2 of a ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2 and the image-side surface S4 is ET2, and the thickness of the second lens L2 on the optical axis Z is d2, the following formula (2) is satisfied. Note that here, the ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2 corresponds to the ray incident on the diagonal length of the image sensor 21. 0.9 <ET2 / d2<1.1 ···(2)

[0014] In other words, it is preferable that the lens power of the second lens L2 is weaker than that of the other lenses in the imaging lens system 11. In an imaging lens system 11 with a relatively long back focus, the chief rays are separated on the object side of the imaging lens system 11, and by arranging such a second lens L2 in the first lens group G1, it is possible to suitably correct field curvature. Specifically, by arranging such a second lens L2 in the first lens group G1, the angle of incidence of the chief rays incident on the second lens L2 becomes large and the chief rays are separated, so that field curvature can be suitably corrected by the second lens L2. That is, in the imaging lens system 11 having a relatively long back focus, the curvature of field can be corrected appropriately. In the first embodiment, the second lens group G2 is composed of four lenses, but the number of lenses is not limited to four and may be any number. If the optical system has a long back focus with BF / L>0.1, the overlap of various light rays at the object-side lens in the optical system is reduced, resulting in the effect of facilitating correction of field curvature for each light ray, and this effect is not affected by the lens configuration of the second lens group G2. Furthermore, the third lens L3 and subsequent lenses are close to the pupil (aperture), and it is sufficient to prioritize imaging performance such as spherical aberration. Furthermore, the focal lengths of the lenses subsequent to the third lens L3 are long, so field curvature is less likely to occur.

[0015] Furthermore, when the distance on the optical axis Z from the image-side surface S2 of the first lens L1 located closest to the object in the first lens group G1 to the object-side surface S5 of the lens (third lens L3) located closest to the object in the second lens group G2 is D24, the following formula (3) is satisfied: 0.3 <D24 / L<0.5 ···(3) If the distance on the optical axis Z from the image-side surface S2 of the lens (first lens L1) adjacent to the object side of the second lens L2 to the object-side surface S3 of the second lens L2 is df, and the distance on the optical axis Z from the image-side surface S4 of the second lens L2 to the object-side surface S5 of the lens (third lens L3) adjacent to the image side of the second lens L2 is dr, it is preferable to satisfy the following equation (4): 0.05 <df / dr<1.0 ···(4)

[0016] By satisfying formula (3), the lenses (first lens L1 to third lens L3) constituting the first lens group G1 can be positioned further toward the object side than the aperture stop position. This makes it possible to more reliably increase the angle of incidence of the chief ray incident on the object-side surface S3 of the second lens L2, and more reliably separate the chief rays incident on the object-side surface S3 of the second lens L2. This makes it possible to more reliably correct field curvature using the second lens L2.

[0017] Furthermore, since df / dr is smaller than 1.0, the second lens L2 can be positioned closer to the first lens L1, which is adjacent to it on the object side, than to the third lens L3, which is adjacent to it on the image side. This reduces the overlap of chief rays incident on the object-side surface S3 of the second lens L2, allowing the second lens L2 to more effectively correct field curvature. Furthermore, if df / dr is smaller than 0.05, that is, if the second lens L2 is too close to the first lens L1, there is a high possibility that the first lens L1 and the second lens L2 will interfere with each other when assembling the imaging lens system 11.

[0018] Furthermore, when the focal length of the entire imaging lens system 11 is F and the focal length of the second lens L2 is f2, it is preferable that the following formula (5) be satisfied. -0.1 <F / f2<0.1 ···(5)

[0019] In other words, it is preferable that the lens power of the second lens L2 is weaker than that of the other lenses in the imaging lens system 11. Therefore, for the same reason as above, the second lens L2 can suitably correct field curvature.

[0020] Furthermore, in the first embodiment, the first lens L1 has negative power, and the object-side surface S1 of the first lens L1 is concave, so that the angle of incidence of off-axis rays (rays that form an image off the optical axis, also called "peripheral rays") incident on the first lens L1 can be significantly changed, and negative distortion (distortion aberration) can be generated. As a result, the image shrinks toward the periphery, and the range detected by the image sensor 21 becomes wider, thereby realizing a wide angle of view and a large angle of view. On the other hand, the second lens L2 has at least one inflection point on at least one of the object-side and image-side surfaces, and therefore can correct field curvature, one of the various aberrations caused by the negative distortion produced by the first lens L1, from the center to the periphery.

[0021] Furthermore, when the second lens L2 has one inflection point on its object-side surface S3, it is preferable that the surface S3 be convex toward the object side in the range from the center position where the object-side surface S3 of the second lens L2 intersects with the optical axis Z to the position of the inflection point of the second lens L2, and that the surface S3 be concave toward the object side in the range from the position of the inflection point to the outer edge of the second lens L2. Furthermore, when the second lens L2 has one inflection point on its image-side surface S4, it is preferable that the surface S4 is concave toward the object side in the range from the central position where the image-side surface S4 of the second lens L2 intersects with the optical axis Z to the position of the inflection point of the second lens L2, and that the surface S4 is convex toward the object side in the range from the position of the inflection point to the outer edge of the second lens L2. By having at least one of the object-side surface S3 of the second lens L2 and the image-side surface S4 of the second lens L2 have the above-mentioned shape, correction can be made to match the characteristics of the field curvature by designing the position of the inflection point from the center position of the second lens L2. Furthermore, it is preferable that the second lens L2 has inflection points on both the object-side surface S3 and the image-side surface S4, and that the second lens L2 is convex toward the object side in the range from the center position where it intersects with the optical axis to a predetermined position, and that the second lens L2 is convex toward the image side in the range from the predetermined position to the outer edge of the second lens L2. This allows correction to be made in accordance with the characteristics of the so-called barrel-shaped or pincushion-shaped field curvature. In the first embodiment, the second lens group G2 is made up of four lenses, but the number of lenses is not limited to this and may be any number. The object of the present invention is achieved with only the first lens L1 for realizing a wide angle of view and the second lens L2 with little power for correcting curvature of field, and the lenses located on the image side of these lenses are simply configured to realize imaging, so it does not matter how many lenses the second lens group G2 may consist of. In other words, the second lens group G2 generates little curvature of field and is close to the pupil (aperture), so it is sufficient to focus on imaging performance such as spherical aberration, and so any configuration is acceptable.

[0022] Next, an example corresponding to the imaging lens system 11 of the first embodiment will be described with reference to the drawings.

[0023] Example 1 FIG. 3 is a cross-sectional view showing an imaging lens system 11 according to Example 1. Specifically, the imaging lens system 11 according to Example 1 includes, in order from the object side to the image side, a first lens L1, a second lens L2, a third lens L3, an aperture stop STOP, a fourth lens L4, a fifth lens L5, and an IR cut filter 12. The first lens L1 has negative power, a concave surface on the object side, and a convex surface on the image side. The second lens has positive power, and an aspherical surface with inflection points on the object side and the image side. The third lens L3 has positive power, a convex aspherical surface on the object side, and an aspherical surface on the image side. The fourth lens L4 has negative power, an aspherical surface on the object side, and a concave aspherical surface on the image side. The fifth lens L5 has positive power, and a convex aspherical surface on the object side and the image side. The sixth lens L6 has positive power, and an aspherical surface on the object side and the image side. The first lens L1 to the third lens L3 are glass lenses, and the fourth lens L4 to the sixth lens L6 are plastic lenses. The imaging lens system 11 also includes an IR cut filter 12 that cuts off light in the infrared region. The characteristic data of the imaging lens system 11 according to Example 1 will be described below.

[0024] Table 1 shows lens data for each lens surface of the imaging lens system 11 according to Example 1. Table 1 presents the lens data, including the radius of curvature (mm) of each surface, the surface spacing (mm) at the central optical axis Z, the refractive index Nd at the d-line, and the Abbe number Vd at the d-line. The imaging lens system 11 according to Example 1 has a half angle of view of 27.0°, an F-number of 1.8, and a focal length F of 8.623 (mm). The refractive index at the d-line and the Abbe number at the d-line shown in Table 1 are values ​​obtained when the ambient temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). In Table 1, surfaces marked with an asterisk (*) are aspherical. The half angle of view of an optical system refers to the angle between a ray of light passing through the center of the pupil and reaching a position on the diagonal length of the sensor (diagonal point) and the optical axis on the object side. [Table 1]

[0025] The aspheric shapes used for the second lens L2, the third lens L3, the fourth lens surface L4, and the lens surfaces of the fifth lens L5 and the sixth lens L6 are expressed by the following formula (6), where Y(h) is the amount of sag in the optical axis direction, c is the reciprocal of the radius of curvature, h is the height from the central optical axis Z in a direction perpendicular to the central optical axis Z, K is the conic coefficient, and A4, A6, A8, A10, A12, A14, and A16 are the aspheric coefficients of the fourth, sixth, eighth, tenth, twelfth, fourteenth, and sixteenth orders, respectively. The meanings of the symbols and the formulas representing the aspheric shapes are the same in the examples described below.

number

[0026] Table 2 shows aspherical coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 1. In Table 2, for example, "-2.33239E-03" corresponds to "-2.33239E×10 -3 " means. [Table 2]

[0027] 4 shows diagrams of spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system 11 of Example 1. As shown in Fig. 4, the imaging lens system 11 of Example 1 has a half angle of view of 27° and an F-number of 1.8. In the longitudinal aberration diagram of FIG. 4A, the horizontal axis indicates the position where the light ray intersects with the optical axis, and the vertical axis indicates the height at the pupil diameter. In the field curvature diagram of Fig. 4B, the horizontal axis represents the distance in the optical axis direction, and the vertical axis represents the image height (angle of view). In the field curvature diagram of Fig. 4B, Sag represents the field curvature on the sagittal plane, and Tan represents the field curvature on the tangential plane. In the distortion diagram of FIG. 4C, the horizontal axis represents the amount of image distortion (%), and the vertical axis represents the image height (angle of view). Moreover, the field curvature diagrams and distortion diagrams of FIGS. 4B and 4C show the results of a simulation using light with a wavelength of 555 nm. FIG. 4 shows diagrams of spherical aberration (longitudinal aberration), field curvature, and distortion when the environmental temperature t (° C.) is 25 (° C.).

[0028] Example 2 5 is a cross-sectional view showing an imaging lens system 11 according to Example 2. The configuration of the imaging lens system 11 according to Example 2 is the same as that of Example 1, and therefore a description thereof will be omitted. Below, characteristic data of the imaging lens system 11 according to Example 2 will be described.

[0029] Table 3 shows lens data for each lens surface of the imaging lens system 11 according to Example 2. The items shown in Table 3 are the same as those in Table 1, and therefore their explanation will be omitted. Here, the imaging lens system 11 according to Example 2 has a half angle of view of 27.0°, an F-number of 1.8, and a focal length F of the entire optical system of 8.923 (mm). [Table 3]

[0030] Table 4 shows aspherical coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of the second embodiment. [Table 4]

[0031] 6 shows diagrams of spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system 11 of Example 2. The explanation of each aberration diagram shown in FIG. 6 is the same as that in FIG. 4, and therefore will not be repeated.

[0032] Example 3 7 is a cross-sectional view showing an imaging lens system 11 according to Example 3. The configuration of the imaging lens system 11 according to Example 3 is the same as that of Example 1, and therefore a description thereof will be omitted. Below, characteristic data of the imaging lens system 11 according to Example 3 will be described.

[0033] Table 5 shows lens data for each lens surface of the imaging lens system 11 according to Example 3. The items shown in Table 5 are the same as those in Table 1, and therefore their explanation will be omitted. Here, the imaging lens system 11 according to Example 3 has a half angle of view of 27.0°, an F-number of 1.8, and a focal length F of the entire optical system of 8.59 (mm). [Table 5]

[0034] Table 6 shows aspherical coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of the third embodiment. [Table 6]

[0035] 8 shows diagrams of spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system 11 of Example 3. The explanation of each aberration diagram shown in Fig. 8 is the same as that in Fig. 4, and therefore will not be repeated.

[0036] Example 4 9 is a cross-sectional view showing an imaging lens system 11 according to Example 4. The configuration of the imaging lens system 11 according to Example 4 is the same as that of Example 1, and therefore a description thereof will be omitted. Below, characteristic data of the imaging lens system 11 according to Example 4 will be described.

[0037] Table 7 shows lens data for each lens surface of the imaging lens system 11 according to Example 4. The items shown in Table 7 are the same as those in Table 1, and therefore their explanation will be omitted. Here, the imaging lens system 11 according to Example 4 has a half angle of view of 27.0°, an F-number of 1.8, and a focal length F of the entire optical system of 8.623 (mm). [Table 7]

[0038] Table 8 shows aspherical coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of the fourth embodiment. [Table 8]

[0039] 10 shows diagrams of spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system 11 of Example 4. The explanation of each aberration diagram shown in Fig. 10 is the same as that in Fig. 4, and therefore will not be repeated.

[0040] 4A, 6A, 8A, and 10A, longitudinal aberrations at wavelengths of 486.1 nm, 587.6 nm, and 656.3 nm are well corrected in the imaging lens systems 11 of Examples 1 to 4. Therefore, the imaging lens system 11 has high resolution.

[0041] 4B, 6B, 8B, and 10B, the curvature of field is well corrected according to the imaging lens systems 11 of Examples 1 to 4. Therefore, the imaging lens system 11 has high resolution.

[0042] 4C, 6C, 8C, and 10C, distortion is well corrected according to the imaging lens systems 11 of Examples 1 to 4. Therefore, the imaging lens system 11 has high resolution.

[0043] Table 9 also lists the focal length F (mm) of the entire imaging lens system 11 according to Examples 1 to 4, the focal lengths f1 to f6 (mm) of the first to sixth lenses L1 to L6, the back focus BF (mm), the total optical length L (mm), the distance ET2 (mm) parallel to the optical axis between the intersection of a ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2 and the image-side surface S4, the thickness d2 (mm) of the second lens L2 on the optical axis Z, the distance D24 (mm) on the optical axis Z from the image-side surface S2 of the first lens L1 to the object-side surface S5 of the third lens L3, The table shows the distance df (mm) along the optical axis Z from the image-side surface S2 of the first lens L1 adjacent to the object side of L2 to the object-side surface S3 of the second lens L2, the distance dr (mm) along the optical axis Z from the image-side surface S4 of the second lens L2 to the object-side surface S5 of the third lens L3 adjacent to the image side of the second lens L2, the composite focal length f4 / 5 (mm) of the cemented lens consisting of the fourth lens L4 and the fifth lens L5, the optical length tol (mm) of the imaging lens system 11, the values ​​of BF / L, ET2 / d2, F / f2, D24 / L, and df / dr. The values ​​shown in Table 9 are those when the wavelength of the light is 555 nm and the ambient temperature t (°C) is 25°C. Note that in Examples 1 to 4, the first lens L1 constituting the first group described above may be replaced with two lenses having negative power by dividing its negative power. The combined power of these two lenses is equal to the power of the first lens L1. In this case, the first lens group G1 is composed of a total of three lenses: the two lenses having the divided negative powers and the second lens L2. [Table 9]

[0044] As shown in Table 9, in Examples 1 to 4, the values ​​of BF / L satisfy the above formula (1). Therefore, the imaging lens systems 11 according to Examples 1 to 4 are optical systems with a relatively long back focus, and can be used in applications such as vehicle-mounted applications that use a relatively thick imaging sensor. Also, as shown in Table 9, in Examples 1 to 4, the values ​​of ET2 / d2 satisfy the above formula (2). In other words, the lens power of the second lens L2 is weaker than that of the other lenses in the imaging lens system 11. In the imaging lens system 11 with a relatively long back focus, the chief rays are separated on the object side of the imaging lens system 11. Therefore, by arranging such a second lens L2 in the first lens group G1, the angle of incidence of the chief rays incident on the second lens L2 becomes large, and since the chief rays are separated, the second lens L2 can suitably correct field curvature. That is, in the imaging lens system 11 having a relatively long back focus, the curvature of field can be corrected appropriately.

[0045] Furthermore, as shown in Table 9, in Examples 1 to 4, the value of D24 / L satisfies the above formula (3). This allows the first lens L1 to the third lens L3 constituting the first lens group G1 to be positioned further toward the object side than the aperture stop STOP position. This more reliably increases the angle of incidence of the chief ray incident on the object-side surface S3 of the second lens L2, and more reliably separates the respective chief rays incident on the object-side surface S3 of the second lens L2. Therefore, the second lens L2 can more reliably correct field curvature.

[0046] Furthermore, as shown in Table 9, in Examples 1 to 4, the values ​​of df / dr satisfy the above formula (4). Therefore, when df / dr is smaller than 1.0, aberration correction can be performed effectively. On the other hand, when df / dr is larger than 0.05, interference between the first lens L1 and the second lens L2 can be prevented during assembly of the imaging lens system 11.

[0047] Furthermore, as shown in Table 9, the values ​​of F / f2 satisfy the above formula (5) in Examples 1 to 4. In other words, in Examples 1 to 4, the lens power of the second lens L2 is weaker than the other lenses in the imaging lens system 11. Therefore, for the same reason as above, the second lens L2 can suitably correct field curvature.

[0048] In addition, in Examples 1 to 4, the second lens L2 has inflection points on both the object-side surface S3 and the image-side surface S4, which allows the second lens L2 to correct field curvature, one of various aberrations caused by negative distortion produced by the first lens L1, from the center to the periphery.

[0049] In addition, in Examples 1 to 4, in the range from the center position where the object-side surface S3 and the image-side surface S4 of the second lens L2 intersect with the optical axis Z to the position of the inflection point of the second lens L2, the surfaces S3 and S4 are convex surfaces toward the object side, and in the range from the position of the inflection point to the outer edge of the second lens L2, the surfaces S3 and S4 are concave surfaces toward the object side. Since the object-side surface S3 of the second lens L2 and the image-side surface S4 of the second lens L2 have the above-described shapes, correction can be made to match the characteristics of the field curvature by designing the position of the inflection point from the center position of the second lens L2.

[0050] It should be noted that the present invention is not limited to the above-described embodiments and can be modified as appropriate without departing from the spirit of the present invention. For example, the applications of the imaging lens system of the present invention are not limited to vehicle-mounted cameras and surveillance cameras, and the imaging lens system may also be used for other applications, such as being mounted on small electronic devices such as mobile phones. [Explanation of symbols]

[0051] 11 Imaging lens system 12 Glass (IR cut filter) 20 Imaging device 21 Image sensor L1 First lens L2 Second lens L3 Third lens L4 4th lens L5 Fifth lens L6 6th lens G1 First lens group G2 Second lens group STOP Aperture IMG Image plane

Claims

1. The lens comprises, in order from the object side to the image side, a first lens group and a second lens group, the first lens group comprises, in order from the object side to the image side, one or two lenses having negative power, and a correction lens having an aspherical surface on the object side and the image side; the second lens group comprises, in order from the object side to the image side, a third lens having positive power, a stop, a fourth lens having negative power, a fifth lens having positive power, and a sixth lens having negative power; When L is an overall optical length that is the distance on the optical axis from the object-side surface of the lens in the first lens group that is located closest to the object to the image-side surface of the lens in the second lens group that is located closest to the image, and BF is a back focus that is the distance on the optical axis from the image-side surface of the lens in the second lens group that is located closest to the image to the object-side surface of the image sensor, the following formula (1) is satisfied: BF / L>0.1...(1) An imaging lens system that satisfies the following formula (2), where ET2 is the distance parallel to the optical axis between the intersection of a ray passing through the outermost diameter side of the object-side surface of the correction lens and the object-side surface of the correction lens, and the intersection of a ray passing through the outermost diameter side of the object-side surface of the correction lens and the image-side surface, and d2 is the thickness on the optical axis of the correction lens. 0.9<ET2 / d2<1.1...(2)

2. 2. The imaging lens system according to claim 1, wherein the following formula (3) is satisfied, where D24 is a distance on the optical axis from the image-side surface of the lens in the first lens group that is located closest to the object to the object-side surface of the lens in the second lens group that is located closest to the object: 0.3<D24 / L<0.5...(3)

3. 2. The imaging lens system according to claim 1, wherein the following formula (4) is satisfied, where df is the distance on the optical axis from the image-side surface of a lens adjacent to the object side of the correction lens to the object-side surface of the correction lens, and dr is the distance on the optical axis from the image-side surface of the correction lens to the object-side surface of the lens adjacent to the image side of the correction lens. 0.05<df / dr<1.0...(4)

4. 2. The imaging lens system according to claim 1, wherein the following formula (5) is satisfied, where F is a focal length of the entire optical system and f2 is a focal length of the correction lens: -0.1<F / f2<0.1...(5)

5. 5. The imaging lens system according to claim 1, wherein the correction lens has at least one inflection point on at least one of the object-side and image-side surfaces.

6. 6. The imaging lens system according to claim 5, wherein the correction lens is convex toward the object side in a range from a center position where it intersects with the optical axis to a predetermined position, and is convex toward the image side in a range from the predetermined position to an outer edge of the correction lens.

7. An imaging lens system according to any one of claims 1 to 6; an imaging element disposed at a focal position of the imaging lens system.

Images