JP2000089106A - Image forming optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized image forming optical system using low- dispersion refractive index distribution material which can be easily manufactured, and capable of satisfactorily correcting various kinds of aberration with a small number of about three lenses, and covering the extent of a lens from a standard lens whose viewing angle is about 50 to 60 deg. to a wide angle lens. SOLUTION: The optical system is a lens system constituted of three groups, and in order from an object side, the system is constituted of a 1st group G1 whose refractive power is negative, a 2nd group G2 which is constituted of one meniscus lens whose concave face faces an image side and a 3rd group G3 including at least one refractive index distribution lens and whose refractive power is positive and the refractive index distribution lens is the radial type one, and the 1st group G1 is constituted of one concave lens, and the 3rd group G3 is constituted of one radial type refractive index distribution lens, and by which -0.005<1/V1<0.01 is satisfied, provided that V1 denotes Abbe's number of the radial type refractive index distribution lens on a d-line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging optical system and, more particularly, to a lens system used for a video camera, a videophone, and the like, and more particularly to a small lens using a radial type gradient index lens. is there.

[0002]

2. Description of the Related Art In recent years, with the spread of video cameras and videophones using a solid-state image pickup device such as a CCD, a lens system having a relatively wide angle of view used for them is required for further miniaturization and production. In order to reduce costs, it is required to reduce the number of lenses. On the other hand, the lens system is required to have higher performance as the number of pixels of the image sensor increases.

As a conventional example of a lens system composed of three lenses using a radial type refractive index distribution, Japanese Patent Application Laid-Open No.
A lens system described in Japanese Patent Application Laid-Open No. 9697 is known.
However, in this conventional example, a radial type refractive index distribution is applied to a microscope objective lens, and the angle of view is narrow.

A lens system disclosed in Japanese Patent Application Laid-Open No. 4-97309 is known as a conventional example in which telecentric incidence on an image plane is taken into consideration in a lens system having a relatively wide angle of view. However, in this conventional example, the refractive index difference between the optical axis of the gradient index lens and the peripheral portion is extremely large, so that it is difficult to produce a gradient index lens material.

[0005] Japanese Patent Application Laid-Open No. 9-281388 discloses a relatively wide-angle lens system composed of two to four lenses using a radial type refractive index distribution and which improves the above-mentioned two conventional examples. There is known a lens system described in Japanese Patent Application Laid-Open (JP-A) no. In this conventional example, as one of the examples, one concave meniscus lens, one plano-convex lens,
A lens system having a three-lens configuration including a brightness stop and a wood lens (refractive index distribution lens) is disclosed. In this embodiment, the angle of view is as wide as about 60 °, and each aberration is well corrected. However, since the material of the gradient index lens has a large negative dispersion (V ₁ : −77) to correct the chromatic aberration of magnification, it is difficult to manufacture the material.

In the other embodiments, in which the chromatic aberration is similarly sufficiently corrected, the refractive index distribution material has a large negative dispersion (1 / V ₁ <−0.0067). Material fabrication is difficult. Further, in the embodiments of the low-dispersion (−0.0067 <1 / V ₁ <0.0067) material other than the above, the correction of the chromatic aberration cannot be said to be sufficient, and there is room for improvement. May be a problem in dealing with

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned situation in the prior art, and an object of the present invention is to use a low-dispersion refractive index distribution material which is relatively easy to produce, It is an object of the present invention to provide a small-sized imaging optical system in which various aberrations are satisfactorily corrected with a small number of lenses, and the angle of view is about 50 ° to 65 °.

[0008]

The image forming optical system according to the present invention for achieving the above object is a lens system composed of three groups. The first group having a negative refractive power and the image side are arranged in order from the object side. And a third group having at least one refractive index distribution lens and having a positive refractive power.

Another image forming optical system according to the present invention comprises, in order from the object side, an interchangeable lens unit and a master lens unit. An imaging optical system, wherein a relative position between at least a part and an imaging surface is changeable.

Hereinafter, the reason why the above configuration is employed in the present invention and the operation thereof will be described. In the present invention, using a refractive index distribution material of a level that can be actually manufactured at present (low dispersion, a small difference in refractive index, etc.), the state is as close to a telecentric state as possible, and the number of lenses is three. In order to realize an optical system in which the aberrations, especially the chromatic aberration of magnification, are well corrected at a wide angle, one concave lens in order from the object side, one meniscus lens having a concave surface facing the image side, and positive refraction. 3 of one radial type gradient index lens with power
A lens system composed of a plurality of lenses and configured to satisfy the following conditions (1) to (4) is provided.

In the above radial type gradient index lens, the medium has a refractive index distribution in a direction perpendicular to the optical axis, and the refractive index distribution n (r) is expressed by the following equation. Things. (A) n (r) = N 0 + N 1 r 2 + N 2 r 4 + N 3 r 6 + ····· where, N ₀ is a refractive index on the optical axis at the wavelength of the reference, N
_i (i = 1, 2, 3,...) is a coefficient representing the refractive index distribution at the reference wavelength, and r is the distance from the optical axis in the vertical direction.

The Abbe number of a radial type gradient index lens is given by the following equation. _{(B) V 0 = (N} 0d -1) / (N 0F -N 0C) (C) V i = (N id -1) / (N iF -N iC) (i = 1,2,3, · ...) where N _0d , N _0F , N _0C and N _id , N _iF , N _iC (i =
..) Are coefficients representing the refractive index distribution at the wavelength λ, and the subscripts d, C, and F represent the d line, the C line, and the F line, respectively.

First, in order to satisfactorily correct lateral chromatic aberration without using a refractive index distribution material having a large negative dispersion, the optical system according to the present invention is designed to reduce lateral chromatic aberration generated by the concave lens of the first group. The difference in the amount of chromatic aberration of magnification between the two surfaces caused by the difference in ray height between the object-side surface and the image-side surface of the two groups of meniscus lenses cancels out the chromatic aberration amounts of the first and second units. By doing so, magnification chromatic aberration correction is realized. In order to favorably correct lateral chromatic aberration with this configuration, the following conditions must be satisfied. This defines the lens thickness and Abbe number of the meniscus lens of the second group.

(1) d ₂ /f>0.2 (2) ν ₂ <50 where d ₂ is the lens thickness of the meniscus lens, ν ₂
Is the Abbe number of the glass material of the meniscus lens, and f is the focal length of the entire system.

Conditional expression (1) is a condition for increasing the difference in ray height between the object-side surface and the image-side surface of the second meniscus lens, and conditional expression (2) is that meniscus lens. This is for increasing the amount of chromatic aberration generated on each surface. Under these two conditions, the difference in the amount of chromatic aberration of magnification occurring between the two surfaces of the meniscus lens can be increased, and a large correction effect can be expected.

Further, it is required that the second group of the meniscus lenses have a concave surface facing the image side and satisfy the following conditions:
It is necessary to favorably correct off-axis chromatic aberration without deteriorating other aberrations. (3) | f / f ₂ | <0.2 where f ₂ is the focal length of the second lens unit, and f is the focal length of the entire system.

If the condition (3) is satisfied, the power of the meniscus lens of the second group is reduced, and the influence on the Petzval sum and the axial aberration is reduced. Therefore, the meniscus lens of the second group has a desired correction effect substantially only for the off-axis aberration, and the off-axis aberration can be satisfactorily corrected.

Further, the refractive index distribution material must also satisfy the following conditions. The following conditions define the dispersion of the medium of the refractive index distribution material. (4) −0.005 <1 / V ₁ <0.01 where V ₁ is the Abbe number of the radial type gradient index lens at the d-line.

As described above, the optical system of the present invention is configured so that the occurrence of chromatic aberration in the first and second units is substantially canceled out. Undesirably, chromatic aberration remains. Condition (4)
Is a condition for reducing the occurrence of chromatic aberration in the medium portion of the radial type gradient index lens and keeping the chromatic aberration favorable. When the value exceeds the upper limit of 0.01 of this condition, chromatic aberration increases, and when the value falls below the lower limit of -0.005, chromatic aberration occurs in the opposite direction. Therefore, it is difficult to produce the material, which is not desirable.

In the lens system of the present invention, in order to satisfactorily correct distortion with a small number of lenses, it is desirable that at least one surface of the first group of concave lenses is aspheric. This makes it possible to satisfactorily correct the negative barrel distortion generated in the first lens unit.

It is desirable that a lens system that requires a higher degree of aberration correction satisfy at least one of the following conditions.

(5) d ₂ /f>0.8 (6) ν ₂ <35 (7) | f / f ₂ | <0.13 (8) −0.003 <1 / V ₁ <0.005 Conditional expression (5) is a condition for further increasing the difference in ray height between the object-side surface and the image-side surface of the meniscus lens, and by satisfying this condition, the two surfaces of the meniscus lens are satisfied. It is possible to further increase the difference in the amount of chromatic aberration of magnification occurring between the lenses, increase the effect of correcting chromatic aberration of magnification in the meniscus lens of the second group, and better correct chromatic aberration of magnification in the entire lens system. it can.

Conditional expression (6) is a condition for further increasing the amount of chromatic aberration generated between the two surfaces of the meniscus lens. By satisfying this condition, the condition of the two surfaces of the meniscus lens is satisfied. It is possible to further increase the difference in the amount of chromatic aberration of magnification occurring between the lenses, and to increase the effect of correcting chromatic aberration of magnification in the meniscus lens of the second group,
Correction of lateral chromatic aberration in the entire lens system can be made better.

Conditional expression (7) is a condition for the meniscus lens of the second group to favorably correct off-axis chromatic aberration without deteriorating other aberrations. By satisfying this condition, the second condition is satisfied. The meniscus lens of the group has a lower power and a smaller influence on Petzval sum and axial aberration. Therefore, the degree of freedom with respect to the correction of the off-axis aberration in the meniscus lens of the second group is further improved, and the off-axis aberration can be corrected more favorably.

Conditional expression (8) is a condition for further reducing the occurrence of chromatic aberration in the medium portion of the radial type gradient index lens element. By satisfying this condition, the refractive index distribution lens element of the third group is obtained. The occurrence of chromatic aberration in the medium is further suppressed, and the amount of occurrence of chromatic aberration at the magnification of the entire system can be reduced.

It is desirable that the following conditions are satisfied as conditions for correcting lateral chromatic aberration. (9) ν ₁ > 50 where ν ₁ is the Abbe number of the concave lens in the first group.

Conditional expression (9) is a condition for reducing the occurrence of chromatic aberration in the concave lens of the first group. By satisfying this condition, the occurrence of chromatic aberration in the concave lens of the first group is further increased. Thus, the amount of chromatic aberration of magnification of the entire system can be reduced.

If it is desired to further increase the amount of chromatic aberration of magnification, the following condition (1) is added in addition to the condition (9).
0) is preferably satisfied. (10) ν ₁ > 65 If the condition (10) is satisfied, the occurrence of chromatic aberration of magnification in the first lens unit can be further suppressed, so that the amount of chromatic aberration of magnification in the entire lens system can be reduced.

Since the optical system of the present invention forms a wide-angle lens system with a small number of lenses, the negative power of the concave lens of the first group is large. Accordingly, it is necessary to increase the positive power of the refractive index distribution lens of the third lens group due to correction of Petzval sum and the like. Moreover, in order to improve the correction of chromatic aberration in the entire system, it is necessary to suppress the occurrence of chromatic aberration in the refractive index distribution lens as described above. Therefore, in order to minimize the occurrence of chromatic aberration of magnification, only a gentle curvature can be provided to the gradient index lens. What Shiga, in order to increase the power of the gradient index lens, it is necessary to increase the refractive power phi _m of the medium.

On the other hand, the refractive power φ of the medium of the refractive index distribution lens
_m is approximated by the following equation (D). (D) provided that _{φ m ≒ -2N 1 d G,} d G is the lens thickness of the radial type refractive index distribution lens.

Therefore, in order to increase φ _m , it is necessary to increase N ₁ or to increase the lens thickness d _{G according} to the equation (D). However, if N ₁ is too large, it becomes difficult to produce the material, which is not preferable. On the other hand, with respect to the lens thickness d _G , a relatively thick lens having a refractive index compared to a conventional homogeneous lens can be easily manufactured due to its manufacturing method. Therefore, in order to increase the power of the gradient index lens, it is desirable to increase the lens thickness d _G because the material can be easily manufactured.

However, when the radial type gradient index lens is extremely thick, it may cause flare and decrease in transmittance. Therefore, it is desirable that the radial type gradient index lens of the lens system of the present invention satisfies the following condition (11). (11) 2 <d _G / f <8, where d _G is the lens thickness of the radial type gradient index lens and f is the focal length of the entire system. The refractive power of the medium of the radial type gradient index lens becomes insufficient, and it becomes difficult to correct the Petzval sum, or ΔN required for the material becomes large, and the material production becomes difficult. On the other hand, when the value exceeds the upper limit of 8, the lens thickness of the refractive index distribution lens becomes large, and flare and transmittance are undesirably reduced.

However, when the thickness of the medium is increased, the medium becomes rod-shaped, and it becomes difficult to perform curved surface processing, and the processing cost of lens polishing increases. Therefore, in order to provide an inexpensive lens system while favorably correcting aberrations, it is desirable that the radial type gradient index lens be a wood lens having two flat surfaces.

In order to improve the optical performance even with a single radial type gradient index lens element, it is important to provide the lens with power so that aberrations are not generated as much as possible, and it is desirable to satisfy the following conditions. . (12) −0.2 <N ₂ · ER ² / N ₁ <0.2 where N ₁ and N ₂ are coefficients representing the refractive index distribution of the radial type gradient index lens, and ER is the radial type refractive index distribution. The effective radius of the lens.

Condition (12) is a condition for maintaining good spherical aberration in the medium portion of the radial type gradient index lens element. If the lower limit of -0.2 of this condition is exceeded, the spherical aberration becomes lower. If the value exceeds the upper limit of 0.2, the spherical aberration becomes excessively large on the over side, which is not preferable.

For a lens system that requires more advanced aberration correction, it is desirable to satisfy the following conditions. (13) -0.05 <N ₂ · ER ² / N ₁ <0.05 The condition (13) is a condition for keeping the spherical aberration in the medium portion of the radial type gradient index lens more satisfactorily.
When the value falls below the lower limit of -0.05 of the condition, the spherical aberration increases toward the under side, and when the value exceeds the upper limit of 0.05, the spherical aberration increases toward the over side.

The position of the aperture stop is preferably provided near the object side surface of the third group of radial type gradient index lenses so that astigmatism does not occur as much as possible and the diameter of the lens does not become too large. Good. At this time, it is desirable to satisfy the following conditions. (14) 0 <| d ₃ / f ₃ | <2 Here, d ₃ is the distance in the optical axis direction from the object side of the radial type gradient index lens of the aperture stop, and f ₃ is the focal length of the third lens unit. is there.

This condition defines the position of the aperture stop. By arranging the stop in this range, astigmatism can be corrected well. This condition (1
If the lower limit of 4) is exceeded, the meridional image plane is significantly tilted in the negative direction. If the upper limit of 2 is exceeded, the meridional image plane is significantly tilted in the positive direction.

In recent years, there has been a high demand for a camera that is small enough to be put in a suit or a shirt pocket, and particularly a thinner camera is desired. However, in order to achieve an optical system with high imaging performance, the overall length of the lens tends to be long, and in order to satisfy the requirements for downsizing and high performance of the camera body, the entire lens system has been shortened, and It is extremely difficult to achieve high imaging performance.

However, if a reflecting surface is used as a part of the lens system, it is possible to achieve a very small lens system while maintaining high imaging performance. FIG. 7 is a view showing that the camera can be downsized by the present invention.
7A is determined by its thickness x, width y, and height z, as shown in FIG. 7A. In general, one of the major factors that determine the thickness x of the camera 1 is the total length t of the lens system 2 (see FIG. 7A). 7
(B)), in order to make the camera 1 smaller and thinner,
It is necessary to shorten the total length t of the lens system 2. However, as described above, there is a limit in shortening the total length t of the lens system 2, and FIG.
As shown in (c), the total length t of the lens system 2 is longer than the thickness x of the camera 1 to be thinned. Compared with the x direction of the camera 1, the need for thinning in the y and z directions is not high. This is because it is not desirable that the camera 1 is difficult to hold the camera 1 at the time of photographing and is apt to be shaken when it is reduced to some extent. Further, in the y direction and the z direction, there are a liquid crystal display element, a battery, a recording device, and the like, and the influence of the lens system 2 on the size is relatively small. Therefore, as shown in FIG.
Is bent without shortening the overall length t of the lens system.
The camera 1 can be reduced in size and thickness (shortened in the x direction).
In the lens system of the present invention, the above requirement can be met without deteriorating performance by adopting a configuration in which the reflecting surface 5 can be arranged between the first group G1 and the second group G2. FIG.
Reference numeral G3 denotes a third unit of the lens system 2, and reference numeral 4 denotes an image plane.

Further, as proposed in Japanese Patent Application Laid-Open No. 9-281388, the lens system is composed of a master lens unit and an interchangeable lens unit, so that the camera can be miniaturized.
FIG. 8 shows that the size of the camera can be reduced by switching the interchangeable lens unit of the lens system according to the present invention. In the figure, the camera is 1, the optical axis of the lens system is 3, the reflection surface is 5, the master lens is 6, and the interchangeable lens units having different focal lengths are slid up, down, 7, 8, 9, 10 up and down. A switching mechanism for exchanging the lens is represented by 11, and a turret for rotating around the central axis 13 and exchanging the interchangeable lens part is represented by 12. FIGS. 8A and 8B show two switched mechanisms when the switching mechanism 11 is used. FIG. 8C is a front view showing the state, FIG. 8C is a front view when the turret 12 is used, and FIG.
(E) is a horizontal sectional view showing two states in which the interchangeable lens unit is switched using the switching mechanism 11 or the turret 12.

As is apparent from FIG. 8, a bifocal lens system or a multifocal lens system can be very easily achieved by switching the interchangeable lens units 7 to 10 having different focal lengths with respect to the master lens unit 6. Even in the lens system of the present invention,
The first lens unit and the second lens unit before the aperture stop are interchangeable lens units 7 to
10, the switching lens system can be achieved by using the third lens group as the master lens unit 6. Further, when switching is performed with the lens system having such a configuration, it is preferable that the light beam incident on the third lens unit be afocal because the image plane position does not move. However, securing a high imaging performance with a lens system having as few as three lenses and making the light rays incident on the third lens group afocal are extremely important design constraints. Therefore, if such a lens system satisfies desired specifications and is made to perform afocal incidence, there is a possibility that the imaging performance of the lens system deteriorates, the cost increases, and the like. Therefore, it is desirable to cope with the movement of the image plane position by changing the relative relationship between the lens or the lens system and the image sensor. In the switching lens system according to the present invention, the relative positions of the imaging surface and a part or all of the lenses of the third group of the master lens unit are changed in conjunction with the movement of the first group and the second group that are the interchangeable lens units. By changing the focus position by changing the focal point, it is possible to slightly shift the light beam incident on the third lens group from the afocal position. Image performance can be ensured.

In this case, it is desirable that the third unit is composed of one radial type gradient index lens element. this is,
As described above, the radial type gradient index lens has a high aberration correction ability, and it is possible to secure a practical level of imaging performance even with one lens.

In this case, it is desirable to satisfy the above condition (4) in order to make chromatic aberration a practically acceptable level. This is a condition for reducing the occurrence of chromatic aberration in the medium portion of the radial type refractive index distribution lens and keeping the chromatic aberration favorable. When the value exceeds the upper limit of 0.01 of this condition, chromatic aberration increases, and when the value falls below the lower limit of -0.005,
Conversely, chromatic aberration occurs in the reverse direction, and in each case, the amount of chromatic aberration is large, which is not preferable in practical use.

Further, in this case, it is desirable that the above-mentioned radial type gradient index lens satisfies the following condition (15). (15) d _G / f _G <2.3 where d _G is the lens thickness of the radial type gradient index lens, and f _G is the focal length of the gradient index lens.

If this condition is not satisfied, an image is formed inside the refractive index distribution lens or in the vicinity of the image-side surface, and the back focus required for the arrangement of the image pickup device and the filter can be secured. Disappears.

It is desirable to make the radial type gradient index lens a wood lens of both planes not only for an inexpensive lens system but also for aberration correction. This is because one is to reduce the Petzval sum, and the other is that a curved surface causes large chromatic aberration on the surface, making correction difficult. The correction can be made by using a negative high-dispersion material, but as described above, it is difficult to produce the material, which is not preferable.

By configuring the image forming optical system of the present invention as described above, it is possible to satisfactorily correct various aberrations of the lens system even with a small number of three. In the optical system described above, the shape of the refractive index distribution lens is preferably a wood lens in terms of cost and the like, but is not necessarily limited to that shape. A curvature may be applied to both sides or one side of the lens.

[0049]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 1 to 8 of the image forming optical system according to the present invention will be described below. Sectional views including optical axes of Examples 1, 4, and 5 are shown in FIGS. 1, 2, and 3, respectively. FIG. 4 is a cross-sectional view showing the wide state and the telescopic state of the eighth embodiment. The lens cross sections of Examples 2 and 3 are substantially the same as FIG. 1 and the lens cross sections of Examples 6 and 7 are substantially the same as FIG. The numerical data of each embodiment will be described later.

Embodiment 1 FIG. 1 is a sectional view including an optical axis of Embodiment 1. This lens system includes, in order from the object side, a first group G1 including one biconcave lens, a second group G2 including one meniscus lens having a concave surface facing the image side, and a wood lens (radial refraction of both planes). (Ratio distribution lens) is a three-group lens system including a third lens group G3 including one lens. First group G1 and second group G2
Are each composed of a homogeneous lens, and the first group G1 has an aspheric surface on the object side. The aperture stop S is arranged on the front surface of the wood lens of the third group G3. The plane-parallel plate disposed on the image side of the lens system is intended for a low-pass filter, an infrared cut filter, a CCD protective glass, or the like.

Embodiment 2 This lens system is arranged in order from the object side in the same manner as in Embodiment 1.
A first group G1 consisting of one biconcave lens, a second group G2 consisting of one meniscus lens having a concave surface facing the image side, and a third group consisting of one wood lens (radial refractive index distribution lens having both planes). The lens system has a three-group configuration including a group G3. Each of the first group G1 and the second group G2 includes a homogeneous lens, and the first group G1 has an aspherical surface on the object side. The aperture stop S is arranged on the front surface of the wood lens of the third group G3. The plane-parallel plate disposed on the image side of the lens system includes a low-pass filter, an infrared cut filter,
D protection glass and the like. In this embodiment, the glass material of the meniscus lens and the wood lens material of the second group G2 in the first embodiment are changed to those having a low negative dispersion.

Embodiment 3 This lens system is, like Embodiment 1, sequentially arranged from the object side.
A first group G1 consisting of one biconcave lens, a second group G2 consisting of one meniscus lens having a concave surface facing the image side, and a third group consisting of one wood lens (radial refractive index distribution lens having both planes). The lens system has a three-group configuration including a group G3. Each of the first group G1 and the second group G2 includes a homogeneous lens, and the first group G1 has an aspherical surface on the object side. The aperture stop S is arranged on the front surface of the wood lens of the third group G3. The plane-parallel plate disposed on the image side of the lens system includes a low-pass filter, an infrared cut filter,
D protection glass and the like. In this embodiment, the biconcave lens of the first group G1 is a plastic molded lens. Compared with the first embodiment, the Abbe number of the concave lens of the first group G1 is on the high dispersion side and the generation amount of chromatic aberration is increased. Therefore, the correction is performed by increasing the lens thickness of the meniscus lens of the second group G2. The first group G1 of this embodiment
FIG. 5 shows a configuration in which a reflection surface (mirror) M is disposed between the first lens unit and the second lens unit G2. The reflection surface M is not limited to a mirror, but may be a prism or the like.

Fourth Embodiment FIG. 2 is a cross-sectional view including an optical axis of a fourth embodiment. This lens system includes, in order from the object side, a first group G1 including one positive meniscus lens having a convex surface facing the object side, a second group G2 including one meniscus lens having a concave surface facing the image side, and a wood. This is a three-group lens system including a third lens group G3 including one lens (radial refractive index distribution lens having two flat surfaces). Both the first group G1 and the second group G2 are composed of homogeneous lenses, and the first group G1 has aspheric surfaces on both sides. The aperture stop S is arranged on the front surface of the wood lens of the third group G3. The basic lens configuration of this embodiment is the same as that of the first embodiment, but both surfaces of the concave lens of the first group G1 are both aspheric. The concave lens of the first group G1 is assumed to be a molded lens made of plastic.

Fifth Embodiment FIG. 3 is a sectional view including an optical axis of a fifth embodiment. This lens system includes, in order from the object side, a first group G1 including one positive meniscus lens having a convex surface facing the object side, a second group G2 including one meniscus lens having a concave surface facing the image side, and a wood. This is a three-group lens system including a third lens group G3 including one lens (radial refractive index distribution lens having two flat surfaces). Each of the first group G1 and the second group G2 includes a homogeneous lens, and the first group G1 has an aspherical surface on the object side. The aperture stop S is arranged between the second group G2 and the third group G3.
In this embodiment, in order to reduce the cost of lens processing, both the first group G1 and the second group G2 assume that the concave lens is a molded lens made of plastic. The plane-parallel plate disposed on the image side of the lens system is intended for a low-pass filter, an infrared cut filter, a CCD protective glass, or the like.

Example 6 This lens system is arranged in order from the object side in the same manner as in Example 4.
A first group G1 composed of one positive meniscus lens having a convex surface facing the object side, and a meniscus lens 1 having a concave surface facing the image side
This is a lens system of a three-group configuration including a second group G2 including a single lens and a third group G3 including a single wood lens (a radial type gradient index lens on both planes). First group G1 and second group
Each of the groups G2 is composed of a homogeneous lens, and the first group G1 has aspheric surfaces on both sides. The aperture stop S is arranged on the front surface of the wood lens of the third group G3. In this embodiment, as in the third embodiment, the concave lens of the first group G1 is formed of a plastic molded lens, and the Abbe number of the concave lens of the first group G1 is higher on the dispersion side than in the first embodiment, and chromatic aberration is obtained. This is a design example in which correction is made by increasing the lens thickness of the meniscus lens of the second group, because the amount of occurrence of is increased.

Embodiment 7 This lens system is, like Embodiment 4, sequentially arranged from the object side.
A first group G1 composed of one positive meniscus lens having a convex surface facing the object side, and a meniscus lens 1 having a concave surface facing the image side
This is a lens system of a three-group configuration including a second group G2 including a single lens and a third group G3 including a single wood lens (a radial type gradient index lens on both planes). First group G1 and second group
Each of the groups G2 includes a homogeneous lens, and the first group G1 has an aspheric surface on the image plane side. The aperture stop S is arranged on the front surface of the wood lens of the third group G3. This embodiment is based on the first
A large space is provided between the group G1 and the second group G2, and a sufficient space is provided for disposing the reflection surface. The aspheric surface is disposed on the image side of the concave lens of the first group G1, and the first group G2 has a negative refractive power.

Embodiment 8 FIG. 4 is a sectional view of this embodiment including the optical axes in the wide state and the telescopic state. In the figure, (W) is a wide state, and (T) is a tele state. In the wide state, in order from the object side,
A first group G1 consisting of one biconcave lens, a second group G2 consisting of one meniscus lens having a concave surface facing the image side, and a third group consisting of one wood lens (radial refractive index distribution lens having both planes). The lens system has a three-group configuration including a group G3. Each of the first group G1 and the second group G2 includes a homogeneous lens, and the first group G1 has an aspherical surface on the object side. The aperture stop S is arranged between the second group G2 and the third group G3.
The parallel plane plate arranged on the image side of the lens system is
It is intended for a low-pass filter, an infrared cut filter, a CCD protective glass, and the like.

This embodiment is a design example in which, in the lens system of the third embodiment, the first unit G1 and the second unit G2 are movable, and tele-wide two-focus switching is possible. FIG. 4 shows a state in which the third group G3 and the aperture stop S have moved to adjust the focus position in response to the removal of the first group G1 and the second group G2 when switching from the wide state to the telephoto state. ing. In this embodiment, the lens system moves, but the image sensor may move. The first group G1 and the second group G2 can be switched by shifting a part of the lens system in one direction as shown in FIGS. ), There is a method in which a part of the lens system is switched by rotation.

The numerical data of the first to eighth embodiments are shown below. In each data, f is the focal length, F _NO is the number, 2
ω is the angle of view, IH is the maximum image height, r ₁ , r ₂ ... are the radii of curvature of the respective lens surfaces, d ₁ , d ₂ .
n _d1 , n _d2 ... are the d-line refractive indices of each lens, v _d1 , v _d2 ...
Is the Abbe number of the d-line, and the aspherical shape can be expressed by the following equation, where the optical axis direction is the Z axis and the direction perpendicular to the optical axis is the Y axis. ^{Z = (Y 2 / R)} / [1 + √ {1-P (Y / R) 2}]
+ A ₄ Y ⁴ + A ₆ Y ⁶ + ... where R is the radius of curvature at the top, P is the conic constant, A ₄ ,
A ₆ is the fourth-order and sixth-order aspherical coefficients, respectively. In addition,
The radial type gradient index lens is indicated by “GRIN”. The refractive index distribution and Abbe number are given by the above formulas (A) and (B),
It is represented by equation (C). Note that the d line is used as a reference wavelength. The symbol φ _s is the refractive power of the surface of the radial type gradient index lens.

[0060] Example _{1 f = 3.14, F NO =} 2.8, 2ω = 57.9 °, IH = 1.6 r 1 = -131.3427 ( _{aspherical) d 1 = 1 n d1 =} 1.48749 ν d1 = 70.23 r 2 = 3.7362 d 2 = 7.5 r ₃ = 5.6325 d ₃ = 3.4 nd ₂ = 1.84666 ν _d2 = 23.78 r ₄ = 4.3095 d ₄ = 0.5 r ₅ = ∞ (aperture) d ₅ = 0 r ₆ = ∞ d ₆ = 15.2011 (GRIN) r _{7 = ∞ d 7 = 0.58 r 8} = ∞ d 8 = 1.83 n d4 = 1.51633 ν d4 = 64.14 r 9 = ∞ d 9 = 0.5 r 10 = ∞ d 10 = 0.75 n d5 = 1.48749 ν d5 = 70.23 r 11 = ∞ d ₁₁ = 0 r ₁₂ = ∞ GRIN i N _i V _i 0 1.664 38.2 1 -9.26 × 10 ^-3 6.55 × 10 ^-2 2 -4.71 × 10 ^-6 6.55 × 10 ^-2 Aspheric surface first surface P =- 5.86 × 10 ⁺⁴ A ₄ = 4.47 × 10 ^-4 A ₆ = 1.04 × 10 ^-5 ν ₁ = 70.23 ν ₂ = 23.78 ν ₁ / ν ₂ = 2.95 | f / f ₂ | = 0.026 d ₂ /f=1.08 _{d 3 / f 3 = 0 d} G / f G = 2.67 d G / f = 4.85 1 / V 0 = 0.026 1 / V 1 = 0.0015 φ m / (φ s + φ m) = 1 N 1 = -0.00926 ^{_{2 = -4.7055 × 10 -6 N 2}} · ER 2 / N 1 = 0.0020.

[0061] Example _{2 f = 3.13, F NO =} 2.8, 2ω = 58.1 °, IH = 1.6 r 1 = -131.3427 ( _{aspherical) d 1 = 1 n d1 =} 1.48749 ν d1 = 70.23 r 2 = 3.7362 d 2 _{= 7.5 r 3 = 5.6325 d 3} = 3.4 n d2 = 1.83481 ν d2 = 42.72 r 4 = 4.3095 d 4 = 0.5 r 5 = ∞ ( _{stop) d 5 = 0 r 6 =} ∞ d 6 = 15.2011 (GRIN) r 7 _{= ∞ d 7 = 0.58 r 8} = ∞ d 8 = 1.83 n d4 = 1.51633 ν d4 = 64.14 r 9 = ∞ d 9 = 0.5 r 10 = ∞ d 10 = 0.75 n d5 = 1.48749 ν d5 = 70.23 r 11 = ∞ d ₁₁ = 0 r ₁₂ = ∞ GRIN i N _i V _i 0 1.664 38.2 1 -9.26 × 10 ^-3 -4.00 × 10 ⁺² 2 -4.71 × 10 ^-6 6.55 × 10 ⁺² Aspheric surface first surface P = -5.86 × 10 ⁺⁴ A ₄ = 4.47 × 10 ^-4 A ₆ = 1.04 × 10 ^-5 ν ₁ = 70.23 ν ₂ = 42.72 ν ₁ / ν ₂ = 1.64 | f / f ₂ | = 0.024 d ₂ / f = 1.09 d ₃ / f ₃ = 0 d _G / f _G = 2.67 d _G / f = 4.86 1 / V ₀ = 0.026 1 / V ₁ = -0.0025 φ _m / (φ _s + φ _m ) = 1 N ₁ = -0.00926 N ₂ = −4.7055 × 10 ⁻⁶ N ₂ · ER ² / N ₁ = 0.0020.

Example 3 f = 3.0, F _NO = 2.7, 2ω = 58.9 °, IH = 1.6 r ₁ = -118.0596 (aspheric surface) d ₁ = 1 nd ₁ = 1.52542 ν _d1 = 55.78 r ₂ = 3.6823 d _{2 = 7.5 r 3 = 7.0684 d 3} = 4.8 n d2 = 1.84666 ν d2 = 23.78 r 4 = 5.9093 d 4 = 0.5 r 5 = ∞ ( _{stop) d 5 = 0 r 6 =} ∞ d 6 = 14.7451 (GRIN) r 7 _{= ∞ d 7 = 0.5 r 8} = ∞ d 8 = 1.83 n d4 = 1.51633 ν d4 = 64.14 r 9 = ∞ d 9 = 0.5 r 10 = ∞ d 10 = 0.75 n d5 = 1.48749 ν d5 = 70.23 r 11 = ∞ d ₁₁ = 0 r ₁₂ = ∞ GRIN i N _i V _i 0 1.664 38.2 1 -9.26 × 10 ^-3 6.55 × 10 ⁺² 2 7.68 × 10 ^-6 6.55 × 10 ⁺² Aspheric surface first surface P = -1.62 × 10 ⁺⁴ A ₄ = 9.17 × 10 ^-4 A ₆ = -7.30 × 10 ^-6 ν ₁ = 55.78 ν ₂ = 23.78 ν ₁ / ν ₂ = 2.35 | f / f ₂ | = 0.063 d ₂ /f=1.60 d ₃ / f ₃ = 0 d _G / f _G = 2.59 d _G / f = 4.92 1 / V ₀ = 0.026 1 / V ₁ = 0.0015 φ _m / (φ _s + φ _m ) = 1 N ₁ = −0.00926 N ₂ = 7.6771 × 10 ⁻⁶ N ₂ · ER ² / N ₁ = −0.0033.

[0063] Example _{4 f = 3.0, F NO =} 2.4, 2ω = 59.6 °, IH = 1.6 r 1 = 11.2433 ( _{aspherical) d 1 = 2.4459 n d1 =} 1.52542 ν d1 = 55.78 r 2 = 3.1445 ( aspherical D ₂ = 10.6667 r ₃ = 6.2257 d ₃ = 4 nd ₂ = 1.864666 ν _d2 = 23.78 r ₄ = 4.6396 d ₄ = 0.5808 r ₅ = ∞ (aperture) d ₅ = 0 r ₆ = ｄ d ₆ = 16.8225 (GRIN _{) r 7 = ∞ d 7 =} 1.1885 r 8 = ∞ GRIN i N i V i 0 1.664 38.2 1 -9.26 × 10 -3 6.55 × 10 +2 2 -4.80 × 10 -6 6.55 × 10 +2 aspherical coefficients a One surface P = -10.041 A ₄ = 0.00 A ₆ = 0.00 Second surface P = 0.1329 A ₄ = -4.89 × 10 ^-4 A ₆ = 4.41 × 10 ^-5 ν ₁ = 55.78 ν ₂ = 23.78 ν ₁ / ν _{2 = 2.35 | f / f 2 |} = 0.022 d 2 / f = 1.33 d 3 / f 3 = 0 d G / f G = 2.89 d G / f = 5.60 1 / V 0 = 0.026 1 / V 1 = 0.0015 φ m / (Φ _s + φ _m ) = 1 N ₁ = -0.00926 N ₂ = -4.7982 × 10 ^-6 N ₂ · ER ² / N ₁ = 0.0021.

[0064] Example _{5 f = 3.0, F NO =} 2.8, 2ω = 58.6 °, IH = 1.6 r 1 = 28.0572 ( _{aspherical) d 1 = 1 n d1 =} 1.52542 ν d1 = 55.78 r 2 = 5.2873 d 2 = 15 r ₃ = 7.7379 d ₃ = 7.2165 nd ₂ = 1.58423 ν _{d 2} = 30.49 r ₄ = 4.9929 d ₄ = 0.4066 r ₅ = ∞ (aperture) d ₅ = 2 r ₆ = ∞ d ₆ = 12.9 (GRIN) r ₇ = _{∞ d 7 = 2.8864 r 8 =} ∞ d 8 = 0.75 n d4 = 1.48749 ν d4 = 70.23 r 9 = ∞ GRIN i n i V i 0 1.664 38.2 1 -9.26 × 10 -3 6.55 × 10 +2 2 -9.05 × 10 ^-6 6.55 × 10 ⁺² Aspheric coefficient First surface P = 1.1318 A ₄ = 2.54 × 10 ^-4 A ₆ = 1.69 × 10 ^-6 ν ₁ = 55.78 ν ₂ = 30.49 ν ₁ / ν ₂ = 1.83 │f / F ₂ | = 0.0038 d ₂ / f = 2.40 d ₃ / f ₃ = 0.34 d _G / f _G = 2.21 d _G / f = 4.30 1 / V ₀ = 0.026178 1 / V ₁ = 0.0015267 φ _m / (φ _s + Φ _m ) = 1 N ₁ = -0.00926 N ₂ = -9.0525 × 10 ^-6 N ₂ · ER ² / N ₁ = 0.0039.

[0065] Example _{6 f = 3.0, F NO =} 2.8, 2ω = 59.5 °, IH = 1.6 r 1 = 101.5395 ( _{aspherical) d 1 = 3.2358 n d1 =} 1.52542 ν d1 = 55.78 r 2 = 4.0003 ( aspherical _{) d 2 = 4.5175 r 3 =} 14.4159 d 3 = 10.5779 n d2 = 1.84666 ν d2 = 23.78 r 4 = 19.6007 d 4 = 2.6984 r 5 = ∞ ( _{stop) d 5 = 0 r 6 =} ∞ d 6 = 13.194 (GRIN _{) r 7 = ∞ d 7 =} 2.9244 r 8 = ∞ GRIN i N i V i 0 1.664 38.2 1 -9.26 × 10 -3 6.55 × 10 +2 2 1.50 × 10 -5 6.55 × 10 +2 aspherical coefficients first surface _{P = -15.5132 A 4 = 0.00 A} 6 = 0.00 second surface _{P = 0.7561 A 4 = -5.28 ×} 10 -4 A 6 = -5.61 × 10 -5 ν 1 = 55.78 ν 2 = 23.78 ν 1 / ν ₂ = 2.35 | f / f ₂ | = 0.09 d ₂ /f=3.52 d ₃ / f ₃ = 0 d _G / f _G = 2.28 d _G / f = 4.31 1 / V ₀ = 0.026 1 / V ₁ = 0.0015 φ _m / (φ _s + φ _m ) = 1 N ₁ = −0.00926 N ₂ = 1.5009 × 10 ⁻⁵ N ₂ · ER ² / N ₁ = −0.0065.

[0066] Example _{7 f = 2.61, F NO =} 2.8, 2ω = 66.8 °, IH = 1.6 r 1 = 17.3914 d 1 = 2 n d1 = 1.52542 ν d1 = 55.78 r 2 = 3.6729 ( aspherical) d ₂ = 14.2809 r ₃ = 5.7708 d ₃ = 3.8 nd ₂ = 1.84666 ν _d2 = 23.78 r ₄ = 3.977 d ₄ = 0.5 r ₅ = ∞ (aperture) d ₅ = 0 r ₆ = ∞ d ₆ = 16.9459 (GRIN) r ₇ = _{∞ d 7 = 1.1593 r 8 =} ∞ GRIN i N i V i 0 1.664 38.2 1 -9.26 × 10 -3 6.55 × 10 +2 2 -2.44 × 10 -5 6.55 × 10 +2 aspherical coefficients second surface P = 0.524 A ₄ = 3.66 × 10 ^-5 A ₆ = -6.24 × 10 ^-6 ν ₁ = 55.78 ν ₂ = 23.78 ν ₁ / ν ₂ = 2.35 | f / f ₂ | = 0.005 d ₂ /f=1.46 d ₃ / _{f 3 = 0 d G / f} G = 2.91 d G / f = 6.50 1 / V 0 = 0.026 1 / V 1 = 0.0015 φ m / (φ s + φ m) = 1 N 1 = -0.00926 N 2 = -2.4358 × 10 ^-5 N ₂ · ER ² / N ₁ = 0.011.

[0067] Example 8 (wide-angle _{state) f = 3.02, F NO =} 2.8, 2ω = 58.9 °, IH = 1.6 r 1 = -131.0731 ( _{aspherical) d 1 = 1 n d1 =} 1.52542 ν d1 = 55.78 r 2 _{= 3.9627 d 2 = 8.9557 r 3} = 7.7902 d 3 = 5 n d2 = 1.84666 ν d2 = 23.78 r 4 = 6.1173 d 4 = 0.5 r 5 = ∞ ( _{stop) d 5 = 1 r 6 =} ∞ d 6 = 12.5 ( _{GRIN) r 7 = ∞ d 7} = 3.8693 r 8 = ∞ d 8 = 0.75 n d4 = 1.48749 ν d4 = 70.23 r 9 = ∞ GRIN i n i V i 0 1.664 38.2 1 -9.26 × 10 -3 6.55 × 10 + ² 2 7.25 × 10 ^-7 6.55 × 10 ⁺² Aspheric coefficient First surface P = -4.99 × 10 ⁺⁴ A ₄ = 5.30 × 10 ^-4 A ₆ = 8.42 × 10 ^-6 ν ₁ = 55.78 ν ₂ = 23.78 ν ₁ / ν ₂ = 2.35 | f / f ₂ | = 0.033 d ₂ /f=1.66 d ₃ / f ₃ = 0.17 d _G / f _G = 2.13 d _G / f = 4.142 1 / V ₀ = 0.026 1 / V _{1 = 0.0015 φ m / (φ} s + φ m) = 1 N 1 = -0.00926 N 2 = 7.2457 × 10 -7 N 2 · ER 2 / N 1 = -0.0003 ( tele state) f = 5.88, F _NO = 2.7, 2ω = 31.5 °, IH = 1.6 r ₁ = ∞ (aperture) d ₁ = 1 r ₂ = ∞ d ₂ = 12.5 (GRIN) r ₃ = ｄ d ₃ = 0.96 r ₄ = _{∞ d 4 = 0.75 n d2 =} 1.48749 ν d2 = 70.23 r 5 = ∞ GRIN i n i V i 0 1.664 38.2 1 -9.26 × 10 -3 6.55 × 10 +2 2 7.25 × 10 -7 6.55 × 10 +2.

FIG. 6 shows aberration diagrams of the eighth embodiment. 6, (a) is an aberration diagram in a wide state, (b) is an aberration diagram in a telephoto state, SA is spherical aberration, AS is astigmatism, and D is
T indicates distortion and CC indicates chromatic aberration of magnification.

The above-described image forming optical system of the present invention can be constituted, for example, as follows. [1] A lens system including three groups, in order from the object side, a first group having a negative refractive power, a second group including one meniscus lens having a concave surface facing the image side, and at least one lens. An imaging optical system comprising a third lens group having a positive refractive power and including a refractive index distribution lens.

[2] The imaging optical system according to [1], wherein the gradient index lens is a radial type gradient index lens.

[3] The image forming optical system according to the above item 2, wherein the first group is composed of one concave lens, and the third group is composed of one radial type refractive index distribution lens.

[4] The above-mentioned item 2, wherein the radial type gradient index lens satisfies the following condition (4).
Or the imaging optical system according to 3. _{(4) -0.005 <1 / V} 1 <0.01 However, V ₁ is the Abbe number of the radial type refractive index distribution lens at d-line.

[5] The imaging optical system as described in any one of [2] to [4] above, wherein the radial type gradient index lens satisfies the following condition (11). (11) 2 <d _G / f <8, where d _G is the lens thickness of the radial type gradient index lens and f is the focal length of the entire system. [6] The radial type gradient index lens has the following conditions. The imaging optical system according to any one of the above items 2 to 5, which satisfies (12). (12) −0.2 <N ₂ · ER ² / N ₁ <0.2 where N ₁ and N ₂ are coefficients representing the refractive index distribution of the radial type gradient index lens, and ER is the radial type refractive index. The effective radius of the distribution lens.

[7] At least one surface of the radial type gradient index lens element is flat.
7. The imaging optical system according to any one of claims 1 to 6.

[8] The above-mentioned item 7, wherein the radial type gradient index lens element is a wood lens having two planes.
An imaging optical system as described in the above.

[0076]

[9] The imaging optical system according to any one of [1] to [8], wherein a brightness stop is arranged between the second group and the third group.

[10] The imaging optical system according to the above item 9, wherein the position of the aperture stop satisfies the following condition (14). (14) 0 <| d ₃ / f ₃ | <2, where d ₃ is the distance of the aperture stop from the object side of the radial type gradient index lens in the optical axis direction, and f ₃ is the focal point of the third lens unit. Distance.

[11] The imaging optical system according to any one of [1] to [10], wherein the following conditions (1) and (2) are satisfied. (1) d ₂ /f>0.2 (2) ν ₂ <50 where d ₂ is the lens thickness of the meniscus lens, ν ₂
Is the Abbe number of the glass material of the meniscus lens, and f is the focal length of the entire system.

[12] The imaging optical system according to any one of [1] to [11], wherein the following condition (3) is satisfied. (3) | f / f ₂ | <0.2 where f ₂ is the focal length of the second lens unit and f is the focal length of the entire system.

[13] The imaging optical system according to any one of [1] to [12], wherein the first unit has a concave lens having an aspherical surface.

[14] The first lens group satisfies the following condition (9):
14. The imaging optical system according to any one of the above items 1 to 13, further comprising a concave lens satisfying the following condition. (9) ν ₁ > 50 where ν ₁ is the Abbe number of the concave lens.

[15] The imaging optical system according to any one of [1] to [14], wherein a reflecting surface is provided between the first group and the second group.

[16] The imaging optical system according to the above item 15, wherein the reflection surface is a mirror.

[17] An interchangeable lens portion and a master lens portion are arranged in this order from the object side, and at least a part of the master lens portion and the image pickup surface are moved relative to the attachment or detachment or change of the interchangeable lens portion. An imaging optical system characterized in that the position can be changed.

[18] The above-mentioned item 17, wherein the master lens section includes a radial type gradient index lens element.
An imaging optical system as described in the above.

[19] The image forming optical system according to the above item 18, wherein the master lens section is composed of one radial type gradient index lens element.

[20] The imaging optical system according to the above item 18 or 19, wherein the radial type gradient index lens satisfies the following condition (4). _{(4) -0.005 <1 / V} 1 <0.01 However, V ₁ is the Abbe number of the radial type refractive index distribution lens at d-line.

[21] The imaging optical system according to any one of items 18 to 20, wherein the radial type gradient index lens satisfies the following condition (15). (15) d _G / f _G <2.3 where d _G is the lens thickness of the radial type gradient index lens, and f _G is the focal length of the gradient index lens.

[22] The imaging optical system according to any one of the above items 18 to 21, wherein at least one surface of the radial type gradient index lens is flat.

[23] The imaging optical system according to the above item 22, wherein the radial type gradient index lens is a wood lens having two planes.

[24] The image forming optical system according to any one of the above items 17 to 23, wherein the interchangeable lens portion comprises two lenses, and the master lens portion comprises one lens.

[25] An imaging apparatus using the imaging optical system according to any one of the above items 1 to 24.

[26] A video camera using the imaging optical system according to any one of [1] to [24].

[0094]

According to the imaging lens of the present invention, various aberrations can be satisfactorily corrected with the number of lenses as small as 3 and the angle of view is about 50 ° to 65 °.
It is possible to obtain a small-sized lens system covering a range from a so-called standard lens to a wide-angle lens.

[Brief description of the drawings]

FIG. 1 is a sectional view including an optical axis of an imaging optical system according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view including an optical axis of an imaging optical system according to a fourth embodiment of the present invention.

FIG. 3 is a sectional view including an optical axis of an imaging optical system according to a fifth embodiment of the present invention.

FIG. 4 is a sectional view including the optical axes of a wide-angle state and a telescopic state of an imaging optical system according to an eighth embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration when a reflecting surface is arranged in an imaging optical system according to a third embodiment of the present invention.

FIG. 6 is an aberration diagram of an image forming optical system according to Example 6 of the present invention.

FIG. 7 is a diagram showing that a camera can be miniaturized by the present invention.

FIG. 8 is a diagram showing that a camera can be miniaturized by making the interchangeable lens unit of the lens system switchable according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Camera 2 ... Lens system 3 ... Optical axis 4 ... Image surface 5 ... Reflection surface 6 ... Master lens part 7, 8, 9, 10 ... Interchangeable lens part 11 ... Switching mechanism 12 ... Turret 13 ... Central axis G1 ... 1st Group G2: Second group G3: Third group S: Aperture stop M: Reflecting surface (mirror)

Claims (2)

[Claims] 1. A lens system comprising three groups, in order from the object side, a first group having a negative refractive power, a second group consisting of one meniscus lens having a concave surface facing the image side, and at least one lens group.
An image forming optical system comprising: a third lens group having a positive refractive power and including two refractive index distribution lenses. 2. An image forming apparatus comprising: an interchangeable lens unit and a master lens unit in order from an object side; and a relative position between at least a part of the master lens unit and an imaging surface in response to attachment / detachment or change of the interchangeable lens unit. An imaging optical system characterized in that it is possible to change the value.