Imaging lens system and imaging device

Abstract

To provide an imaging lens system and imaging device that can suitably correct field curvature in an imaging lens system with a relatively long back focus. [Solution] The imaging lens system 11 consists of a first lens group G1 comprising a first lens L1 having negative power and a second lens L2 having an aspherical shape, and a second lens group G2 comprising at least two third lenses L3 and a fourth lens L4. When the total optical length is L, the back focus is BF, the thickness of the outer edge and the thickness on the optical axis of the second lens L2 are ET2 and d2, respectively, the distance from the first lens L1 to the third lens L3 is D24, and the distance from the first lens L1 to the second lens L2 and the distance from the second lens L2 to the third lens L3 are df and dr, respectively, the following equations (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

Description

Technical Field

[0001] The present invention relates to an imaging lens system and an imaging device, and more particularly to an imaging lens system and an imaging device for vehicle-mounted use, for example.

Background Art

[0002] Conventionally, in order to correct field curvature, in an imaging lens system, a lens having a inflection point on at least one of the object side and the image side surfaces is disposed at the position closest to the image side. For example, in Patent Documents 1 and 2, an imaging lens system including a first lens to a sixth lens arranged in order from the object side to the image side uses, as the sixth lens, a lens having an inflection point on the object side and image side surfaces.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, Patent Documents 1 and 2 assume an imaging lens system mounted on a mobile phone or the like, and the back focus of the imaging lens system mounted on a mobile phone or the like is short. Therefore, the incident angle of the chief ray incident on the imaging sensor becomes large, and since each chief ray is separated, the field curvature can be preferably corrected by the aspherical lens disposed on the most image side of the imaging lens system. However, for an in-vehicle imaging lens system, the back focus is relatively long, and even if an aspherical lens having an inflection point on the most image side of the imaging lens system is disposed, it is difficult to preferably correct the field curvature.

[0005] This invention has been made in view of the above problems, and aims to provide an imaging lens system and 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] In one embodiment, the imaging lens system consists of a first lens group, an aperture, and a second lens group, arranged in order from the object side to the image side. The first lens group consists of, in order from the object side to the image side, one or two negative power lenses and a corrective lens having an aspherical shape on both the object side and the image side. The second lens group consists of at least one lens, Let L be the total optical length, which is the distance along the optical axis from the object-side surface of the lens closest to the object in the first lens group to the image-side surface of the lens closest to the image in the second lens group, and let BF be the back focus, which is the distance along the optical axis from the image-side surface of the lens closest to the image in the second lens group to the object-side surface of the image sensor. Then, the following equation (1) is satisfied, BF / L>0.1 ···(1) When ET2 is the distance parallel to the optical axis between the intersection point of the light ray passing through the outermost diameter side of the object-side surface of the corrector lens and the object-side surface of the corrector lens, and the intersection point of the light ray passing through the outermost diameter side of the object-side surface of the corrector lens and the image-side surface, and d2 is the thickness of the corrector lens along the optical axis, then the following equation (2) is satisfied, 0.9 <ET2 / d2<1.1 ···(2) When D24 is the distance along the optical axis from the image-side surface of the lens closest to the object in the first lens group to the object-side surface of the lens closest to the object in the second lens group, the following equation (3) is satisfied, 0.3 <D24 / L<0.5 ···(3) When df is the distance along the optical axis from the image-side surface of the lens adjacent to the object-side of the corrective lens to the object-side surface of the corrective lens, and dr is the distance along the optical axis from the image-side surface of the corrective lens to the object-side surface of the lens adjacent to the image-side of the corrective lens, 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 suitably correct field curvature by using a corrective lens arranged in the front group in an imaging lens system with a relatively long back focus. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view showing the configuration of the imaging lens system and imaging device and the light rays according to Embodiment 1. [Figure 2] This is a diagram illustrating the distance ET2. [Figure 3] This is a cross-sectional view showing the configuration of the imaging lens system and imaging device according to Example 1. [Figure 4] These are the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system of Example 1. [Figure 5] This is a cross-sectional view showing the configuration of the imaging lens system and imaging device according to Example 2. [Figure 6] These are the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system of Example 2. [Figure 7] This is a cross-sectional view showing the configuration of the imaging lens system and imaging device according to Example 3. [Figure 8] These are the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system of Example 3. [Figure 9] This is a cross-sectional view showing the configuration of the imaging lens system and imaging device according to Example 4. [Figure 10] These are the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system of Example 4. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 (Imaging Lens System and Imaging Device) FIG. 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. For example, a CCD image sensor or a CMOS image sensor is used. The imaging element 21 is disposed at the imaging position of the imaging lens system 11. Hereinafter, the imaging lens system 11 will be described in detail.

[0010] As shown in FIG. 1, the imaging lens system 11 of Embodiment 1 includes, in order from the object side to the image side, a first lens group G1, an aperture STOP, and a second lens group G2. The first lens group G1 includes, in order from the object side to the image side, one or two lenses having negative power and a correction lens having an aspherical shape on the object side and the image side. The second lens group G2 includes at least two lenses, and the aperture STOP is disposed on the image side of the most object-side lens of the second lens group G2. Specifically, the first lens group G1 consists of, in order from the object side to the image side, a first lens L1 which is a meniscus lens with negative power and a concave surface on the object side, and a second lens L2 which has aspherical surfaces on both the object side and the image side and functions as a corrective lens. The second lens group G2 consists of a third lens L3 which has positive power and a convex surface on the object side, a fourth lens L4 which has negative power and a concave surface on the image side, a fifth lens L5 which has positive power and convex surfaces on both the object side and the image side, and a sixth lens L6 which has negative power and aspherical surfaces on both the object side and the image side. Furthermore, as shown in Figure 1, the imaging lens system 11 may also include a wavelength filter glass 12 between the image-side lens surface S13 of the sixth lens L6 and the imaging plane IMG. Also, the fourth lens L4 and the fifth lens L5 constitute a cemented lens. In addition, there is a cover glass 22 on the image sensor 21, and the imaging plane of the imaging lens system 11 is indicated by IMG. The first lenses L1 to the third lenses L3 are preferably glass lenses, and the fourth lenses L4 to the sixth lenses L6 are preferably plastic lenses. Alternatively, the first lens L1 constituting the first group may be replaced by two lenses having negative powers, 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 each having a split negative power, and the second lens L2.

[0011] Furthermore, the first lens L1 and the second lens L2, positioned on the object side of the aperture stop, constitute the first lens group G1, while the third lens L3, positioned adjacent to the object side of the aperture stop, and the fourth lens L4, fifth lens L5, and sixth lens L6, positioned adjacent to the image side of the aperture stop, 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 is the aperture that determines the F-number (Fno) of the imaging lens system 11.

[0012] Also, when the optical overall 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 side of the first lens group G1 to the image-side surface S13 of the lens (sixth lens L6) located closest to the image side of the second lens group G2, is denoted 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 side of the second lens group G2 to the object-side surface IMG of the imaging device 21, is denoted as BF, the following formula (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] Also, as shown in FIG. 2, when the distance parallel to the optical axis Z between the intersection point P1 of the light ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2, which is a correction lens, and the object-side surface S3 of the second lens L2, and the intersection point P2 of the light 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 denoted as ET2, and the thickness of the second lens L2 on the optical axis Z is denoted as d2, the following formula (2) is satisfied. Here, the light ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2 corresponds to the incident light ray to the diagonal length of the imaging device 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 of the imaging lens system 11. In the imaging lens system 11 with a relatively long back focus, on the object side of the imaging lens system 11, each chief ray is separated. By arranging such a second lens L2 in the first lens group G1, field curvature can be suitably corrected. Specifically, by arranging such a second lens L2 in the first lens group G1, the incident angle of the chief ray incident on the second lens L2 becomes large, and since each chief ray is separated, the field curvature can be suitably corrected by the second lens L2. That is, in the imaging lens system 11 with a relatively long back focus, field curvature can be suitably corrected. In Embodiment 1, the second lens group G2 consists of four lenses, but it is not limited to this configuration; any number of lenses is acceptable. If the optical system has a long back focus and BF / L > 0.1, the overlap of various light rays in the object-side lens within the optical system will be reduced, making it easier to correct for field curvature for each light ray. This effect is not dependent on the lens configuration of the second lens group G2. Furthermore, the third lens L3 and subsequent lenses are close to the pupil (aperture), so it is sufficient to prioritize imaging performance such as spherical aberration. Also, field curvature is less pronounced for the lenses from the third lens L3 onward due to their longer focal lengths.

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

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

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

[0018] Furthermore, when the total focal length of the imaging lens system 11 is F and the focal length of the second lens L2 is f2, it is preferable that the following equation (5) is 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. For the same reasons as above, the second lens L2 can suitably correct field curvature.

[0020] Furthermore, in this embodiment 1, the first lens L1 has negative power, and the object-side surface S1 of the first lens L1 is concave. This allows for a large change in the angle of incidence of off-axis rays (rays that form an image outside the optical axis; also called "peripheral rays") incident on the first lens L1, thereby generating negative distortion. As a result, the image shrinks towards the edges, widening the range detected by the image sensor 21, enabling a high angle of view and a wide angle of view. On the other hand, the second lens L2 has at least one inflection point on at least one of its surfaces, the object side and the image side, thereby correcting the field curvature, one of the various aberrations caused by the negative distortion generated by the first lens L1, from the center to the periphery.

[0021] Furthermore, if the second lens L2 has one inflection point on the object-side surface S3, it is preferable that the surface S3 is convex toward the object in the range from the center position where the object-side surface S3 of the second lens L2 intersects the optical axis Z to the position of the inflection point of the second lens L2, and concave toward the object in the range from the position of the inflection point to the outer edge of the second lens L2. Furthermore, if the second lens L2 has one inflection point on the image-side surface S4, it is preferable that the surface S4 is concave toward the object in the range from the center position where the image-side surface S4 of the second lens L2 intersects 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 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 and the image-side surface S4 of the second lens L2 have the above-described shape, the position of the inflection point from the center position of the second lens L2 can be designed to correct for the characteristics of field curvature. 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 towards the object side in the range from the central position intersecting the optical axis to a predetermined position, and convex towards 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 to match the characteristics of field curvature known as barrel distortion or pincushion distortion. In this embodiment 1, the second lens group G2 is composed of four lenses, but it is not limited to this configuration; any number of lenses is acceptable. The present invention achieves its objective with only a first lens L1 for achieving a wide angle of view and a second lens L2 with low power for correcting field curvature. The lenses located on the image side of these lenses are simply configured to achieve image formation, so the configuration of the second lens group G2 does not matter. In other words, since the second lens group G2 has little field curvature and is close to the pupil (aperture), it is sufficient to prioritize imaging performance such as spherical aberration, so any configuration is acceptable.

[0022] Next, an embodiment corresponding to the imaging lens system 11 of Embodiment 1 will be described with reference to the drawings.

[0023] (Example 1) Figure 3 is a cross-sectional view showing the imaging lens system 11 according to Embodiment 1. Specifically, the imaging lens system 11 according to Embodiment 1 consists of, 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, 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 both 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 both the object side and the image side. The sixth lens L6 has positive power and an aspherical surface on both the object side and the image side. The first to third lenses L1 to L3 are glass lenses, and the fourth to sixth lenses L4 to L6 are plastic lenses. The imaging lens system 11 also includes an IR cut filter 12, which is a filter for cutting out light in the infrared region. The characteristic data of the imaging lens system 11 according to Embodiment 1 will be described below.

[0024] Table 1 shows the lens data for each lens surface of the imaging lens system 11 according to Example 1. In Table 1, the lens data for each surface includes the radius of curvature (mm), the interplanar 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. Here, the half-angle of view of the imaging lens system 11 according to Example 1 is 27.0°, the F-number is 1.8, and the focal length F of the entire optical system is 8.623 (mm). Also, the refractive index at the d line and the Abbe number at the d line shown in Table 1 are values ​​when the ambient temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). Furthermore, in Table 1, surfaces marked with an asterisk (*) indicate that they are aspherical surfaces. Note that the half-angle of view of the optical system refers to the angle that a light ray passing through the center of the pupil and reaching the position (diagonal point) at the diagonal length of the sensor makes with the optical axis on the object side. [Table 1]

[0025] Furthermore, the aspherical shapes adopted for the lens surfaces of the second lens L2, third lens L3, fourth lens surface L4, fifth lens L5, and sixth lens L6 are expressed by the following equation (6), where Y(h) is the sag amount 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 the direction perpendicular to the central optical axis Z, K is the conicity coefficient, and A4, A6, A8, A10, A12, A14, and A16 are the 4th, 6th, 8th, 10th, 12th, 14th, and 16th order aspherical coefficients, respectively. Note that the meaning of each symbol and the equations representing the aspherical shapes are the same in the embodiments described later.

number

[0026] Table 2 shows the aspheric coefficients used to define the aspheric shape of the aspheric lens surface in the imaging lens system 11 of Example 1. In Table 2, for example, "-2.33239E-03" is equivalent to "-2.33239E×10 -3 It means "...". [Table 2]

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

[0028] (Example 2) Figure 5 is a cross-sectional view showing the 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, so its explanation will be omitted. The characteristic data of the imaging lens system 11 according to Example 2 will be described below.

[0029] Table 3 shows the 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, so their explanation is omitted. Here, the half-angle of view of the imaging lens system 11 according to Example 2 is 27.0°, the F-number is 1.8, and the focal length F of the entire optical system is 8.923 (mm). [Table 3]

[0030] Table 4 shows the aspheric coefficients used to define the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 2. [Table 4]

[0031] Figure 6 shows the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system 11 of Example 2. The explanation of each aberration diagram shown in Figure 6 is the same as in Figure 4, so the explanation is omitted.

[0032] (Example 3) Figure 7 is a cross-sectional view showing the 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, so its explanation will be omitted. The characteristic data of the imaging lens system 11 according to Example 3 will be described below.

[0033] Table 5 shows the 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, so their explanation is omitted. Here, the half-angle of view of the imaging lens system 11 according to Example 3 is 27.0°, the F-number is 1.8, and the focal length F of the entire optical system is 8.59 (mm). [Table 5]

[0034] Table 6 shows the aspheric coefficients used to define the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 3. [Table 6]

[0035] Figure 8 shows the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system 11 of Example 3. The explanation of each aberration diagram shown in Figure 8 is the same as in Figure 4, so the explanation is omitted.

[0036] (Example 4) Figure 9 is a cross-sectional view showing the 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, so its explanation will be omitted. The characteristic data of the imaging lens system 11 according to Example 4 will be described below.

[0037] Table 7 shows the 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, so their explanation is omitted. Here, the half-angle of view of the imaging lens system 11 according to Example 4 is 27.0°, the F-number is 1.8, and the focal length F of the entire optical system is 8.623 (mm). [Table 7]

[0038] Table 8 shows the aspheric coefficients used to define the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 4. [Table 8]

[0039] Figure 10 shows the spherical aberration diagram (longitudinal aberration diagram), field curvature diagram, and distortion aberration diagram for the imaging lens system 11 of Example 4. The explanation of each aberration diagram shown in Figure 10 is the same as in Figure 4, so the explanation is omitted.

[0040] As shown in the longitudinal aberration diagrams of Figures 4A, 6A, 8A, and 10A, the imaging lens system 11 of these embodiments 1 to 4 effectively corrects longitudinal aberrations at wavelengths of 486.1 nm, 587.6 nm, and 656.3 nm. Therefore, the imaging lens system 11 achieves high resolution.

[0041] Furthermore, as shown in the field curvature diagrams of Figures 4B, 6B, 8B, and 10B, the field curvature is well corrected in the imaging lens system 11 of these embodiments 1 to 4. Therefore, the imaging lens system 11 achieves high resolution.

[0042] Furthermore, as shown in the distortion diagrams in Figures 4C, 6C, 8C, and 10C, distortion is well corrected in the imaging lens system 11 of these embodiments 1 to 4. Therefore, the imaging lens system 11 achieves high resolution.

[0043] Table 9 also shows the total focal length F (mm) of the imaging lens system 11 according to Examples 1 to 4, the focal lengths f1 to f6 (mm) of the first lens L1 to the sixth lens L6, the back focus BF (mm), the total optical length L (mm), the distance ET2 (mm) parallel to the optical axis between the light ray transmitted through the outermost diameter side of the object-side surface S3 of the second lens L2 and the intersection point with 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, and the second lens Table 9 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 combined 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 BF / L value, the ET2 / d2 value, the F / f2 value, the D24 / L value, and the df / dr value. The values ​​shown in Table 9 are for when the wavelength of the light is 555 nm and the ambient temperature t (°C) is 25 (°C). 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 each possessing the divided negative power, and the second lens L2. [Table 9]

[0044] As shown in Table 9, in Examples 1 to 4, the BF / L value satisfies equation (1) above. Therefore, the imaging lens system 11 according to Examples 1 to 4 is an optical system with a relatively long back focus and can be used for applications such as automotive use where a relatively thick image sensor is used. Also, as shown in Table 9, in Examples 1 to 4, the ET2 / d2 value satisfies equation (2) above. 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 an imaging lens system 11 with a relatively long back focus, each principal ray is 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 incident angle of the principal rays incident on the second lens L2 becomes large, and since each principal ray is separated, the field curvature can be suitably corrected by the second lens L2. In other words, in an imaging lens system 11 with a relatively long back focus, field curvature can be suitably corrected.

[0045] Furthermore, as shown in Table 9, in Examples 1 to 4, the value of D24 / L satisfies equation (3) above. This allows the first lenses L1 to the third lenses L3 constituting the first lens group G1 to be positioned further toward the object than the aperture stop position. This makes it possible to more reliably increase the incident angle of the principal rays incident on the object-side surface S3 of the second lens L2, and to more reliably separate each principal ray incident on the object-side surface S3 of the second lens L2. Therefore, the field curvature can be more reliably corrected by the second lens L2.

[0046] Furthermore, as shown in Table 9, in Examples 1 to 4, the df / dr value satisfies the above equation (4). Therefore, if df / dr is less than 1.0, aberration correction can be effectively performed. On the other hand, if df / dr is greater than 0.05, interference between the first lens L1 and the second lens L2 can be prevented during the assembly of the imaging lens system 11.

[0047] Furthermore, as shown in Table 9, in Examples 1 to 4, the F / f2 value satisfies equation (5) above. In other words, in Examples 1 to 4, 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 reasons as above, the second lens L2 can suitably correct field curvature.

[0048] Furthermore, 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. This allows the second lens L2 to correct the field curvature, one of the various aberrations caused by the negative distortion generated by the first lens L1, from the center to the periphery.

[0049] Furthermore, in Examples 1 to 4, in the range from the center position where the object-side surface S3 and image-side surface S4 of the second lens L2 intersect the optical axis Z to the position of the inflection point of the second lens L2, the surfaces S3 and S4 are convex towards the object, 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 towards the object. Because the object-side surface S3 and the image-side surface S4 of the second lens L2 have the above-described shapes, the position of the inflection point from the center position of the second lens L2 can be designed to correct for the characteristics of field curvature.

[0050] It should be noted that the present invention is not limited to the embodiments described above, and can be modified as appropriate without departing from the spirit of the invention. For example, the applications of the imaging lens system of the present invention are not limited to in-vehicle cameras and surveillance cameras, but may also be used for other applications such as mounting 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 3rd lens L4 4th lens L5 5th lens L6 6th lens G1 First Lens Group G2 2nd lens group STOP aperture IMG imaging plane

Claims

1. It consists of a first lens group and a second lens group, arranged in order from the object side towards the image side. The first lens group consists of, in order from the object side to the image side, one or two negative power lenses and a corrective lens having an aspherical shape on both the object side and the image side. The second lens group consists of, in order from the object side to the image side, a third lens having positive power, a fourth and fifth lens forming a cemented lens, and a sixth lens having negative power. When the total optical length is L, which is the distance along the optical axis from the object-side surface of the lens closest to the object in the first lens group to the image-side surface of the lens closest to the image in the second lens group, and the back focus is BF, which is the distance along the optical axis from the image-side surface of the lens closest to the image in the second lens group to the object-side surface of the image sensor, then the following equation (1) is satisfied, BF / L>0.1...(1) An imaging lens system that satisfies the following equation (2), where ET2 is the distance parallel to the optical axis between the intersection point of the light ray passing through the outermost diameter side of the object-side surface of the corrector lens and the object-side surface of the corrector lens, and d2 is the thickness of the corrector lens along the optical axis. 0.9<ET2 / d2<1.1...(2)

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

3. The imaging lens system according to claim 1, wherein when df is the distance along the optical axis from the image-side surface of the lens adjacent to the object-side of the correcting lens to the object-side surface of the correcting lens, and dr is the distance along the optical axis from the image-side surface of the correcting lens to the object-side surface of the lens adjacent to the image-side of the correcting lens, the following equation (4) is satisfied. 0.05<df / dr<1.0...(4)

4. The imaging lens system according to claim 1, wherein when the focal length of the entire optical system is F and the focal length of the corrective lens is f2, the following equation (5) is satisfied. -0.1<F / f2<0.1...(5)

5. The imaging lens system according to any one of claims 1 to 4, wherein the corrective lens has at least one inflection point on at least one of the object-side and image-side surfaces.

6. The imaging lens system according to claim 5, wherein the corrective lens is convex toward the object in the range from a central position intersecting the optical axis to a predetermined position, and convex toward the image in the range from the predetermined position to the outer edge of the corrective lens.

7. An imaging lens system according to any one of claims 1 to 6, An imaging device comprising an image sensor positioned at the focal position of the imaging lens system.

Images