JP7621798B2 - Imaging lens system and imaging device - Google Patents

Description

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.

Recently, there has been an increasing demand for in-vehicle front sensing devices to recognize distant objects (telephoto) and capture images of a wider range (wide-angle). To achieve this, it is possible to use a high-resolution sensor and a wide-angle imaging lens. This means that the lens system is wide-angle to view a wider range, and a high-resolution sensor is used to capture distant objects. As an example, Patent Document 1 describes an imaging device that uses a wide-angle lens system and a high-resolution imaging element to meet the demands of both telephoto and wide-angle.

International Publication No. 2013/018306

However, in Patent Document 1, it is not possible to achieve a higher resolution (i.e., a telephoto lens) than the resolution of the image sensor. In addition, there are cost and technical constraints on increasing the resolution of the image sensor.

The present invention was made in consideration of these problems, and aims to provide an imaging lens system and imaging device that can capture a wide range of images and can identify distant objects without relying on the resolution of the imaging element.

An imaging lens system of one embodiment includes, in order from the object side to the image side, a first lens group including a first lens having a concave surface facing the object side and having negative power, a second lens having positive power, and a second lens group including a third lens and subsequent lenses,
When the focal length of the first lens is f1 and the composite focal length of all the lenses other than the first lens is Fr, the following formula (1) is satisfied:
-6.0<f1/Fr<-1.5...(1)
When the distance on the optical axis between the image-side surface of the first lens and the object-side surface of the second lens is d12 and the focal length of the entire optical system is F, the following formula (2) is satisfied.
0.7<d12/F<1.5...(2)
By having the first lens have negative power and a concave surface on the object side, negative distortion is generated, and a wide-angle lens system can be realized. Furthermore, by satisfying the above expressions (1) and (2), negative distortion can be effectively generated in the first lens.

An imaging lens system according to another embodiment includes, in order from the object side to the image side, a first lens group including a first lens having a concave surface on the object side and negative power, a second lens having an aspheric shape on the object side and on the image side, and a third lens having positive power, and a second lens group including a fourth lens and subsequent lenses,
the second lens has an inflection point on at least one of an object side surface and an image side surface,
When the focal length of the first lens is f1 and the composite focal length of all the lenses other than the first lens is Fr, the following formula (1) is satisfied:
-6.0<f1/Fr<-1.5...(1)
When the distance on the optical axis between the image side surface of the first lens and the object side surface of the second lens is d12, the thickness on the optical axis of the second lens is d2, the distance on the optical axis between the image side surface of the second lens and the object side surface of the third lens is d23, the refractive index of the second lens is n2, D = d12 + d2/n2 + d23, and the focal length of the entire optical system is F, the following equation (4) is satisfied.
0.7<D/F<1.5...(4)
By having the first lens have negative power and a concave surface on the object side, negative distortion is generated, and a wide-angle imaging lens system can be realized. Furthermore, by satisfying the above expressions (1) and (4), negative distortion can be effectively generated in the first lens.

The present invention provides an imaging lens system and imaging device that can capture a wide range of images and can identify distant objects without relying on the resolution of the imaging element.

1 is a cross-sectional view showing the configurations and light rays of an imaging lens system and an imaging device according to a first embodiment. 1 is a cross-sectional view showing a configuration of an imaging lens system and an imaging device according to a first embodiment. 3A to 3C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 1. FIG. 11 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to a second embodiment. 11A to 11C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 2. FIG. 11 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to a third embodiment. 11A to 11C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 3. FIG. 11 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to a fourth embodiment. 11A to 11C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 4. FIG. 13 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to a fifth embodiment. 13A to 13C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 5. 11 is a cross-sectional view showing the configurations and light rays of an imaging lens system and an imaging device according to a second embodiment. FIG. 13 is a diagram illustrating a distance ET2. FIG. 13 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to Example 6. 13A to 13C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 6. FIG. 13 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to Example 7. 13A to 13C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 7. FIG. 13 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to Example 8. 13A to 13C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 8. FIG. 13 is a cross-sectional view showing the configuration of an imaging lens system and an imaging device according to Example 9. 13A to 13C are diagrams showing spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system of Example 9.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
First embodiment
(Imaging Lens System and Imaging Device)
1 is a cross-sectional view showing the configurations and light rays of an imaging lens system 11 and an imaging device 20 according to embodiment 1. The imaging device 20 includes the 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 image sensor 21 is an element that converts received light into an electric signal, and is, for example, a CCD image sensor or a CMOS image sensor. The image sensor 21 is disposed at the image forming position of the imaging lens system 11.
The imaging lens system 11 will now be described in detail.

As shown in FIG. 1, the imaging lens system 11 of the first embodiment comprises, in order from the object side to the image side, a first lens group G1 including a first lens L1 having a concave surface facing the object side and negative power, a second lens L2 having positive power, and a second lens group G2 including a third lens L3 and subsequent lenses.
Specifically, the first lens group G1 is composed of, in order from the object side to the image side, a first lens L1, which is a meniscus lens having negative power and a concave surface on the object side, and a second lens L2 having positive power and a convex surface on the object side. In addition, the second lens group G2 is composed of a third lens L3 having negative power and a concave surface on the image side, a fourth lens L4 having positive power and a convex surface on the object side and the image side, and a fifth lens L5 having negative power and an aspheric surface on the object side and the image side, sandwiching the aperture stop STOP. In addition, the imaging lens system 11 may include a wavelength filter glass 12 between the image side lens surface S11 of the fifth lens L5 and the imaging surface IMG, as shown in FIG. In addition, the third lens L3 and the fourth lens L4 form a cemented lens. In addition, a cover glass 22 is provided on the imaging element 21, and the imaging surface of the imaging lens system 11 is indicated by IMG. The first lens L1 and the second lens L2 are preferably glass lenses, and the third lens L3 to the fifth lens L5 are preferably plastic lenses.

The first lens L1 and second lens L2, which are arranged on the object side of the aperture STOP, constitute the first lens group G1, and the third lens L3, fourth lens L4, and fifth lens L5, which are arranged on 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 an aperture that determines the F-number (Fno) of the imaging lens system 11.

Because the first lens L1 has negative power and the object-side surface S1 of the first lens L1 is concave, 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 greatly changed, generating negative distortion (distortion aberration). As a result, the image shrinks towards the periphery, and the range detected by the image sensor 21 becomes wider, allowing a wide angle of view to be achieved, and a wide-angle imaging lens system 11 can be realized. In addition, because the second lens L2 has positive power, light-collecting performance can be ensured.

Furthermore, when the focal length of the first lens L1 is f1 and the combined focal length of the lenses L2, L3, L4, and L5 other than the first lens L1 is Fr, it is preferable to satisfy the following formula (1).
-6.0<f1/Fr<-1.5...(1)
The above formula (1) expresses the condition for effectively producing negative distortion with the first lens L1 as the ratio of the focal length of the first lens L1 to the composite focal length of the subsequent lenses. If the upper limit is exceeded, the negative power will be too strong, causing distortion to become too large and difficult to correct, and if the lower limit is exceeded, the generation of negative distortion will be insufficient, making it difficult to obtain a desired angle of view.
Therefore, it is possible to realize a wide-angle imaging lens system 11, and also a telephoto imaging lens system 11 without requiring a high-resolution imaging element. Therefore, it is possible to provide an imaging lens system and an imaging device that can recognize distant objects and capture images of a wide range without relying on the resolution of the imaging element.

Furthermore, when the distance on the optical axis Z between the image-side surface S2 of the first lens L1 and the object-side surface S3 of the second lens L2 is d12 and the overall focal length of the imaging lens system 11 is F, the following equation (2) is satisfied.
0.7<d12/F<1.5...(2)

If the distance d12 on the optical axis Z between the image-side surface S2 of the first lens L1 and the object-side surface S3 of the second lens L2 is too small, the incident position of the off-axis light beam relative to the axial light beam (a light beam that forms an image on the optical axis, also called the "center ray") on the object-side surface S1 of the first lens L1 is close, and the incident angle of the off-axis light beam becomes large, so that it is not possible to generate a large negative distortion. On the other hand, if d12 is relatively large, the incident position of the off-axis light beam relative to the axial light beam on the object-side surface S1 of the first lens can be moved away, and the incident angle of the off-axis light beam becomes smaller, so that it is possible to generate a large negative distortion. Therefore, it is preferable that d12/F is greater than 0.7. On the other hand, if d12/F is greater than 1.5, the lens diameter of the first lens L1 becomes too large, and it is not possible to make the imaging lens system 11 compact.

The half angle of view of the optical system is 40° or less, and preferably 30° or less. By making the half angle of view of the optical system 40° or less, the focal length of the imaging lens system can be increased. Here, the half angle of view of the optical system refers to the angle that a light ray that passes through the center of the pupil and reaches the diagonal length position (diagonal point) of the sensor makes with the optical axis on the object side.

In the first embodiment, the second lens group G2 is composed of three lenses, but is not limited to this and may be composed of any number of lenses.
In the present invention, the object of the invention is achieved by only the first lens L1 for obtaining the angle of view and the second lens L2 having a light collecting function, 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 there are. In other words, from the third lens L3 onwards, there is little occurrence of curvature of field, and since they are close to the pupil (aperture), it is sufficient to place importance on imaging performance such as spherical aberration, so any configuration is acceptable.

Furthermore, when the radius of curvature of the object-side surface S1 of the first lens L1 is R11, it is preferable that the following formula (3) be satisfied.
-2.5<R11/F<-1...(3)

Furthermore, when R11/F is smaller than -2.5, in other words, when the radius of curvature R11 of the object-side surface S1 of the first lens L1 is relatively small, the distortion aberration generated by the first lens L1 becomes small, making it difficult to ensure a sufficiently large angle of view. On the other hand, when R11/F is larger than -1, in other words, when the radius of curvature R11 of the object-side surface S1 of the first lens L1 is relatively large, the field curvature becomes too large, making it difficult to correct the imaging performance of the imaging lens system 11. Therefore, by satisfying the above formula (3), it is possible to ensure sufficient imaging performance while ensuring a sufficiently large angle of view.

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

Example 1
2 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 is composed 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 STOP, a fourth lens L4, a fifth lens L5, and an IR cut filter 12. The first lens L1 has a negative power, a concave surface on the object side, and a convex surface on the image side. The second lens L2 has a positive power, a convex aspheric surface on the object side, and an aspheric surface on the image side. The third lens L3 has a negative power, an aspheric surface on the object side, and a concave aspheric surface on the image side. The fourth lens L4 has a positive power, and a convex aspheric surface on the object side and the image side. The fifth lens L5 has a positive power, and an aspheric surface on the object side and the image side. The first lens L1 and the second lens L2 are glass lenses, and the third lens L3 to the fifth lens L5 are plastic lenses. The imaging lens system 11 also includes an IR cut filter 12, which is a filter for cutting light in the infrared region. Hereinafter, characteristic data of the imaging lens system 11 according to the first embodiment will be described.

Table 1 shows lens data of each lens surface of the imaging lens system 11 according to Example 1. In Table 1, the lens data includes 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. 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 8.269 (mm). The refractive index at the d-line and the Abbe number at the d-line shown in Table 1 are values when the environmental temperature t (°C), which is the temperature around the imaging lens system 11, is 25 (°C). In Table 1, the surfaces marked with an asterisk (*) are aspheric.

The aspheric shape adopted for the lens surfaces of the second lens L2, the third lens L3, the fourth lens surface L4, and the fifth lens L5 is expressed by the following formula (8), where the sag amount in the optical axis Z direction is Y(h), c is the inverse 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 conical coefficient, and A4, A6, A8, A10, A12, A14, and A16 are aspheric coefficients of the fourth, sixth, eighth, tenth, twelfth, fourteenth, and sixteenth orders, respectively. The meanings of the symbols and the formulas expressing the aspheric shapes are the same in the examples described later.

Table 2 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of Example 1. In Table 2, for example, "-6.88301E-06" means "-6.88301E×10 ^-6 ".

3 shows spherical aberration (longitudinal aberration), field curvature, and distortion in the imaging lens system 11 of Example 1. As shown in Fig. 3, the imaging lens system 11 of Example 1 has a half angle of view of 27.0° and an F-number of 1.8.
In the longitudinal aberration diagram of FIG. 3A, the horizontal axis indicates the position where the light ray intersects with the optical axis Z, and the vertical axis indicates the height at the pupil diameter.
In the field curvature diagram of Fig. 3B, the horizontal axis indicates the distance in the direction of the optical axis Z, and the vertical axis indicates the image height (angle of view). In the field curvature diagram of Fig. 3B, Sag indicates the field curvature on the sagittal plane, and Tan indicates the field curvature on the tangential plane.
In the distortion diagram of FIG. 3C, 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 aberration diagrams in FIG. 3B and FIG. 3C show the results of a simulation using a light beam with a wavelength of 555 nm.
FIG. 3 shows diagrams of spherical aberration (longitudinal aberration), field curvature, and distortion when the environmental temperature t (° C.) is 25 (° C.).

Example 2
4 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 similar to 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.

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 8.506 (mm).

Table 4 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the second embodiment.

Figure 5 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 2. The explanation of each aberration diagram shown in Figure 5 is the same as that in Figure 3, so the explanation will be omitted.

Example 3
6 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 similar to 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.

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 8.29 (mm).

Table 6 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the third embodiment.

Figure 7 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 3. The explanation of each aberration diagram shown in Figure 7 is the same as that in Figure 3, so the explanation will be omitted.

Example 4
8 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 similar to 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.

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 7.98 (mm).

Table 8 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the fourth embodiment.

Figure 9 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 4. The explanation of each aberration diagram shown in Figure 9 is the same as that in Figure 3, so the explanation will be omitted.

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

Table 9 shows lens data for each lens surface of the imaging lens system 11 according to Example 5. The items shown in Table 9 are the same as those in Table 1, and therefore the description thereof will be omitted. Here, the imaging lens system 11 according to Example 5 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 8.006 (mm).

Table 10 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the fifth embodiment.

Figure 11 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 5. The explanation of each aberration diagram shown in Figure 11 is the same as that in Figure 3, so the explanation will be omitted.

As shown in the longitudinal aberration diagrams of Figures 3A, 5A, 7A, 9A, and 11A, the imaging lens system 11 of the first to fifth embodiments satisfactorily corrects longitudinal aberrations at wavelengths of 486.1 nm, 587.6 nm, and 656.3 nm. Therefore, the imaging lens system 11 has high resolution.

In addition, as shown in the field curvature diagrams of Figures 3B, 5B, 7B, 9B, and 11B, the imaging lens system 11 of the first to fifth embodiments effectively corrects the field curvature. Therefore, the imaging lens system 11 has high resolution.

In addition, as shown in the distortion diagrams of Figures 3C, 5C, 7C, 9C, and 11C, the imaging lens system 11 of the first to fifth embodiments effectively corrects distortion. Therefore, the imaging lens system 11 has high resolution.

Table 11 also shows the focal length F (mm) of the entire imaging lens system 11 according to Examples 1 to 5, the focal lengths f1 to f5 (mm) of the first lens L1 to the fifth lens L5, the composite focal length Fr (mm) of all the lenses other than the first lens, the radius of curvature R11 (mm) of the object-side surface S1 of the first lens L1, the distance d12 (mm) on the optical axis Z between the image-side surface S2 of the first lens L1 and the object-side surface S3 of the second lens L2, the composite focal length f3/4 (mm) of the cemented lens consisting of the third lens L3 and the fourth lens L4, the optical length tol (mm) of the imaging lens system 11, the value of d12/F, the value of f1/Fr, and the value of R11/F. The values shown in Table 11 are values when the wavelength of the light is 555 nm and the environmental temperature t (°C) is 25 (°C).

In Examples 1 to 5, the first lens L1 has negative power and a concave surface on the object side, thereby realizing a wide-angle imaging lens system 11 with a half angle of view of 27.0°. Also, as shown in Table 11, in Examples 1 to 5, the value of f1/Fr satisfies the above formula (1). This makes it possible to effectively generate negative distortion in the first lens L1, thereby realizing a wide-angle imaging lens system 11.
In addition, in Examples 1 to 5, the half angle of view of the entire optical system is 40° or less, and therefore the focal length F of the imaging lens system 11 according to Examples 1 to 5 is 7.98 to 8.51, which makes it possible to realize a telephoto imaging lens system 11. In other words, it is possible to realize a telephoto imaging lens system 11 without relying on the resolution of the image sensor 21.

Furthermore, as shown in Table 11, in Examples 1 to 5, the value of d12/F satisfies the above formula (2). Therefore, since d12/F is greater than 0.7, a sufficiently large negative distortion (distortion aberration) can be generated by the first lens L1. This allows the imaging lens system 11 to have a wide angle. In fact, the half angle of view of the imaging lens system 11 in Examples 1 to 5 is 27.0°, which is sufficiently large. On the other hand, since d12/F is smaller than 1.5, an increase in the lens diameter of the first lens L1 can be prevented, and the imaging lens system 11 can be made compact.
In addition, the second lens L2 has a positive power, and the second lens group G2 has a positive power, thereby ensuring the light-collecting performance of the imaging lens system 11. For example, as shown in Figures 3, 5, 7, 9, and 11, in Examples 1 to 5, the longitudinal aberration, the field curvature, and the distortion aberration have good values. Therefore, in Examples 1 to 5, the telephoto of the imaging lens system 11 can be realized.

Also, as shown in Table 11, in Examples 1 to 5, the value of R11/F satisfies the above formula (3). Because R11/F is greater than -2.5, the radius of curvature R11 of the object-side surface S1 of the first lens L1 can be made sufficiently large, and the distortion aberration generated by the first lens L1 can be made sufficiently large. Therefore, the angle of view of the imaging lens system 11 can be made sufficiently large. On the other hand, because R11/F is smaller than -1, the radius of curvature R11 of the object-side surface S1 of the first lens L1 can be prevented from becoming too large, and an increase in field curvature can be prevented. Therefore, it is easier to correct the imaging performance of the imaging lens system 11.

Embodiment 2
(Imaging Lens System and Imaging Device)
12 is a cross-sectional view showing the configurations and light rays of an imaging lens system 11 and an imaging device 20 according to embodiment 2. 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).

12, the imaging lens system 11 of the second embodiment has a configuration in which a correction lens is inserted between the first lens L1 and the second lens L2 in the imaging lens system 11 of the first embodiment. That is, from the object side to the image side, the imaging lens system 11 is made up of a first lens group G1 including a first lens L1 having a concave surface facing the object side and having negative power, a second lens L2 serving as a correction lens, and a third lens L3 having positive power, and a second lens group G2 including a fourth lens L4 and subsequent lenses.
Specifically, the first lens group G1 is composed of, in order from the object side to the image side, a first lens L1 which is a meniscus lens having negative power and a concave surface on the object side, a second lens L2 which is a correction lens having aspheric surfaces on the object side and the image side, and a third lens L3 which has positive power and a convex surface on the object side. In addition, the second lens group G2 is composed of 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 a convex surface on the object side and the image side, and a sixth lens L6 which has negative power and has aspheric surfaces on the object side and the image side, sandwiching the aperture stop STOP. In addition, 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 the image forming surface IMG, as shown in FIG. In addition, the fourth lens L4 and the fifth lens L5 form a cemented lens. In addition, there is a cover glass 22 on the imaging element 21, and the image forming surface 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 second lens L2 as a correction lens preferably has at least one inflection point on either the object side or the image side, which makes it possible to correct the field curvature from the center to the periphery, among the various aberrations caused by the negative distortion produced by the first lens L1.

The first lens L1, second lens L2, and third lens L3, which are arranged on the object side of the aperture STOP, constitute the first lens group G1, and the fourth lens L4, fifth lens L5, and sixth lens L6, which are arranged on 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 an aperture that determines the F-number (Fno) of the imaging lens system 11.

Because the first lens L1 has negative power and the object-side surface S1 of the first lens L1 is concave, the angle of incidence of off-axis light rays incident on the first lens L1 can be significantly changed, generating negative distortion. This causes the image to shrink toward the periphery, widening the range detected by the image sensor 21, realizing a wide angle of view and realizing a wide-angle imaging lens system 11. In addition, because the second lens L2 has positive power, light-collecting performance can be ensured.

Furthermore, when the focal length of the first lens L1 is f1 and the combined focal length of the lenses L2, L3, L4, L5, and L6 other than the first lens L1 is Fr, it is preferable to satisfy the following formula (1).
-6.0<f1/Fr<-1.5...(1)
The above formula (1) expresses the condition for effectively producing negative distortion with the first lens L1 as the ratio of the focal length of the first lens L1 to the composite focal length of the subsequent lenses. If the upper limit is exceeded, the negative power will be too strong, causing distortion to become too large and difficult to correct, and if the lower limit is exceeded, the generation of negative distortion will be insufficient, making it difficult to obtain a desired angle of view.
Therefore, it is possible to realize a wide-angle imaging lens system 11, and also a telephoto imaging lens system 11 without requiring a high-resolution imaging element. Therefore, it is possible to provide an imaging lens system and an imaging device that can recognize distant objects and capture images of a wide range without relying on the resolution of the imaging element.

Furthermore, when the distance on the optical axis Z between the image side surface S2 of the first lens L1 and the object side surface S3 of the second lens L2 is d12, the thickness on the optical axis Z of the second lens L2 is d2, the distance on the optical axis Z between the image side surface S4 of the second lens L2 and the object side surface S5 of the third lens L3 is d23, the refractive index of the second lens L2 is n2, D = d12 + d2/n2 + d23, and the overall focal length of the imaging lens system 11 is F, the following equation (4) is satisfied.
0.7<D/F<1.5...(4)

If the distance D on the optical axis Z between the image-side surface S2 of the first lens L1 and the object-side surface S5 of the third lens L3 is too small, the incident position of the off-axis light beam relative to the on-axis light beam on the object-side surface S1 of the first lens L1 is close, and the incident angle of the off-axis light beam becomes large, so that it is not possible to generate a large negative distortion. On the other hand, if the distance D is relatively large, the incident position of the off-axis light beam relative to the on-axis light beam on the object-side surface S1 of the first lens L1 can be moved away, and the incident angle of the off-axis light beam becomes smaller, so that it is possible to generate a large negative distortion. Therefore, it is preferable that D/F is greater than 0.7. On the other hand, if D/F is greater than 1.5, the lens diameter of the first lens L1 becomes too large, and it is not possible to make the imaging lens system 11 compact.

The half angle of view of the optical system is 40° or less, and preferably 30° or less. By making the half angle of view of the optical system 40° or less, the focal length of the imaging lens system can be increased. Here, the half angle of view of the optical system refers to the angle that a light ray that passes through the center of the pupil and reaches the diagonal length position (diagonal point) of the sensor makes with the optical axis on the object side.

In the second embodiment, the second lens group G2 is composed of three lenses, but is not limited to this and may be composed of any number of lenses.
In the present invention, the object of the invention is achieved by only the first lens L1 for obtaining the angle of view and the third lens L3 having a light collecting function, and the lenses located on the image side of these lenses are merely configured to realize imaging, so it does not matter how many lenses there are. In other words, from the fourth lens L4 onwards, there is little occurrence of curvature of field, and since they are close to the pupil (aperture), it is sufficient to place importance on imaging performance such as spherical aberration, so any configuration is acceptable.

Furthermore, when the radius of curvature of the object-side surface S1 of the first lens L1 is R11, it is preferable that the following formula (3) be satisfied.
-2.5<R11/F<-1...(3)

When R11/F is smaller than -2.5, in other words, when the radius of curvature R11 of the object-side surface S1 of the first lens L1 is relatively small, the distortion generated by the first lens L1 becomes small, making it difficult to ensure a sufficiently large angle of view. On the other hand, when R11/F is larger than -1, in other words, when the radius of curvature R11 of the object-side surface S1 of the first lens L1 is relatively large, the field curvature becomes too large, making it difficult to correct the imaging performance of the imaging lens system 11. Therefore, by satisfying the above formula (3), it is possible to ensure sufficient imaging performance while ensuring a sufficiently large angle of view.

In addition, it is preferable that the following formula (5) is satisfied.
0.05<d12/d23<1.0...(5)

When d12/d23 is smaller than 1.0, the second lens L2 is disposed closer to the first lens L1 than the third lens L3, and the overlap of the incident rays on the surface of the second lens L2 is reduced, making it possible to correct each incident ray separately, and thus allowing for effective aberration correction. It is also preferable that d12/d23 is greater than 0.05. If d12/d23 is smaller than 0.05, i.e., 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.

Furthermore, when the focal length of the second lens L2 is f2, it is preferable that the following formula (6) be satisfied.
-0.1<F/f2<0.1...(6)
By satisfying this condition, the power of the second lens L2 can be limited, and the image plane can be efficiently corrected as a correction lens.

Furthermore, as shown in FIG. 13, 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 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 defined as ET2, it is preferable to satisfy the following formula (7):
0.9<ET2/d2<1.1 (7)

It is preferable that at least one of the above formulas (6) and (7) is satisfied. In other words, it is preferable that the lens power of the second lens L2 is weaker than the other lenses in the imaging lens system 11. This makes it possible for the second lens L2 to suitably correct various aberrations caused by the negative distortion produced by the first lens L1.

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

Example 6
14 is a cross-sectional view showing an imaging lens system 11 according to Example 6. Specifically, the imaging lens system 11 according to Example 1 is composed 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 STOP, a fourth lens L4, a fifth lens L5, and an IR cut filter 12. The first lens L1 has a negative power, a concave surface on the object side, and a convex surface on the image side. The second lens has a positive power, and an aspheric surface with an inflection point on the object side and the image side. The third lens L3 has a positive power, a convex aspheric surface on the object side, and an aspheric surface on the image side. The fourth lens L4 has a negative power, an aspheric surface on the object side, and a concave aspheric surface on the image side. The fifth lens L5 has a positive power, and a convex aspheric surface on the object side and the image side. The sixth lens L6 has a positive power, and an aspheric 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 6 will be described below.

Table 12 shows lens data for each lens surface of the imaging lens system 11 according to Example 6. The items shown in Table 12 are the same as those in Table 1, and therefore their explanation will be omitted. Here, the imaging lens system 11 according to Example 6 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 13 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the sixth embodiment.

Figure 15 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 6. The explanation of each aberration diagram shown in Figure 15 is the same as that in Figure 3, so the explanation will be omitted.

(Example 7)
16 is a cross-sectional view showing an imaging lens system 11 according to Example 7. The configuration of the imaging lens system 11 according to Example 7 is similar to that of Example 6, and therefore a description thereof will be omitted. Below, characteristic data of the imaging lens system 11 according to Example 7 will be described.

Table 14 shows lens data for each lens surface of the imaging lens system 11 according to Example 7. The items shown in Table 14 are the same as those in Table 1, and therefore their explanation will be omitted. Here, the imaging lens system 11 according to Example 7 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 15 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the seventh embodiment.

Figure 17 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 7. The explanation of each aberration diagram shown in Figure 17 is the same as that in Figure 3, so the explanation will be omitted.

(Example 8)
18 is a cross-sectional view showing an imaging lens system 11 according to Example 8. The configuration of the imaging lens system 11 according to Example 8 is similar to that of Example 6, and therefore a description thereof will be omitted. Below, characteristic data of the imaging lens system 11 according to Example 8 will be described.

Table 16 shows lens data for each lens surface of the imaging lens system 11 according to Example 8. The items shown in Table 16 are the same as those in Table 1, and therefore their explanations are omitted. Here, the imaging lens system 11 according to Example 8 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 17 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the eighth embodiment.

Figure 19 shows the spherical aberration diagram (longitudinal aberration diagram), the field curvature diagram, and the distortion aberration diagram for the imaging lens system 11 of Example 8. The explanation of each aberration diagram shown in Figure 19 is the same as that in Figure 3, so the explanation will be omitted.

(Example 9)
20 is a cross-sectional view showing an imaging lens system 11 according to Example 9. The configuration of the imaging lens system 11 according to Example 9 is similar to that of Example 6, and therefore a description thereof will be omitted. Below, characteristic data of the imaging lens system 11 according to Example 9 will be described.

Table 18 shows lens data for each lens surface of the imaging lens system 11 according to Example 9. The items shown in Table 18 are the same as those in Table 1, and therefore their explanation will be omitted. Here, the imaging lens system 11 according to Example 9 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 19 shows aspheric coefficients for defining the aspheric shape of the lens surface that is made aspheric in the imaging lens system 11 of the ninth embodiment.

Figure 21 shows the spherical aberration (longitudinal aberration), field curvature, and distortion diagrams for the imaging lens system 11 of Example 9. The explanation of each aberration diagram shown in Figure 21 is the same as that in Figure 3, so the explanation will be omitted.

As shown in the longitudinal aberration diagrams of Figures 15A, 17A, 19A, and 21A, the imaging lens system 11 of Examples 6 to 9 effectively corrects longitudinal aberrations at wavelengths of 486.1 nm, 587.6 nm, and 656.3 nm. Therefore, the imaging lens system 11 has high resolution.

In addition, as shown in the field curvature diagrams of Figures 15B, 17B, 19B, and 21B, the imaging lens system 11 of Examples 6 to 9 corrects the field curvature well. Therefore, the imaging lens system 11 has high resolution.

In addition, as shown in the distortion diagrams of Figures 15C, 17C, 19C, and 21C, the imaging lens system 11 of Examples 6 to 9 corrects distortion well. Therefore, the imaging lens system 11 has high resolution.

Table 20 also lists the focal length F (mm) of the entire imaging lens system 11 according to Examples 6 to 9, the focal lengths f1 to f6 (mm) of the first lens L1 to the sixth lens L6, the composite focal length Fr (mm) of all the lenses other than the first lens, the radius of curvature R11 (mm) of the object-side surface S1 of the first lens L1, the distance d12 (mm) on the optical axis Z between the image-side surface S2 of the first lens L1 and the object-side surface S3 of the second lens L2, the thickness d2 (mm) on the optical axis Z of the second lens L2, and the optical axis distance between the image-side surface S4 of the second lens L2 and the object-side surface S5 of the third lens L3. The distance d23 (mm) on Z, the distance D (mm) on the optical axis Z between the image side surface S2 of the first lens L1 and the object side surface S5 of the third lens L3, the distance ET2 (mm) parallel to the optical axis Z between the intersection of the light beam passing through the outermost diameter side of the object side surface S3 of the second lens L2 and the image side surface S4, 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 value of D/F, the value of f1/Fr, the value of R11/F, the value of d12/d23, the value of F/f2, and the value of ET2/d2. The values shown in Table 20 are values when the wavelength of the light beam is 555 nm and the environmental temperature t (°C) is 25 (°C).

In Examples 6 to 9, the first lens L1 has negative power and a concave surface on the object side, thereby realizing a wide-angle imaging lens system 11 with a half angle of view of 27.0°. Also, as shown in Table 20, in Examples 6 to 9, the value of f1/Fr satisfies the above formula (1). This makes it possible to effectively generate negative distortion in the first lens L1, thereby realizing a wide-angle imaging lens system 11.
In addition, in Examples 6 to 9, the half angle of view of the entire optical system is 40° or less, so the focal length F of the imaging lens system 11 according to Examples 6 to 9 is 8.55 to 8.93, and it is possible to realize a telephoto imaging lens system 11. In other words, it is possible to realize a telephoto imaging lens system 11 without relying on the resolution of the image sensor 21. In addition, in Examples 6 to 9, the first lens L1 has negative power and has a concave surface on the object side, so it is possible to realize a wide-angle imaging lens system 11 with a half angle of view of 27.0°.
In addition, in Examples 6 to 9, the second lens L2 as a correction lens has one inflection point on the object side and the image side surface. This makes it possible to correct the field curvature, which is one of the various aberrations caused by the negative distortion caused by the first lens L1, from the center to the periphery. For example, as shown in Figures 15B, 17B, 19B, and 21B, in Examples 6 to 9, the field curvature has a good value.

As shown in Table 20, in Examples 6 to 9, the value of D/F satisfies the above formula (4). Therefore, since D/F is greater than 0.7, a sufficiently large negative distortion (distortion aberration) can be generated by the first lens L1. This allows the imaging lens system 11 to have a wide angle. In fact, the half angle of view of the imaging lens system 11 in Examples 6 to 9 is 27.0, which is sufficiently large. On the other hand, since D/F is smaller than 1.5, an increase in the lens diameter of the first lens L1 can be prevented, and the imaging lens system 11 can be made compact.
In addition, the third lens L3 has a positive power, and the second lens group G2 has a positive power, thereby ensuring the light-collecting performance of the imaging lens system 11. For example, as shown in Figures 15, 17, 19, 21, and 23, in Examples 6 to 9, the longitudinal aberration, the field curvature, and the distortion aberration have good values. Therefore, in Examples 6 to 9, the telephoto mode of the imaging lens system 11 can be realized.

Also, as shown in Table 20, in Examples 6 to 9, the value of R11/F satisfies the above formula (3). Because R11/F is greater than -2.5, the radius of curvature R11 of the object-side surface S1 of the first lens L1 can be made sufficiently large, and the distortion aberration generated by the first lens L1 can be made sufficiently large. Therefore, the angle of view of the imaging lens system 11 can be made sufficiently large. On the other hand, because R11/F is smaller than -1, the radius of curvature R11 of the object-side surface S1 of the first lens L1 can be prevented from becoming too large, and an increase in field curvature can be prevented. Therefore, it is easier to correct the imaging performance of the imaging lens system 11.

Furthermore, as shown in Table 20, in Examples 6 to 9, the value of d12/d23 satisfies the above formula (5). Therefore, when d12/d23 is smaller than 1.0, aberration correction can be performed effectively. On the other hand, when d12/d23 is larger than 0.05, interference between the first lens L1 and the second lens L2 can be prevented.

Also, as shown in Table 20, in Examples 6 to 9, the values of F/f2 satisfy the above formula (6), and the values of ET2/d2 satisfy the above formula (7). Therefore, the lens power of the second lens L2 is weaker than the other lenses in the imaging lens system 11, and the second lens L2 can effectively correct various aberrations caused by the negative distortion produced by the first lens L1.

The present invention is not limited to the above-mentioned embodiments, and can be modified as appropriate without departing from the spirit of the invention. For example, the use of the imaging lens system of the present invention is not limited to vehicle-mounted cameras and surveillance cameras, and it may also be used for other purposes, such as being mounted on small electronic devices such as mobile phones.

11 imaging lens system 12 glass (IR cut filter)
20 Imaging device 21 Imaging element 22 Cover glass L1 First lens L2 Second lens L3 Third lens L4 Fourth lens L5 Fifth lens L6 Sixth lens G1 First lens group G2 Second lens group STOP Aperture IMG Imaging surface

Claims (4)

a first lens group including, in order from the object side to the image side, a first lens having a concave surface facing the object side and having negative power, and a second lens having positive power; and a second lens group including, in order from the object side to the image side, a third lens having negative power , a fourth lens having positive power, and a fifth lens having negative power .
When the focal length of the first lens is f1 and the composite focal length of all the lenses other than the first lens is Fr, the following formula (1) is satisfied:
-6.0<f1/Fr<-1.5...(1)
An imaging lens system which satisfies the following formula (2), where d12 is a distance on the optical axis between an image-side surface of the first lens and an object-side surface of the second lens, and F is a focal length of the entire optical system.
0.7<d12/F<1.5...(2) The imaging lens system according to claim 1 , wherein the following formula (3) is satisfied when the radius of curvature of the object side of the first lens is R11:
-2.5<R11/F<-1...(3) The imaging lens system according to claim 1, wherein the half angle of view of the entire optical system is 40° or less. An imaging lens system according to any one of claims 1 to 3 ;
an imaging element disposed at a focal position of the imaging lens system.

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