JP2000056217A - Wide angle lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wide angle lens, which shortens back focus and includes a wide angle area, suitable for a miniaturized high-performance camera by making the various parameters of a lens system satisfy a specified conditional expression. SOLUTION: A first lens group G1 is provided with a positive lens L11 turning a sharp convex face toward the side of an object and a negative meniscus lens L12 (L13) arranged on the image side of that lens while turning a sharp convex face toward the side of the object. A second lens group G2 is provided with paired achromatic lenses L22 and L33 having a positive focal distance, and that pair is composed of joint or separate lenses composed of negative lens L22 and positive lens L23 successively from the side of the object. A third lens group G3 is provided with a negative lens L31 turning a concave face toward the object side and a positive lens L32 (L33) arranged on the image side of that lens. Such a lens system satisfies the conditions of Bf/f<1.45, Bf/y<2.5 and 9.5>TL/f>5. In the expressions, Bf is the back focus of the entire system, (f) is the focal distance of the entire system, TL is a distance from the most object side face of the wide angle lens to the image surface and (y) is a peak image height.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus wide-angle lens suitable for a small camera using a solid-state image pickup device such as a CCD.

[0002]

2. Description of the Related Art In recent years, a wide-angle lens has been required in a camera using a solid-state imaging device such as a CCD. In particular, in recent years, in order to increase the resolution, for example, a CCD having 1.3-1.6 million pixels in 1/3 inch has been used.

[0003]

However, when using such a small CCD having a large number of pixels,
Higher performance is also required of the taking lens, and the configuration of the optical system must be complicated. In particular, in the case of the retrofocus type, since it is necessary to insert filters such as an optical low-pass filter and an infrared cut filter between the lens and the CCD, it is necessary to secure a long back focus. At this time, the negative power of the front group of the optical system must be increased, and the asymmetry of the power with respect to the stop in the optical system is increased.

In such a lens system having a strong asymmetry with respect to the diaphragm, it is very difficult to correct distortion, as well as field curvature, astigmatism, asymmetric coma, and chromatic aberration of magnification. This is not preferable for achieving lens performance. Conventionally, as an optical low-pass filter for preventing color moiré when using an imaging device such as a CCD, a filter made of crystal having a large thickness has been used.
An optical low-pass filter comprising lithium niobate has been proposed. The optical low-pass filter made of lithium niobate can fulfill its function with a thickness of about 1/6 to 1/7 compared with that made of quartz.

[0005] The inventor of the present application has made it possible to further improve the optical system if the camera system can be constructed even if the back focus is shorter than in the past, or to achieve the same level of performance as in the past. It has been found that further miniaturization can be achieved while securing. An object of the present invention is to achieve a wide-angle lens having a short back focus and suitable for a high-performance and small-sized digital still camera including a wide-angle region.

[0006]

In order to achieve the above-mentioned object, a wide-angle lens according to the present invention comprises:
A wide-angle lens system including a first negative lens unit, a second positive lens unit, and a third positive lens unit;
The lens group includes a positive lens having a strong convex surface facing the object side, and a negative meniscus lens disposed on the image side of the positive lens in the first lens group and having a strong convex surface facing the object side. The two lens groups include an achromatic lens pair having a positive focal length. The third lens group includes a negative lens having a concave surface facing the object side and an image side of the negative lens in the third lens group. The achromatic lens pair in the second lens group includes, in order from the object side, a cemented lens or a separation lens composed of a negative lens and a positive lens, and satisfies the following conditional expression: Things. (1) Bf / f <1.45 (2) Bf / y <2.5 (3) 9.5> TL / f> 5 where Bf: back focus of the whole system f: focal length of the whole system TL: Distance from the most object side surface of the wide-angle lens to the image plane y: The maximum image height.

[0007] Based on the above configuration, the wide-angle lens according to the present invention preferably satisfies the following conditional expression. (4) 0.6 <| f1 / f | <1.6, where f1: the focal length of the first lens group.

The first lens group includes, in order from the object side, a positive lens having a strong convex surface facing the object side, a negative meniscus lens having a strong concave surface facing the image side, and a negative lens having a strong concave surface facing the image side. It is preferable that the zoom lens is composed of a lens and satisfies the following conditional expression. (5) 0.9 <q11 <3.5 where q11 is a shape factor of a positive lens whose strong convex surface faces the object side. The shape factor q11 is
It is expressed by the following equation. q11 = (r2 + r1) / (r2-r1) where r1 = radius of curvature of the object side surface of the positive lens r2 = curvature radius of the image side surface of the positive lens.

Preferably, the second lens group includes, in order from the object side, a positive lens, a stop, a negative lens, and a biconvex positive lens, and satisfies the following conditional expression. (6) 1.1 <f2 / f <1.8 (7) 1.2 <f2c / f2 <2.0 where f2: focal length of the second lens group f2C: from the aperture in the second lens group This is the composite focal length of the lens arranged on the image side.

It is preferable that the positive lens disposed closest to the image in the third lens group is a positive lens having a convex surface having a higher curvature directed to the object side and satisfies the following conditional expression. (8) 0.8 <q33 <4.5 where q33 is the shape factor of the positive lens. The shape factor q33 is represented by the following equation. q33 = (r2 + r1) / (r2-r1) where r1 = curvature radius of the object-side surface of the positive lens r2 = curvature radius of the image-side surface of the positive lens.

The negative lens closest to the object side in the third lens group is preferably a negative lens having a strong concave surface facing the object side, and preferably satisfies the following conditional expression. (9) 0.6 <q31 <1.5, where q31 is the shape factor of the negative lens. The shape factor q31 is represented by the following equation. q31 = (r2 + r1) / (r2-r1) where r1 = the radius of curvature of the object-side surface of the negative lens, and r2 = the radius of curvature of the image-side surface of the negative lens.

It is preferable that the following conditional expression is satisfied. (10) −2.0 <f3 / f33I <0.1 (11) 0.2 <f31 / f3 <0.5, where f33I: the positive lens located closest to the image side in the third lens group F31: focal length of the negative lens of the third lens group which is disposed closest to the object side.

Preferably, the inclination of the principal ray corresponding to the maximum image height emitted from the most image side surface of the wide-angle lens system with respect to the optical axis satisfies the following conditional expression. (12) −0.1 <m <0.1, where m is the direction cosine of the inclination of the principal ray.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been made based on the discovery that there is a trade-off between securing a back focus and improving the performance and miniaturization of an optical system in the design of a wide-angle lens system. The present inventors have found that the configuration described below can solve various problems unique to the retrofocus type and achieve a high-performance and small-sized wide-angle lens.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a lens cross-sectional view of a wide-angle lens system according to an embodiment of the present invention. In the following, the embodiment shown in FIG. 1 will be described as an example, but the following description is also valid for the wide-angle lens system according to the embodiment shown in FIGS. 3, 5, and 7. The wide-angle lens system in FIG. 1 includes, in order from the object side, a negative first lens group G1,
It has a positive second lens group G2 and a positive third lens group G3. The first lens group G1 includes a positive lens L11 having a strong convex surface facing the object side, and a negative meniscus lens disposed on the image side of the positive lens L11 in the first lens group G1 and having a strong convex surface facing the object side. L12 (L13). The second lens group G2 includes an achromatic lens pair L22, L23 having a positive focal length. Second lens group G
The achromatic lens pair in 2 is a negative lens L in order from the object side.
It consists of a cemented lens or a separation lens composed of the positive lens 22 and the positive lens L23. The third lens group G3 includes a negative lens L31 having a concave surface facing the object side, and a positive lens L32 (L3) disposed on the image side of the negative lens L31 in the third lens group G3.
3) is included.

The wide-angle lens system of FIG. 1 satisfies the following conditional expression. (1) Bf / f <1.45 (2) Bf / y <2.5 (3) 9.5> TL / f> 5 where Bf: back focus of the whole system f: focal length of the whole system TL: Distance from the most object-side surface of the wide-angle lens to the image plane. Y: Maximum image height.

In the present invention, the back focus is configured to be shorter than that of the conventional one, but it is still necessary to secure a certain back focus. For this reason, a retrofocus type will be adopted,
As long as a certain amount of negative power is arranged in the first lens group G1, essentially large negative distortion occurs. In order to correct this distortion as efficiently as possible, in the first lens group, it is necessary to arrange at least one positive lens L11 having a strong convex surface facing the object side closest to the object side. It is also important that at least one of the subsequent negative lenses L12 (L13) has a meniscus shape with the convex surface facing the object side to minimize the generation of negative distortion. Especially when shortening the back focus and miniaturizing the lens system, the power of each lens element becomes stronger,
The configuration of the first lens group is particularly important.

The second lens group G2 having a positive refractive power functions as an imaging lens group to form an image of an object on the image plane I. The second lens group G2 efficiently corrects the positive spherical aberration generated in the first lens group G1, and has axial chromatic aberration and spherical aberration because the stop S is disposed near the second lens group. Has a function of correcting a difference due to the color of the image.

The third lens group G3 having a positive refractive power has a function of configuring the optical system to be image-side telecentric. In the third lens group G3, large negative distortion tends to occur in order to secure good telecentricity. In order to efficiently correct this negative distortion, it is necessary to arrange at least one negative meniscus lens L31 having a concave surface facing the object side closest to the object side in the third lens group.

Next, conditional expressions (1) to (3) will be described.
The conditional expressions (1) and (2) define an optimum back focus range with respect to the focal length and image height of the entire system, and correspond to the degree of downsizing of the lens system. If the values deviate from the ranges of the conditional expressions (1) and (2), it is impossible to achieve the small lens system which is the object of the present invention.

The conditional expression (3) defines a preferable ratio of the total length to the focal length of the entire system, and corresponds to the degree of the telephoto ratio (telephoto ratio) of the optical system. When the value exceeds the upper limit of the conditional expression (3), the size reduction as the object of the present invention cannot be achieved. On the other hand, if the lower limit of conditional expression (3) is not reached, the telephoto ratio of the optical system will be too high, which will lead to deterioration of the imaging performance.

The wide-angle lens system according to the embodiment shown in FIG. 1 satisfies the following conditional expression (4). (4) 0.6 <| f1 / f | <1.6, where f1: the focal length of the first lens group.

The conditional expression (4) is a condition for securing an appropriate back focus and achieving good correction of distortion. When the value exceeds the upper limit of the conditional expression (4), the total length of the lens becomes long, which is not preferable because it is contrary to miniaturization. Conversely, if the lower limit of conditional expression (4) is exceeded,
This is not preferable because the generation of negative distortion is large and correction becomes difficult. In this case, particularly when the lens system is miniaturized, the power of each lens element constituting the lens system increases, so that it becomes difficult to correct distortion.

In the wide-angle lens system according to the embodiment shown in FIG. 1, the first lens group G1 includes, in order from the object side, a positive lens L11 having a strong convex surface facing the object side, and a negative meniscus having a strong concave surface facing the image side. It consists of a lens L12 and a negative lens L13 with a strong concave surface facing the image side, and satisfies the following conditional expression (5). (5) 0.9 <q11 <3.5 where q11: shape factor of the positive lens L11 with the strong convex surface facing the object side The shape factor q11 is represented by the following equation. q11 = (r2 + r1) / (r2-r1) where r1 = curvature radius of the object-side surface of the positive lens L11 r2 = curvature radius of the image-side surface of the positive lens L11.

The above conditional expression (5) is a condition for favorably correcting distortion while suppressing occurrence of astigmatism.
If the upper limit of the conditional expression (5) is exceeded, the contribution of the positive lens L11 on the object-side lens surface to correct the distortion positively decreases, and the distortion occurs largely negatively. . Conversely, falling below the lower limit of conditional expression (5) is not preferable because astigmatism increases.

In the wide-angle lens system according to the embodiment shown in FIG. 1, the second lens group G2 includes, in order from the object side, a positive lens L21, a stop S, a negative lens L22, and a biconvex positive lens L23. And the following conditional expressions (6) and (7) are satisfied. (6) 1.1 <f2 / f <1.8 (7) 1.2 <f2c / f2 <2.0 where f2: focal length of the second lens group G2 f2C: aperture S in the second lens group G2 This is the composite focal length of the lenses L22 and L23 disposed closer to the image.

The conditional expression (6) is a condition relating to correction of spherical aberration. Since the second lens group G2 is an imaging lens group, it greatly contributes to spherical aberration. Conditional expression (6) above
If the lower limit is exceeded, the focal length of the second lens group G2 becomes too short, which makes it difficult to correct spherical aberration, which is not preferable. On the other hand, when the value exceeds the upper limit of the conditional expression (6), the focal length of the second lens group G2 becomes too long, which is not preferable for downsizing.

The condition (7) is a condition for achieving good achromatism. If the lower limit of conditional expression (7) is exceeded, the overall power of the lens element for achromatism will be too strong, and accordingly, the chromatic aberration correction surface (the image side of the positive lens L21 and the object side of the negative lens L23) Surface)
Is also undesirably sharp, and changes in color such as spherical aberration and astigmatism cause high-order bending. Conversely, when the value exceeds the upper limit of the conditional expression (7), the combined focal length of the lens elements for achromatism becomes too long, so that a burden is imposed on the lens before the stop, and as a result, the curvature of the achromatizing surface also increases. This is not preferable because the chromatic change of the spherical aberration becomes large because it becomes strong.

In the wide-angle lens system according to the embodiment shown in FIG. 1, the positive lens L33 disposed closest to the image in the third lens group G3 is a positive lens having a convex surface having a stronger curvature directed to the object side. And satisfies the following conditional expression (8). (8) 0.8 <q33 <4.5 where q33 is a shape factor of the positive lens L33.
The shape factor q33 is represented by the following equation. q33 = (r2 + r1) / (r2-r1) where r1 = the radius of curvature of the object-side surface of the positive lens L33, and r2 = the radius of curvature of the image-side surface of the positive lens L33.

The above conditional expression (8) is a condition for achieving good correction of telecentricity and distortion. Here, while suppressing the occurrence of distortion in the third lens group G3,
In order to ensure telecentricity, it is important to divide the positive power of the third lens group G3 and refract the principal ray slowly. However, when the entire optical system is reduced in size and the power of each lens element constituting the third lens group G3 becomes stronger, the surface closest to the image side has negative power to correct distortion. Is effective. Conditional expression (8) is defined from that viewpoint. If the upper limit of conditional expression (8) is exceeded, the object-side surface of the positive lens L33 has too strong positive power, so that the negative distortion is large. It is not preferable. Conversely, if the lower limit of conditional expression (8) is exceeded, it becomes difficult to secure telecentricity, which is not preferable.

In the wide-angle lens system according to the embodiment shown in FIG. 1, the negative lens L31 located closest to the object side in the third lens group G3 is a negative lens having a strong concave surface facing the object side. In addition, the following conditional expression (9) is satisfied. (9) 0.6 <q31 <1.5, where q31 is a shape factor of the negative lens L31.
The shape factor q31 is represented by the following equation. q31 = (r2 + r1) / (r2-r1) Here, r1 = curvature radius of the object-side surface of the negative lens L31 r2 = curvature radius of the image-side surface of the negative lens L31.

The conditional expression (9) is a condition for achieving good correction of telecentricity and distortion. If the lower limit of the conditional expression (9) is exceeded, the fluctuation of the distortion due to the angle of view will occur, and at the same time, the outward coma will be large, and the field curvature will also increase positively, making it difficult to correct. . If the upper limit of conditional expression (9) is exceeded, the angle of view of the distortion will also fluctuate, and intrinsic coma will occur, making it difficult to correct.

The wide-angle lens system according to the embodiment shown in FIG. 1 satisfies the following conditional expressions (10) and (11). (10) -2.0 <f3 / f33I <0.1 (11) 0.2 <| f31 / f3 | <0.5, where f33I: the third lens group disposed closest to the image side Focal length f31 of the image-side surface of the positive lens f31: This is the focal length of the negative lens located closest to the object side in the third lens group.

The above conditional expressions (10) and (11) are conditions for simultaneously achieving the telecentricity and good correction of the distortion. Conditional expression (10) defines the power of the most image side lens surface of the positive lens L33 closest to the image side in the third lens group G3. If the upper limit of conditional expression (10) is exceeded, the power of the lens surface becomes strong in the positive direction, and it is not preferable because the distortion remaining negative can not be corrected. Conversely, if the lower limit of conditional expression (10) is exceeded, the power of this lens surface becomes too negative and too strong, and extrinsic coma is also generated, which makes it impossible to balance distortion and telecentricity.

If the lower limit of conditional expression (11) is exceeded, the negative lens L located closest to the object side in the third lens group G3 will be described.
The power of the lens 31 becomes too strong, and the distortion varies depending on the angle of view, which is not preferable because balance with telecentricity cannot be secured. Conversely, if the upper limit of conditional expression (11) is exceeded,
The power of the negative lens is too weak, and the ability to correct negative distortion is low, and the balance between telecentricity and distortion correction cannot be maintained.

In the wide-angle lens system according to the embodiment shown in FIG. 1, the inclination of the principal ray corresponding to the maximum image height emitted from the most image-side surface of the wide-angle lens system with respect to the optical axis is expressed by the following conditional expression. Is pleased. (12) −0.1 <m <0.1, where m is the direction cosine of the inclination of the principal ray.

The above conditional expression (12) is a condition relating to telecentricity on the image side. When the value is out of the range of the conditional expression (12), the inclination of the principal ray corresponding to the optical axis and the maximum image height increases, and the position of an image sensor such as a CCD becomes more sensitive, and color shading occurs. Therefore, it is not preferable.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Numerical embodiments will be described below. Here, FIG. 1, FIG. 3, FIG. 5, and FIG.
FIG. 2, FIG. 4, FIG. 6, and FIG. 8 are various aberration diagrams of the wide-angle lens systems of the first to fourth embodiments, respectively. Although the wide-angle lens systems of the first to fourth embodiments described below are examples designed for a 1/3 inch CCD, other sizes, for example, 1/2 inch, can be used by proportionally enlarging / reducing the optical system. , 1/5 inch or the like.

The wide-angle lens system of each embodiment shown in FIGS. 1, 3, 5 and 7 includes, in order from the object side, a first lens group G1 having a negative refractive power, a second lens group G2 having a positive refractive power, and This is a three-group configuration of a third lens group G3 having a positive refractive power. [First Embodiment] In FIG. 1, a first lens group G1 having a positive refractive power includes, in order from the object side, a positive lens L11 having a strong convex surface facing the object side, and a negative meniscus lens L12 having a strong concave surface facing the image side. And a negative lens L1 having a strong concave surface facing the image side
Three.

The second lens group G2 includes, in order from the object side, a thick positive lens L21 having a biconvex shape, a stop S, a negative meniscus lens L22 having a concave surface facing the image side, and a positive lens L23 having a biconvex shape. The negative lens L22 and the positive lens L23 form an achromatic lens pair. The third lens group G3 includes, in order from the object side, a biconcave negative lens L31 having a strong concave surface facing the object side, a meniscus-shaped positive lens L32 having a convex surface facing the image side, and a convex surface having a higher curvature being closer to the object side. And a positive lens L33 having a biconvex shape.

[Second Embodiment] In the second embodiment shown in FIG. 3, the first and second lens groups G1 and G2 have the same configuration as in the first embodiment, and a description thereof will be omitted. The third lens group G3 includes, in order from the object side, a biconcave negative lens L31 having a strong concave surface facing the object side, and a biconvex positive lens L3
2 and a meniscus-shaped positive lens L33 with the convex surface having a stronger curvature facing the object side.

[Third Embodiment] In the third embodiment shown in FIG. 5, the first to third lens groups G1 to G3 have the same configuration as that of the above-described second embodiment, and therefore description thereof will be omitted. [Fourth Embodiment] In the fourth embodiment shown in FIG. 7, the first and second lens groups G1 and G2 have the same configuration as in the first embodiment, and a description thereof will be omitted.

The third lens group G3 includes, in order from the object side, a biconcave negative lens L31 having a strong concave surface facing the object side, a meniscus-shaped positive lens L32 having a convex surface facing the image side, and a convex surface having a higher curvature. And a meniscus-shaped positive lens L33 with the lens facing the object side. Hereinafter, Tables 1 to 4 show lens data of the first to fourth examples, respectively. In each table, ri is the radius of curvature of the lens surface Ri, di + 1 is the distance between the lens surface Ri and the lens surface Ri + 1 on the optical axis, and N (d) i is the lens surface Ri and the lens surface Ri + 1. The refractive index of the d-line between
Is the Abbe number for the d-line between the lens surface Ri and the lens surface Ri + 1, f is the focal length, FN is the F number, Y is the image height, and Bf is the back focus.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4] FIGS. 2, 4, 6, and 8 show various aberration diagrams of the wide-angle lens system shown in Tables 1 to 4. In each aberration diagram, H indicates an incident height, FN indicates an F number, A indicates a half angle of view, and Y indicates an image height. In the astigmatism diagram, a solid line indicates a sagittal image plane, and a dotted line indicates a meridional image plane.

The following Table 5 shows the first to fourth data shown in Tables 1 to 4.
10 shows condition-corresponding values of the wide-angle lens system of the example.

[0049]

Table 5 Example 1 Example 2 Example 3 Example 4 Example 4 (1) 1.433 0.875 0.673 1.205 (2) 1.938 1.210 0.931 1.668 (3) 8.662 6.658 6.456 8.653 (4) 1.356 1.002 0.852 1.427 (5) ) 2.556 2.109 2.048 2.327 (6) 1.664 1.357 1.340 1.625 (7) 1.567 1.751 1.748 1.502 (8) 0.846 2.616 3.831 1.023 (9) 0.903 0.873 0.865 0.903 (10) 0.085 -0.873 -1.689 -0.013 (11) 0.360 0.393 0.321 0.375 (12) 0.0045 -0.0200 -0.0202 -0.0065 As is clear from the specifications and the aberration diagrams, the wide-angle lens system according to each embodiment of the present invention has a total angle of view (2ω) of about 74 °. It has a large angle of view and an F-number of 2.
Although it is a bright and compact optical system of about 8, it is well corrected for aberrations, and is suitable for a digital still camera or a video camera using a solid-state imaging device such as a CCD. That is, according to the embodiments of the present invention, the back focus is 1.45 times or less of the focal length and the half angle of view is suitable for a high-performance and compact digital still camera including a wide-angle region of about 36 °. A wide-angle lens can be realized.

In each of the above embodiments, an object located at infinity is assumed. However, when focusing on an object at a short distance, for example, the entire optical system (first to third lens groups G1 to G3) is used. ) To the object side.
A front lens feeding method in which only the first lens group G1 is fed to the object side, an inner focus method in which at least one lens group of the second and third lens groups is moved,
A focusing method such as a floating method in which at least two lens groups of the lens groups are moved while changing the moving ratio can be applied.

[0051]

As described above, according to the present invention, a high-performance and compact wide-angle lens having a short back focus and including a wide-angle region can be realized.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram of a first embodiment according to the present invention.

FIG. 2 is a diagram illustrating various aberrations of the first example.

FIG. 3 is a lens configuration diagram of a second embodiment according to the present invention.

FIG. 4 is a diagram illustrating various aberrations of the second example.

FIG. 5 is a lens configuration diagram of a third embodiment according to the present invention.

FIG. 6 is a diagram illustrating various aberrations of the third example.

FIG. 7 is a lens configuration diagram of a fourth embodiment according to the present invention.

FIG. 8 is a diagram illustrating various aberrations of the fourth example.

[Explanation of symbols]

 G1: first lens group, G2: second lens group, G3: third lens group, S: stop I: image plane

Claims (8)

[Claims] 1. A wide-angle lens system comprising a negative first lens group, a positive second lens group, and a positive third lens group in order from the object side, wherein the first lens group is located on the object side. A positive lens having a strong convex surface, and a negative meniscus lens disposed on the image side of the positive lens in the first lens group and having a strong convex surface facing the object side; An achromatic lens pair having a focal length, wherein the third lens group includes a negative lens having a concave surface facing the object side, and a positive lens disposed on the image side of the negative lens in the third lens group. The achromatic lens pair in the second lens group includes, in order from the object side, a cemented lens or a separation lens composed of a negative lens and a positive lens, and satisfies the following conditional expression: system. (1) Bf / f <1.45 (2) Bf / y <2.5 (3) 9.5> TL / f> 5 where Bf: back focus of the whole system f: focal length of the whole system TL: The distance y from the most object side surface of the wide-angle lens to the image plane is y: maximum image height. 2. The wide-angle lens system according to claim 1, wherein the following conditional expression is satisfied. (4) 0.6 <| f1 / f | <1.6, where f1: focal length of the first lens group. 3. The first lens group includes, in order from the object side, a positive lens having a strong convex surface facing the object side, a negative meniscus lens having a strong concave surface facing the image side, and a negative lens having a strong concave surface facing the image side. 3. The wide-angle lens system according to claim 2, comprising a lens and satisfying the following conditional expression. (5) 0.9 <q11 <3.5 where q11 is a shape factor of a positive lens whose strong convex surface faces the object side. The shape factor q11 is
It is expressed by the following equation. q11 = (r2 + r1) / (r2-r1) where r1 = radius of curvature of the object side surface of the positive lens r2 = curvature radius of the image side surface of the positive lens. 4. The second lens group comprises, in order from the object side, a positive lens, a stop, a negative lens, and a biconvex positive lens, and satisfies the following conditional expression. Or the lens system according to 3. (6) 1.1 <f2 / f <1.8 (7) 1.2 <f2c / f2 <2.0 where f2: focal length of the second lens group f2C: from the aperture in the second lens group This is the composite focal length of the lens arranged on the image side. 5. A positive lens disposed on the image side in the third lens group, the convex lens having a stronger curvature being directed to the object side, and satisfying the following conditional expression. The wide-angle lens system according to claim 1, 3 or 4, wherein: (8) 0.8 <q33 <4.5 where q33 is the shape factor of the positive lens. The shape factor q33 is represented by the following equation. q33 = (r2 + r1) / (r2-r1) where r1 = curvature radius of the object-side surface of the positive lens r2 = curvature radius of the image-side surface of the positive lens. 6. The negative lens disposed in the third lens group closest to the object side is a negative lens having a strong concave surface facing the object side, and satisfies the following conditional expression. 5. The wide-angle lens system according to 5. (9) 0.6 <q31 <1.5, where q31 is the shape factor of the negative lens. The shape factor q31 is represented by the following equation. q31 = (r2 + r1) / (r2-r1) where r1 = the radius of curvature of the object-side surface of the negative lens, and r2 = the radius of curvature of the image-side surface of the negative lens. 7. The wide-angle lens system according to claim 6, wherein the following conditional expression is satisfied. (10) -2.0 <f3 / f33I <0.1 (11) 0.2 <| f31 / f3 | <0.5, where f33I: the third lens group disposed closest to the image side Focal length f31 of the image-side surface of the positive lens f31: This is the focal length of the negative lens located closest to the object side in the third lens group. 8. The tilt of the principal ray corresponding to the maximum image height emitted from the most image-side surface of the wide-angle lens system with respect to the optical axis satisfies the following conditional expression. 7. The wide-angle lens system according to 7. (12) −0.1 <m <0.1, where m is the direction cosine of the inclination of the principal ray.