JP7621799B2 - 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.

Conventionally, in order to correct field curvature, a lens having an inflection point on at least one of the object-side and image-side surfaces has been placed in a position closest to the image side in an imaging lens system. For example, Patent Documents 1 and 2 describe an imaging lens system that is made up of a first lens to a sixth lens arranged in order from the object side to the image side, and uses a lens having an inflection point on the object-side and image-side surfaces as the sixth lens.

JP 2020-027256 A JP 2019-032462 A

However, Patent Documents 1 and 2 assume that an imaging lens system to be mounted on a mobile phone or the like has a short back focus. Therefore, the angle of incidence of the chief ray incident on the imaging sensor becomes large, and each chief ray is separated, so that the field curvature can be suitably corrected by the aspherical lens arranged closest to the image side of the imaging lens system. However, an imaging lens system for vehicle use has a relatively long back focus, and even if an aspherical lens having an inflection point is arranged closest to the image side of the imaging lens system, it is difficult to suitably correct the field curvature.

The present invention has been made in consideration of these problems, and aims to provide an imaging lens system and an imaging device that can effectively correct field curvature in an imaging lens system with a relatively long back focus.

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

The present invention provides an imaging lens system and an imaging device that can effectively correct field curvature using a correction lens arranged in the front group in an imaging lens system with a relatively long back focus.

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. FIG. 13 is a diagram illustrating a distance ET2. 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.

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, an aperture STOP, and a second lens group G2. The first lens group G1 comprises, in order from the object side to the image side, one or two lenses having negative power, and a correction lens having an aspheric shape on the object side and the image side. The second lens group G2 comprises at least two lenses, and the aperture STOP is located on the image side of the lens in the second lens group G2 that is closest to the object side.
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 which has aspheric surfaces on the object side and the image side and functions as a correction lens. The second lens group G2 is composed 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 a convex surface on the object side and the image side, and a sixth lens L6 which has negative power and an aspheric surface on the object side and the image side. In addition, as shown in FIG. 1, the imaging lens system 11 may include a wavelength filter glass 12 between the image side lens surface S13 of the sixth lens L6 and the imaging surface IMG. In addition, the fourth lens L4 and the fifth lens L5 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 to the third lens L3 are preferably glass lenses, and the fourth lens L4 to the sixth lens L6 are preferably plastic lenses. The first lens L1 constituting the first group may be replaced with two lenses having negative power by 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, including the two lenses having the divided negative power and the second lens L2.

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

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

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

In other words, it is preferable that the lens power of the second lens L2 is weaker than that of the other lenses in the imaging lens system 11. In the imaging lens system 11 having a relatively long back focus, the respective chief rays are separated on the object side of the imaging lens system 11, and by arranging such a second lens L2 in the first lens group G1, it is possible to suitably correct the field curvature. Specifically, by arranging such a second lens L2 in the first lens group G1, the incidence angle of the chief rays incident on the second lens L2 becomes large, and the respective chief rays are separated, so that it is possible to suitably correct the field curvature by the second lens L2.
That is, in the imaging lens system 11 having a relatively long back focus, the curvature of field can be suitably corrected.
In the first embodiment, the second lens group G2 is composed of four lenses, but it may be composed of any number of lenses. If BF/L>0.1 and the back focus is long, the overlap of various light rays is reduced in the lens on the object side of the optical system, and the effect of easily correcting the curvature of field for each light ray is obtained, and this effect is not affected by the lens configuration of the second lens group G2. In addition, the third lens L3 and subsequent lenses are close to the pupil (aperture), and it is sufficient to focus on the imaging performance such as spherical aberration. In addition, the focal length of the lenses subsequent to the third lens L3 is long, so the occurrence of curvature of field is small.

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

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

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

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

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. Therefore, for the same reason as above, the second lens L2 can effectively correct the field curvature.

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

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

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

Example 1
3 is a cross-sectional view showing an imaging lens system 11 according to Example 1. Specifically, the imaging lens system 11 according to Example 1 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 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 aspheric surface with an inflection point on the object side and the image side. The third lens L3 has positive power, a convex aspheric surface on the object side, and an aspheric surface on the image side. The fourth lens L4 has negative power, an aspheric surface on the object side, and a concave aspheric surface on the image side. The fifth lens L5 has positive power, and a convex aspheric surface on the object side and the image side. The sixth lens L6 has 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 is a filter for cutting light in the infrared region. The characteristic data of the imaging lens system 11 according to Example 1 will be described below.

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 8.623 (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 "*" are aspheric. 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 position of the diagonal length of the sensor (diagonal point) makes with the optical axis on the object side.

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 and the sixth lens L6 is expressed by the following formula (6), where the sag amount in the optical axis 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 aspheric lens surface in the imaging lens system 11 of Example 1. In Table 2, for example, "-2.33239E-03" means "-2.33239E×10 ^-3 ".

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

Example 2
5 is a cross-sectional view showing an imaging lens system 11 according to Example 2. The configuration of the imaging lens system 11 according to Example 2 is 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 8.923 (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 6 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 6 is the same as that in Figure 4, so the explanation will be omitted.

Example 3
7 is a cross-sectional view showing an imaging lens system 11 according to Example 3. The configuration of the imaging lens system 11 according to Example 3 is 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 8.59 (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 8 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 8 is the same as that in Figure 4, so the explanation will be omitted.

Example 4
9 is a cross-sectional view showing an imaging lens system 11 according to Example 4. The configuration of the imaging lens system 11 according to Example 4 is 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 8.623 (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 10 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 10 is the same as that in Figure 4, so the explanation will be omitted.

As shown in the longitudinal aberration diagrams of Figures 4A, 6A, 8A, and 10A, the imaging lens system 11 of the first to fourth 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 4B, 6B, 8B, and 10B, the imaging lens system 11 of the first to fourth 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 4C, 6C, 8C, and 10C, the imaging lens system 11 of the first to fourth embodiments effectively corrects distortion. Therefore, the imaging lens system 11 has high resolution.

Table 9 also lists the focal length F (mm) of the entire imaging lens system 11 according to Examples 1 to 4, the focal lengths f1 to f6 (mm) of the first 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 intersection of a ray passing through the outermost diameter side of the object-side surface S3 of the second lens L2 and the image-side surface S4, the thickness d2 (mm) on the optical axis Z of the second lens L2, 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 thickness t2 (mm) of the second lens L2. The distance df (mm) on 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) on the optical axis Z from the image side surface S4 of the second lens L2 to the object side surface S5 of the third lens L3 adjacent to the image side of the second lens L2, the composite focal length f4/5 (mm) of the cemented lens consisting of the fourth lens L4 and the fifth lens L5, the optical length tol (mm) of the imaging lens system 11, the value of BF/L, the value of ET2/d2, the value of F/f2, the value of D24/L, and the value of df/dr are shown. The values shown in Table 9 are values when the wavelength of the light is 555 nm and the environmental temperature t (°C) is 25 (°C). Note that in Examples 1 to 4, the first lens L1 constituting the first group 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, namely, the two lenses each having the divided negative power and the second lens L2.

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

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

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

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

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

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

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 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 (5)

The lens comprises, in order from the object side to the image side, a first lens group and a second lens group,
the first lens group comprises, in order from the object side to the image side, one or two lenses having negative power and a correction lens having an aspheric shape on the object side and the image side;
the second lens group includes, in order from the object side to the image side, a third lens having positive power, a stop, a fourth lens having negative power, a fifth lens having positive power, and a sixth lens having negative power;
When a total optical length that is the distance on the optical axis from the object-side surface of the lens located closest to the object in the first lens group to the image-side surface of the lens located closest to the image in the second lens group is denoted by L, and a back focus that is the distance on the optical axis from the image-side surface of the lens located closest to the image in the second lens group to the object-side surface of an image sensor is denoted by BF, the following formula (1) is satisfied:
BF/L>0.1...(1)
When the distance parallel to the optical axis between the intersection point of a light ray passing through the outermost diameter side of the object-side surface of the correction lens and the object-side surface of the correction lens and the intersection point of a light ray passing through the outermost diameter side of the object-side surface of the correction lens and the image-side surface is defined as ET2, and the thickness on the optical axis of the correction lens is defined as d2, the following formula (2) is satisfied:
0.9<ET2/d2<1.1...(2)
When a distance on the optical axis from an image-side surface of a lens located closest to the object in the first lens group to an object-side surface of a lens located closest to the object in the second lens group is D24, the following formula (3) is satisfied:
0.3<D24/L<0.5...(3)
An imaging lens system which satisfies the following formula (4), where df is the distance on the optical axis from the image-side surface of the lens adjacent to the object side of the correction lens to the object-side surface of the correction lens, and dr is the distance on the optical axis from the image-side surface of the correction lens to the object-side surface of the lens adjacent to the image side of the correction lens.
0.05<df/dr<1.0...(4) 2. The imaging lens system according to claim 1, wherein the following formula (5) is satisfied, where F is a focal length of the entire optical system, and f2 is a focal length of the correction lens:
-0.1<F/f2<0.1...(5) The imaging lens system according to claim 1 or 2, wherein the correction lens has at least one inflection point on at least one of the object side and image side surfaces. The imaging lens system according to claim 3, wherein the correction lens is convex toward the object side in a range from a center position intersecting the optical axis to a predetermined position, and is convex toward the image side in a range from the predetermined position to an outer edge of the correction lens. An imaging lens system according to any one of claims 1 to 4;
an imaging element disposed at a focal position of the imaging lens system.

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