Image capturing lens system and image capturing device

Abstract

To provide an image capturing lens system which can be produced at low cost and offers more stable performance against temperature variations, and to provide an image capturing device having the same.SOLUTION: An image capturing lens system 11 provided herein comprises a front group lens system and a rear group lens system arranged in order form the object side to the image side. The front group lens system consists of, in order from the object side to the image side, a first lens L1 with negative power having a concave surface on the image side, a second lens L2 with negative power having a convex surface on the image side, and a third ens L3 with positive power having a convex surface on the image side. The rear group lens system consists of, in order from the object side to the image side, a fourth lens L4 with positive power having a convex surface of the image side, a fifth lens L5 with negative power, and a sixth lens L6. The fourth lens L4 and the fifth lens L5 constitute a cemented lens. The first lens L1 and the third lens L3 are glass lenses. The second lens L2, the fourth lens L4, and the fifth lens L5 are plastic lenses.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an imaging lens system and an imaging device.

Background Art

[0002] In recent years, the applications of wide-angle lenses mounted on vehicles have changed from viewing to sensing. Since sensing requires a resolution necessary for image analysis, high-resolution images corresponding to megapixels are required. Also, a wide angle of view is required.

[0003] As described above, in an in-vehicle imaging device, it is required to image a distant object in the traveling direction with high resolution and image a nearby object with a wide angle. Furthermore, it is required to be a bright optical system. In particular, for an imaging lens system for an in-vehicle camera, compactness is also required.

[0004] For example, Patent Document 1 describes an imaging device that switches between a wide-angle optical system and a telephoto optical system arranged so that their optical axes are orthogonal to each other with a mirror, and forms an optical image by either optical system on a common image sensor.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the imaging device of Patent Document 1, since two types of lenses, a telephoto optical system for long-distance imaging and a wide-angle optical system for short-distance imaging, are used, the size of the unit becomes large and the cost also increases. In addition, when all lenses are made of plastic lenses to reduce costs, there is a problem that the performance stability is lacking with respect to changes in environmental temperature.

[0007] Thus, in the conventional imaging lens system, there has been a problem that it is impossible to realize an imaging lens system and an imaging apparatus that suppress costs and have stable performance against temperature changes.

Means for Solving the Problem

[0008] An imaging lens system according to an embodiment is an imaging lens system including a front group lens system and a rear group lens system in order from the object side to the image side, The front group lens system includes, in order from the object side to the image side, a first lens having a negative power and a concave shape on the image side, a second lens having a negative power and a concave shape on the object side, and a third lens having a positive power and a convex shape on the object side and a convex shape on the image side. The rear group lens system includes, in order from the object side to the image side, a fourth lens having a positive power and a convex shape on the object side and a convex shape on the image side, a fifth lens having a negative power, and a sixth lens. The fourth lens and the fifth lens are cemented lenses, the first lens and the third lens are glass lenses, and the second lens, the fourth lens, and the fifth lens are plastic lenses. When the focal length of the second lens is f2, the combined focal length of the plastic lenses in the rear group lens system is frp, and the focal length of the entire lens system is F, the following equations (1) and (2) are satisfied. 2 < |f2 / F| ···(1) 2 < |frp / F| ···(2)

[0009] According to this configuration, it is possible to realize an imaging lens system that suppresses costs by using a large number of plastic lenses and has stable performance against temperature changes.

Effects of the Invention

[0010] According to the present invention, it is possible to provide an imaging lens system and an imaging apparatus that suppress costs and have stable performance against temperature changes.

Brief Description of the Drawings

[0011]

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Mode for Carrying Out the Invention

[0012] Hereinafter, the optical lens and the imaging device according to the present embodiment will be described. (Embodiment 1: Imaging Lens System) The imaging lens system of Embodiment 1 is an imaging lens system including a front group lens system and a rear group lens system in order from the object side to the image side, The front group lens system includes, in order from the object side to the image side, a first lens having a negative power and a concave shape on the image side, a second lens having a negative power and a concave shape on the object side, and a third lens having a positive power, a convex shape on the object side, and a convex shape on the image side. The rear group lens system includes, in order from the object side to the image side, a fourth lens having a positive power, a convex shape on the object side, and a convex shape on the image side, a fifth lens having a negative power, and a sixth lens. The fourth lens and the fifth lens are cemented lenses, the first lens and the third lens are glass lenses, the second lens, the fourth lens, and the fifth lens are plastic lenses. When the focal length of the second lens is f2, the combined focal length of the plastic lenses in the rear group lens system is frp, and the focal length of the entire lens system is F, the following formulas (1) and (2) are satisfied. 2 < |f2 / F| ···(1) 2 < |frp / F| ···(2)

[0013] Thus, according to the imaging lens system of Embodiment 1, an imaging lens system with reduced cost and stabilized performance against temperature changes can be realized.

[0014] In the imaging lens system of the above-described Embodiment 1, when the sixth lens is a glass lens and the focal length of the second lens is f2, the following formula (3) may be satisfied. 0.5 < |f2 / frp| < 2 ···(3)

[0015] According to these configurations, between the plastic lens in the front lens group system and the plastic lens group in the rear lens group system, the refractive power changes generated by both due to temperature changes can be compensated for each other, and the performance change of the entire optical system due to temperature changes can be suppressed.

[0016] Next, an example corresponding to the imaging lens system of Embodiment 1 will be described with reference to the drawings. (Example 1) FIG. 1 is a cross-sectional view showing the configuration of the imaging lens system according to Example 1. In FIG. 1, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a negative power, a second lens L2 having a negative power, a third lens L3 having a positive power, a diaphragm STOP, a fourth lens L4 having a positive power, a fifth lens L5 having a negative power, and a sixth lens L6. The imaging surface of the imaging lens system 11 is indicated by IMG. Further, the imaging lens system 11 includes an IR cut filter 12.

[0017] The first lens L1 is an aspherical glass lens having a negative power. The object-side lens surface S1 of the first lens L1 has a convex curved surface portion on the object side. The image-side lens surface S2 of the first lens L1 has a concave curved surface portion on the image side.

[0018] The second lens L2 is an aspherical plastic lens having a negative power. The object-side lens surface S3 of the second lens L2 has a concave curved surface portion on the object side. Also, the image-side lens surface S4 of the second lens L2 has a convex curved surface portion on the image side.

[0019] The third lens L3 is an aspherical glass lens having a positive power. Also, the object-side lens surface S5 faces a convex surface toward the object side, and the image-side lens surface S6 faces a convex surface toward the image side.

[0020] The aperture STOP is an aperture that determines the F-number (Fno) of the lens system. The aperture STOP is disposed between the third lens L3 and the fourth lens L4.

[0021] The fourth lens L4 is an aspherical plastic lens having a positive power. The object-side lens surface S8 faces a convex surface toward the object side, and the image-side lens surface S9 faces a convex surface toward the image side.

[0022] The fifth lens L5 is an aspherical plastic lens having a negative power. The object-side lens surface S10 faces a concave surface toward the object side, and the image-side lens surface S11 faces a convex surface toward the image side.

[0023] The fourth lens L4 and the fifth lens L5 constitute a cemented lens in which the image-side lens surface S9 of the fourth lens L4 and the object-side lens surface S10 of the fifth lens L5 are cemented together.

[0024] The sixth lens L6 is an aspherical lens having a positive power. The object-side lens surface S12 faces a convex surface toward the object side, and the image-side lens surface S13 faces a concave surface toward the image side. The sixth lens L6 is a glass lens.

[0025] The IR cut filter 12 is a filter for cutting light in the infrared region. The IR cut filter 12 is treated as being integral with the imaging lens system 11 at the time of designing the imaging lens system 11. However, the IR cut filter 12 is not an essential component of the imaging lens system 11.

[0026] Table 1 shows the lens data of each lens surface in the imaging lens system 11 of Example 1. In Table 1, as the lens data, the radius of curvature (mm) of each surface, the surface interval (mm), the refractive index at the d line, and the Abbe number at the d line are presented. The refractive index at the d line and the Abbe number at the d line are the values when the environmental temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). The surface marked with "*" indicates that it is an aspherical surface. [Table 1]

[0027] The aspherical shape adopted for the lens surface is expressed by the following equation when Z is the sag amount, c is the reciprocal of the radius of curvature, k is the conic coefficient, r is the ray height from the optical axis Z, and the aspherical coefficients of the 4th, 6th, 8th, 10th, and 12th orders are α4, α6, α8, α 10 , α 12 respectively. [Equation]

[0028] Table 2 shows the aspherical coefficients for defining the aspherical shape of the lens surface that is aspherical in the imaging lens system 11 of Example 1. In Table 2, for example, "-6.522528E-03" means "-6.522528×10 -3 ". [Table 2]

[0029] FIG. 2 is a spherical aberration diagram of the imaging lens system of Example 1. FIG. 3 is a field curvature diagram of the imaging lens system of Example 1. FIG. 4 is a distortion aberration diagram of the imaging lens system of Example 1. As shown in FIGS. 2 to 4, in the imaging lens system 11 of Example 1, the half field angle ω is 65° and the F number is 1.6. In the spherical aberration diagram of FIG. 2, the horizontal axis indicates the position where the ray intersects the optical axis Z, and the vertical axis indicates the relative height of the ray at the pupil diameter. In the field curvature diagram of FIG. 3, the horizontal axis indicates the distance of the imaging point in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In FIG. 4, Sag indicates the field curvature in the sagittal plane, and Tan indicates the field curvature in the tangential plane. In the distortion aberration diagram of FIG. 4, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (field angle). In FIG. 2, the simulation results by rays with wavelengths of 0.42 μm, 0.55 μm, and 0.68 μm are shown. In FIGS. 3 and 4, the simulation results by the ray with a wavelength of 0.55 μm are shown.

[0030] Table 11 below shows the calculation results of the characteristic values of the imaging lens systems of the respective examples. In Table 11, when the focal length of the first lens L1 in the imaging lens system 11 is f1, the focal length of the second lens L2 is f2, the focal length of the third lens L3 is f3, the focal length of the fourth lens L4 is f4, the focal length of the fifth lens L5 is f5, the focal length of the sixth lens L6 is f6, the focal length of the entire lens system is F, the combined focal length of the fourth lens L4 and the fifth lens L5 is f45, the combined focal length of the fourth lens L4, the fifth lens L5, and the sixth lens L6 is f456, the combined focal length of the plastic lenses in the front group is ffp, and the combined focal length of the plastic lenses in the rear group is frp, each characteristic value is shown. Also, the various focal lengths in Table 11 were calculated using light rays with a wavelength of 555 nm. In Example 1, since the sixth lens is a glass lens, the combined focal length frp of the plastic lenses in the rear group lens system becomes the combined focal length f45 of the fourth lens L4 and the fifth lens L5.

[0031] FIG. 5 is a graph showing the relationship between the spatial frequency and the MTF at 25° C. in the imaging lens system of Example 1. FIG. 6 is a graph showing the relationship between the spatial frequency and the MTF at -40° C. in the imaging lens system of Example 1. FIG. 7 is a graph showing the relationship between the spatial frequency and the MTF at 115° C. in the imaging lens system of Example 1. In FIGS. 5 to 7, the vertical axis represents the MTF (Modulation Transfer Function). The horizontal axis represents the spatial frequency (cycles / mm). FIGS. 5 to 7 show the relationship between the spatial frequency and the MTF for each angle of view. In FIGS. 5 to 7, the dashed line represents the relationship between the spatial frequency and the MTF in the tangential plane. The solid line represents the relationship between the spatial frequency and the MTF in the sagittal plane.

[0032] Note that in this specification, the angle of view means, in a cross-sectional view along the optical axis, the angle at which the outermost marginal light beam that can actually pass through the first lens L1 intersects the lines obtained by extending the incident light on the object-side lens surface. Specifically, it corresponds to the "angle of view" in FIG. 8.

[0033] As shown in FIGS. 5 to 7, in the imaging lens system of Example 1, the relationship between the spatial frequency and the MTF maintains substantially the same relationship even at different temperatures.

[0034] (Example 2) FIG. 9 is a cross-sectional view showing the configuration of the imaging lens system of Example 2. In FIG. 9, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a negative power, a second lens L2 having a negative power, a third lens L3 having a positive power, a diaphragm STOP, a fourth lens L4 having a positive power, a fifth lens L5 having a negative power, and a sixth lens L6 having a positive power. In Embodiment 2, the sixth lens L6 is a glass lens. The imaging surface of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12. In FIG. 9, the same components as those in FIG. 1 are denoted by the same reference numerals and the description thereof is omitted.

[0035] Table 3 shows the lens data of each lens surface in the imaging lens system 11 of Example 2. In Table 3, as the lens data, the radius of curvature (mm) of each surface, the air space (mm), the refractive index at the d line, and the Abbe number at the d line are presented. The refractive index at the d line and the Abbe number at the d line are the values when the environmental temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). The surface marked with an asterisk (*) indicates an aspherical surface. [Table 3]

[0036] Table 4 shows the aspherical coefficients for defining the aspherical shape of the lens surfaces that are aspherical in the imaging lens system 11 of Example 2. In Table 4, the aspherical shape adopted for the lens surfaces is represented by the same formula as in Example 1. [Table 4]

[0037] FIG. 10 is a spherical aberration diagram of the imaging lens system in Example 2. FIG. 11 is a field curvature diagram of the imaging lens system in Example 2. FIG. 12 is a distortion aberration diagram of the imaging lens system in Example 2. As shown in FIGS. 10 to 12, in the imaging lens system 11 of Example 2, the semi-field angle ω is 65° and the F number is 1.6. In the spherical aberration diagram of FIG. 10, the horizontal axis indicates the position where the light ray intersects the optical axis Z, and the vertical axis indicates the relative height of the light ray at the pupil diameter. In the field curvature diagram of FIG. 11, the horizontal axis indicates the distance of the imaging point in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In FIG. 12, Sag indicates the field curvature in the sagittal plane, and Tan indicates the field curvature in the tangential plane. In the distortion aberration diagram of FIG. 12, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (field angle). In FIG. 10, the simulation results for light rays with wavelengths of 0.42 μm, 0.55 μm, and 0.68 μm are shown. In FIGS. 11 and 12, the simulation results for light rays with a wavelength of 0.55 μm are shown.

[0038] In Table 11, the results of calculating the characteristic values of the imaging lens system 11 of Example 2 are shown. In Example 2, since the sixth lens is a glass lens, the combined focal length frp of the plastic lenses in the rear lens group is the combined focal length f45 of the fourth lens L4 and the fifth lens L5.

[0039] FIG. 13 is a graph showing the relationship between the spatial frequency and MTF at 25° C. in the imaging lens system of Example 2. FIG. 14 is a graph showing the relationship between the spatial frequency and MTF at -40° C. in the imaging lens system of Example 2. FIG. 15 is a graph showing the relationship between the spatial frequency and MTF at 115° C. in the imaging lens system of Example 2. In FIGS. 13 to 15, the vertical axis represents the MTF (Modulation Transfer Function). The horizontal axis represents the spatial frequency (cycles / mm). FIGS. 13 to 15 show the relationship between the spatial frequency and MTF for each angle of view. In FIGS. 13 to 15, the broken line represents the relationship between the spatial frequency and MTF in the tangential plane. The solid line represents the relationship between the spatial frequency and MTF in the sagittal plane.

[0040] As shown in FIGS. 13 to 15, the imaging lens system of Example 2 maintains substantially the same relationship between the spatial frequency and MTF even at different temperatures.

[0041] (Example 3) FIG. 16 is a cross-sectional view showing the configuration of the imaging lens system of Example 3. In FIG. 16, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a negative power, a second lens L2 having a negative power, a third lens L3 having a positive power, a diaphragm STOP, a fourth lens L4 having a positive power, a fifth lens L5 having a negative power, and a sixth lens L6 having a positive power. In Embodiment 2, the sixth lens L6 is a glass lens. The imaging plane of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12. In FIG. 16, the same components as those in FIG. 1 are denoted by the same reference numerals and the description thereof is omitted.

[0042] Table 5 shows the lens data of each lens surface in the imaging lens system 11 of Example 3. In Table 5, as the lens data, the radius of curvature (mm) of each surface, the surface interval (mm), the refractive index at the d-line, and the Abbe number at the d-line are presented. The refractive index at the d-line and the Abbe number at the d-line are the values when the environmental temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). The surface marked with "*" indicates that it is an aspherical surface.

Table 5

[0043] Table 6 shows the aspherical coefficients for defining the aspherical shape of the lens surfaces that are aspherical in the imaging lens system 11 of Example 3. In Table 6, the aspherical shape adopted for the lens surface is represented by the same formula as in Example 1.

Table 6

[0044] Figure 17 is a spherical aberration diagram of the imaging lens system in Example 3. Figure 18 is a field curvature diagram of the imaging lens system in Example 3. Figure 19 is a distortion aberration diagram of the imaging lens system in Example 3. As shown in Figures 17 to 19, in the imaging lens system 11 of Example 3, the semi-field angle ω is 65° and the F-number is 1.6. In the spherical aberration diagram of Figure 17, the horizontal axis indicates the position where the light ray intersects the optical axis Z, and the vertical axis indicates the relative height of the light ray at the pupil diameter. In the field curvature diagram of Figure 18, the horizontal axis indicates the distance of the imaging point in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In Figure 19, Sag indicates the field curvature in the sagittal plane, and Tan indicates the field curvature in the tangential plane. In the distortion aberration diagram of Figure 19, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (field angle). In Figure 17, the simulation results for light rays with wavelengths of 0.42 μm, 0.55 μm, and 0.68 μm are shown. In Figures 18 and 19, the simulation results for light rays with a wavelength of 0.55 μm are shown.

[0045] Table 11 shows the calculated results of the characteristic values of the imaging lens system 11 of Example 3. In Example 3, since the sixth lens is a glass lens, the combined focal length frp of the plastic lenses in the rear group lens system is the combined focal length f45 of the fourth lens L4 and the fifth lens L5.

[0046] FIG. 20 is a graph showing the relationship between the spatial frequency and MTF at 25° C. in the imaging lens system of Example 3. FIG. 21 is a graph showing the relationship between the spatial frequency and MTF at -40° C. in the imaging lens system of Example 3. FIG. 22 is a graph showing the relationship between the spatial frequency and MTF at 115° C. in the imaging lens system of Example 3. In FIGS. 20 to 22, the vertical axis represents MTF (Modulation Transfer Function). The horizontal axis represents the spatial frequency (cycles / mm). FIGS. 20 to 22 show the relationship between the spatial frequency and MTF for each angle of view. In FIGS. 20 to 22, the broken line shows the relationship between the spatial frequency and MTF in the tangential plane. The solid line shows the relationship between the spatial frequency and MTF in the sagittal plane.

[0047] As shown in FIGS. 20 to 22, the imaging lens system of Example 3 maintains substantially the same relationship between the spatial frequency and MTF even at different temperatures.

[0048] (Example 4) FIG. 23 is a cross-sectional view showing the configuration of the imaging lens system of Example 4. In FIG. 23, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a negative power, a second lens L2 having a negative power, a third lens L3 having a positive power, a diaphragm STOP, a fourth lens L4 having a positive power, a fifth lens L5 having a negative power, and a sixth lens L6 having a positive power. In Embodiment 4, the sixth lens L6 is a plastic lens. The imaging surface of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12. In FIG. 23, the same components as those in FIG. 1 are denoted by the same reference numerals and the description thereof is omitted.

[0049] Table 7 shows the lens data of each lens surface in the imaging lens system 11 of Example 4. In Table 7, as the lens data, the radius of curvature (mm) of each surface, the surface interval (mm), the refractive index at the d line, and the Abbe number at the d line are presented. The refractive index at the d line and the Abbe number at the d line are the values when the environmental temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). The surface marked with "*" indicates that it is an aspherical surface. [Table 7]

[0050] Table 8 shows the aspherical coefficients for defining the aspherical shape of the lens surfaces that are aspherical in the imaging lens system 11 of Example 4. In Table 8, the aspherical shape adopted for the lens surface is represented by the same formula as in Example 1. [Table 8]

[0051] FIG. 24 is a spherical aberration diagram of the imaging lens system in Example 4. FIG. 25 is a field curvature diagram of the imaging lens system in Example 4. FIG. 26 is a distortion aberration diagram of the imaging lens system in Example 4. As shown in FIGS. 24 to 26, in the imaging lens system 11 of Example 4, the semi-field angle ω is 65° and the F number is 1.6. In the spherical aberration diagram of FIG. 24, the horizontal axis indicates the position where the light ray intersects the optical axis Z, and the vertical axis indicates the relative height of the light ray at the pupil diameter. In the field curvature diagram of FIG. 25, the horizontal axis indicates the distance of the imaging point in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In FIG. 26, Sag indicates the field curvature in the sagittal plane, and Tan indicates the field curvature in the tangential plane. In the distortion aberration diagram of FIG. 26, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (field angle). In FIG. 24, the simulation results by light rays with wavelengths of 0.42 μm, 0.55 μm, and 0.68 μm are shown. In FIGS. 25 and 26, the simulation results by the light ray with a wavelength of 0.55 μm are shown.

[0052] Table 11 shows the calculation results of the characteristic values of the imaging lens system 11 of Example 4. In Example 4, since the sixth lens is a plastic lens, the combined focal length frp of the plastic lenses in the rear group lens system is the combined focal length f456 of the fourth lens L4, the fifth lens L5, and the sixth lens.

[0053] FIG. 27 is a graph showing the relationship between the spatial frequency and MTF at 25° C. in the imaging lens system of Example 4. FIG. 28 is a graph showing the relationship between the spatial frequency and MTF at -40° C. in the imaging lens system of Example 4. FIG. 29 is a graph showing the relationship between the spatial frequency and MTF at 115° C. in the imaging lens system of Example 4. In FIGS. 27 to 29, the vertical axis represents MTF (Modulation Transfer Function). The horizontal axis represents the spatial frequency (cycles / mm). FIGS. 27 to 29 show the relationship between the spatial frequency and MTF for each angle of view. In FIGS. 27 to 29, the dashed line shows the relationship between the spatial frequency and MTF in the tangential plane. The solid line shows the relationship between the spatial frequency and MTF in the sagittal plane.

[0054] As shown in FIGS. 27 to 29, the imaging lens system of Example 4 maintains substantially the same relationship between the spatial frequency and MTF even at different temperatures.

[0055] (Example 5) FIG. 30 is a cross-sectional view showing the configuration of the imaging lens system of Example 5. In FIG. 30, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a negative power, a second lens L2 having a negative power, a third lens L3 having a positive power, a diaphragm STOP, a fourth lens L4 having a positive power, a fifth lens L5 having a negative power, and a sixth lens L6 having a positive power. In Embodiment 5, the sixth lens L6 is a plastic lens. The imaging surface of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12. In FIG. 30, the same components as those in FIG. 1 are denoted by the same reference numerals and the description thereof is omitted.

[0056] Table 9 shows the lens data of each lens surface in the imaging lens system 11 of Example 5. In Table 9, as the lens data, the radius of curvature (mm) of each surface, the air space (mm), the refractive index at the d-line, and the Abbe number at the d-line are presented. The refractive index at the d-line and the Abbe number at the d-line are the values when the environmental temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). The surfaces marked with an asterisk (*) indicate aspherical surfaces. [Table 9]

[0057] Table 10 shows the aspherical coefficients for defining the aspherical shape of the lens surfaces that are aspherical in the imaging lens system 11 of Example 5. In Table 10, the aspherical shape adopted for the lens surface is represented by the same formula as in Example 1. [Table 10]

[0058] Figure 31 is a spherical aberration diagram of the imaging lens system in Example 5. Figure 32 is a field curvature diagram of the imaging lens system in Example 5. Figure 33 is a distortion aberration diagram of the imaging lens system in Example 5. As shown in Figures 31 to 33, in the imaging lens system 11 of Example 5, the semi-field angle ω is 65° and the F-number is 1.6. In the spherical aberration diagram of Figure 31, the horizontal axis indicates the position where the ray intersects the optical axis Z, and the vertical axis indicates the relative height of the ray at the pupil diameter. In the field curvature diagram of Figure 32, the horizontal axis indicates the distance of the imaging point in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In Figure 33, Sag indicates the field curvature in the sagittal plane, and Tan indicates the field curvature in the tangential plane. In the distortion aberration diagram of Figure 33, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (field angle). In Figure 31, the simulation results for rays with wavelengths of 0.42 μm, 0.55 μm, and 0.68 μm are shown. In Figures 32 and 33, the simulation results for rays with a wavelength of 0.55 μm are shown.

[0059] Table 11 shows the calculation results of the characteristic values of the imaging lens system 11 of Example 5. In Example 5, since the sixth lens is a plastic lens, the combined focal length frp of the plastic lenses in the rear group lens system is the combined focal length f456 of the fourth lens L4, the fifth lens L5, and the sixth lens.

[0060] FIG. 34 is a graph showing the relationship between the spatial frequency and MTF at 25° C. in the imaging lens system of Example 5. FIG. 35 is a graph showing the relationship between the spatial frequency and MTF at -40° C. in the imaging lens system of Example 5. FIG. 36 is a graph showing the relationship between the spatial frequency and MTF at 115° C. in the imaging lens system of Example 5. In FIGS. 34 to 36, the vertical axis represents MTF (Modulation Transfer Function). The horizontal axis represents the spatial frequency (cycles / mm). FIGS. 34 to 36 show the relationship between the spatial frequency and MTF for each angle of view. In FIGS. 34 to 36, the broken line represents the relationship between the spatial frequency and MTF in the tangential plane. The solid line represents the relationship between the spatial frequency and MTF in the sagittal plane.

[0061] As shown in FIGS. 34 to 36, the imaging lens system of Example 5 maintains substantially the same relationship between the spatial frequency and MTF even at different temperatures.

[0062] Table 11 lists the numerical values corresponding to the respective conditional expressions in the present embodiment according to the above conditional expressions. Apparently, the imaging lens system of the present embodiment satisfies the above conditional expressions.

Table 11

[0063] (Embodiment 2: Application Example to an Imaging Apparatus) FIG. 37 is a cross-sectional view of an imaging apparatus according to Embodiment 2. The imaging apparatus 21 includes an imaging lens system 11, a cover glass 22, and an imaging element 23. The imaging lens system 11, the cover glass 22, and the imaging element 23 are housed in a housing (not shown).

[0064] The imaging element 23 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 23 is disposed at the imaging position of the imaging lens system 11. Note that the horizontal angle of view is the angle of view corresponding to the horizontal direction of the imaging element 23.

[0065] The cover glass 22 is provided on the imaging element 23 to protect the imaging element 23 from foreign matter.

[0066] Note that the present invention is not limited to the above-described embodiments, and can be appropriately modified without departing from the spirit thereof.

Explanation of Reference Numerals

[0067] 11 Imaging lens system 12 Cutoff filter 21 Imaging device 22 Cover glass 23 Imaging element L1, L2, L3, L4, L5, L6 Lenses STOP Diaphragm IMG Imaging plane

Claims

1. An imaging lens system consisting of a front lens group and a rear lens group, in that order from the object side to the image side, the front lens system comprises, in order from the object side to the image side, a first lens having negative power and a concave shape on the image side, a second lens having negative power and a convex shape on the image side, and a third lens having positive power and a convex shape on the image side; the rear lens system comprises, in order from the object side to the image side, a fourth lens having positive power and a convex shape facing the image side, a fifth lens having negative power, and a sixth lens; the fourth lens and the fifth lens are cemented lenses, the first lens and the third lens are glass lenses, The imaging lens system, wherein the second lens, the fourth lens, and the fifth lens are plastic lenses.

2. 2. The imaging lens system according to claim 1, wherein the following formula (1) is satisfied, where f2 is a focal length of the second lens and F is a focal length of the entire lens system: 2<|f2 / F| ...(1)

3. 2. The imaging lens system according to claim 1, wherein the following formula (2) is satisfied, where frp is a composite focal length of the plastic lenses in the rear lens group system, and F is a focal length of the entire lens system: 2<|frp / F| ...(2)

4. 2. The imaging lens system according to claim 1, wherein the following formula (3) is satisfied, where f2 is a focal length of the second lens and frp is a composite focal length of the plastic lenses in the rear lens group system: 0.5<|f2 / frp|<2...(3)

5. The imaging lens system of claim 1 , wherein the sixth lens is a glass lens.

6. The imaging lens system of claim 1 , wherein the sixth lens is a plastic lens.

7. An imaging lens system according to any one of claims 1 to 6; an imaging element disposed on the image side of the imaging lens system.

Images