JP2006119574A - Variable magnification optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable magnification optical system which is compact and has a large aperture ratio and wider angle than heretofore while satisfactorily keeping an optical performance. <P>SOLUTION: The variable magnification optical system is constituted so that a negative first lens group G<SB>1</SB>, a diaphragm 1 and a positive second lens group G<SB>2</SB>are arranged in this order from the object side and, as proceeding from a wide angle end to a telephoto end, the variable power is performed by moving the second lens group to the object side on the optical axis and the movement of the image surface accompanying the variable power operation is corrected by moving the first lens group to the image forming surface side. The first lens group G<SB>1</SB>is composed of a first lens L<SB>1</SB>of a negative meniscus lens directing a convex face to the object side, a second lens L<SB>2</SB>of biconcave lens and a third lens L<SB>3</SB>of a positive lens directing a convex face to the object side. On the side closest to the object, two single lenses of a first lens L2-1 which has positive refractive power and has at least one surface made aspheric and a second lens L2-2 which has positive refractive power and is made of biconvex lens are arranged in this order from the object side. The variable magnification optical system satisfies the following conditional relations; nd21 > 1.55 (1), nd22 < 1.75 (2) and 1.0 < ¾f2/f1¾ < 1.8 (3), wherein nd21 is refractive index for d-ray of the first lens L2-1 of the second lens group, nd22 is refractive index for d-ray of the second lens L2-2 of the second lens group, f1 is focal distance of the first lens group and f2 is focal distance of the second lens group. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a variable magnification optical system having an angle of view of about 140 degrees used for various video cameras and electronic still cameras equipped with a CCD, and more particularly to a small, large aperture ratio and wide angle suitable for use in a surveillance camera. The present invention relates to a variable magnification optical system.

For example, the demand for surveillance cameras is increasing in order to operate an unmanned facility even at night, but in recent years, there is a demand for a camera that can be scaled. Such a variable power optical system for a surveillance camera is particularly required to be able to identify a subject even in a low-light environment and to have a wide angle, but is further required to be compact and have good optical performance. It is getting to be.
As an example of such a variable magnification optical system, there is a wide-angle zoom lens described in Patent Document 1, for example.

  The one described in this publication has a compact two-group configuration in which negative and positive lens groups are arranged from the object side. Such a configuration preceded by a lens unit having a negative refractive power is suitable for widening the angle and has a feature that it is relatively easy to secure a back focus. However, if a negative and positive two-group lens configuration is used from the object side, the entire lens system becomes asymmetrical, and aberration fluctuations accompanying zooming increase. It has been a challenge to improve the performance.

  In this publication, in the positive second lens group, an aspherical lens is arranged on the image forming surface side of a positive lens having a convex surface on the object side, so that at that time, the zooming ratio is small and has a large aperture ratio. An optical system is realized.

JP 2003-287777 A

  However, at present, there is a demand for a wide-angle variable magnification optical system capable of monitoring a wider range than the above-described conventional example for the surveillance camera lens. Of course, it is also necessary to make it the same level as the conventional one in terms of small size, large aperture ratio, and good optical performance.

  Here, looking at recent advances in molding technology for aspheric lenses, the aspherical surface is formed into a lens with a relatively small outer diameter and center thickness in consideration of the molding technology, as in the conventional example above. Therefore, it is now possible to mold a relatively large lens. Under such a technical background, a selection range of lenses capable of forming an aspheric surface is now widened.

  The present invention has been made in view of the above circumstances, and by using an aspheric lens effectively under the current technical background, it is compact, maintains a good optical performance, has a large aperture ratio, and has been conventionally used. Is intended to provide a variable magnification optical system with a wide angle.

In the variable power optical system of the present invention, in order from the object side, a first lens group having a negative refractive power, a stop, and a second lens group having a positive refractive power are arranged.
As the zoom lens moves from the wide-angle end to the telephoto end, zooming is performed by moving the second lens group to the object side on the optical axis, and the first lens group is moved to the imaging plane side. Is configured to correct the movement of the image plane due to this scaling operation,
The first lens group includes, in order from the object side, a first lens L1-1 having a negative refractive power, a second lens L1-2 having a negative refractive power, and a third lens L1-3 having a positive refractive power. Consisting of three single lenses
The second lens group includes a first lens L2-1 having a positive refractive power and at least one aspherical surface in order from the object side to the most object side, and a biconvex lens having a positive refractive power. Two single lenses of the second lens L2-2 are arranged,
The following conditional expressions (1) to (3) are satisfied.
nd ₂₁ > 1.55 (1)
nd ₂₂ <1.75 (2)
_{1.0 <| f 2 / f 1} | <1.8 ... (3)
However,
nd _21: refractive index nd ₂₂ at the d-line of the first lens L2-1 of the second lens group refractive index at the d-line of second lens L2-2 of the second lens group f _1: focal length of the first lens group f ₂ : Focal length of the second lens group

In the first lens group, the first lens L1-1 is a meniscus lens, the second lens L1-2 is a biconcave lens,
The second lens group has, on the image plane side of the second lens L2-2, a third lens L2-3 composed of a meniscus lens having a negative refractive power in order from the object side, and a positive refractive power. And a fourth lens L2-4 made of a biconvex lens is arranged, and is composed of four single lenses,
It is preferable that the following conditional expressions (4) and (5) are satisfied.
νd ₁₃ <27 (4)
νd ₂₃ <27 (5)
However,
[nu] d _13: Abbe number [nu] d ₂₃ for the d-line of the third lens L1-3 of the first lens group: Abbe number at the d-line of the third lens L2-3 of the second lens group

Moreover, it is preferable that the following conditional expression (6) is satisfied.
_{| Rf 22 / rb 22 | <} 1.0 ... (6)
However,
rf _22: curvature of the object side surface of the second lens L2-2 of the second lens group rb _22: curvature of the surface of the image plane side of the second lens L2-2 of the second lens group

  In addition, it is preferable that the first lens L2-1 of the second lens group is a biconvex lens whose both surfaces are aspheric.

  According to the variable magnification optical system according to the present invention, two single lenses including a first lens group including three predetermined single lenses and having a negative refractive power, a positive aspherical lens and a biconvex lens closest to the object side. The zoom lens is composed of a second lens unit having a positive refractive power in which a lens is arranged, and is configured to satisfy a predetermined conditional expression, thereby being compact and having a large aperture ratio while maintaining good optical performance. Although it is an optical system, it is possible to achieve a wider angle than the conventional one and to reduce the eccentricity sensitivity.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings and specific examples. FIG. 1 is a diagram showing the configuration of a variable magnification optical system according to Example 1 of the present invention, which will be described later.

As shown in the drawing, in this zoom optical system, in order from the object side, a first lens group G ₁ having negative refractive power, a diaphragm 1, and a second lens group G ₂ having positive refractive power are arranged. Being done. Further, in FIG. 1 is a filter unit 2 such as an infrared cut filter is disposed on the image plane side of the second lens group G _2, the light flux incident along the object side in the optical axis X to the solid-state image pickup element ( The image is formed at an imaging position on the imaging surface 3 of the CCD).

Further, the zoom lens system, upon zooming, the first lens group G ₁ and second lens group G ₂ is moved along the optical axis X. That is, as shown by arrow in FIG. 1, toward the telephoto end from the wide angle end (WIDE), so that the zooming is performed by moving the second lens group G ₂ on the object side on the optical axis X to, and is configured to correct the movement of the image plane accompanying the zooming operation by moving the first lens group G ₁ to the image plane side. Further, focusing is performed by moving along the first lens group G ₁ to the optical axis X.

The first lens group G ₁ of the zoom lens system, in order from the object side, a first lens having a negative refractive power L1-1, the second lens L1-2 having negative refractive power, and a positive refractive power The third lens L1-3 having a single lens. In the variable magnification optical system shown in FIG. 1, in order from the object side, a first lens L ₁ composed of a negative meniscus lens having a convex surface facing the object side, a second lens L ₂ composed of a biconcave lens, and a convex surface facing the object side. the third lens L ₃ are formed of a positive lens toward the equivalent. The shape of the first lens L _1, and has a shape suitable for wider angle.

The second lens group G ₂ of the zoom lens system, the most object side, in order from the object side, a positive first lens at least one surface has a refractive power is aspheric L2-1, and Two single lenses of a second lens L2-2 having a positive refractive power and made of a biconvex lens are arranged. In the variable magnification optical system shown in FIG. 1, the fourth lens L _{4 made of} a biconvex lens whose both surfaces are aspherical surfaces and the fifth lens L ₅ made of a biconvex lens correspond to these. Biconvex shape of the fourth lens L ₄ is a convex since the object side has a convex shape with a stronger curvature to gradually converges the light flux of choice, imaging surface side in order to obtain a good chromatic aberration correction It is determined because the shape is suitable.

Aspherical surface formed on the first lens L2-1 of the second lens group G ₂ is represented by the aspheric surface expression shown in the following Expression 1.

Further, in the zoom lens system shown in FIG. 1, the imaging surface side of the second lens L2-2 of the second lens group G _2, the third lens formed of a meniscus lens having a negative refractive power L2-3 , and a fourth lens L2-4 made of a biconvex lens having a positive refractive power, the sixth lens L _6, and the seventh lens L ₇ formed of a biconvex lens of a negative meniscus lens having a convex surface directed toward the object side Are arranged in order.
Thus, the variable magnification optical system includes a first lens group G ₁ of the three-element is a second lens group G ₂ and the sequence of the four lenses becomes, the seven compact lens as a whole structure and Has been.

In the variable power optical system of the present invention, the second lens group G ₂ is closest to the object side, a first lens L2-1, the positive refractive power of the lens in the order of the second lens L2-2 are arranged two, object The first lens L2-1 on the side becomes larger. Since the first lens L2-1 having a large diameter is a positive lens having a large refractive power, and the object side surface is a convex surface having a strong curvature, the entire lens system can be a bright lens system. .

  As described above, since the light beam that has passed through a high position of the first lens L2-1 having a large diameter conventionally passes through the aspherical surface in the second lens L2-2, the aberration in the lens peripheral portion is deteriorated. Ingenuity to suppress was carried out. However, this is unavoidably formed on the second lens L2-2 based on the design limitation that the aspherical surface is formed into a lens having a relatively small outer diameter and center thickness in consideration of molding technology. However, it is not always the best at present when an aspherical surface can be formed up to a relatively large lens. At present, the selection range of lenses capable of forming an aspheric surface has expanded, and it is possible to enjoy the merits of forming an aspheric surface on the first lens L2-1.

  In the variable magnification optical system of the present invention, by forming an aspherical surface on the first lens L2-1 that is a positive lens having a large diameter and a large refractive power, it is possible to suppress the deterioration of aberration in the lens peripheral portion. When the first lens L2-1 is an aspheric lens, there is a margin in performance, such as better correction of spherical aberration, than when the second lens L2-2 is an aspheric lens. Is possible.

  As a result, according to the variable magnification optical system of the present invention, the optical performance can be maintained satisfactorily while the lens is bright, and the decentration sensitivity can be relaxed, which is very advantageous in manufacturing.

In the variable power optical system of the present invention, the first lens L2-1 also first in the second lens group G ₂ than other lens, since the emphasis the benefits due to the aspherical lens, the optical system The overall configuration is also suitable for making the first lens L2-1 an aspherical lens. The second lens group G _2, the shape of the third lens L2-3 made of meniscus lens having a negative refractive power are examples.

In the prior art in the present invention, in order to converge the light flux diverged by the first lens group G ₁ having a negative refractive power, the first lens L2-1 of the second lens group G ₂ is large curvature of the object side It has become. Accordingly, the positive refractive power increases as the distance from the optical axis increases, and the spherical aberration becomes excessively corrected toward the periphery.

In the above prior art, and therefore, the third lens L2-3 having negative refractive power in the second lens group G _2, since the required stronger negative refractive power with distance from the optical axis, the lens both It is a concave lens.

  On the other hand, in the variable magnification optical system of the present invention, even if the spherical aberration is excessively corrected in the peripheral portion due to the curvature on the object side of the first lens L2-1, the shape that weakens the positive refractive power in the peripheral portion. Is provided on the first lens L2-1, the generation amount of spherical aberration can be suppressed. Therefore, in the variable magnification optical system of the present invention, the third lens L2-3 having negative refractive power does not need to have strong negative refractive power in the peripheral portion, and has a meniscus shape.

  In addition, this variable power optical system is compact and maintains a good optical performance by satisfying the following conditional expressions (1) to (3), while being a variable power optical system having a large aperture ratio, A wider angle than before can be achieved.

nd ₂₁ > 1.55 (1)
nd ₂₂ <1.75 (2)
_{1.0 <| f 2 / f 1} | <1.8 ... (3)
However,
nd _21: second lens group index at the d-line of the first lens L2-1 of _{G 2} nd _22: refractive index at the d-line of the second lens L2-2 of the second lens group _{G 2} f _1: the first lens Focal length of group G ₁ f ₂ : Focal length of second lens group G ₂

  In conditional expression (1) below this lower limit, the curvature in the vicinity of the optical axis increases, making it difficult to manufacture an aspheric lens.

In conditional expression (2) above, if this upper limit is exceeded, spherical aberration will be overcorrected, making it difficult to maintain good optical performance. The aberration of the second lens L2-2 as a single lens can be reduced by reducing the curvature by using a material having a high refractive index. However, this may deteriorate the performance of the entire optical system. is there. For these reasons, the upper limit value of the refractive index is defined to improve the optical performance of the entire optical system. The present inventors have confirmed that such good optical performance is maintained up to about 1.75. If the conditional expression nd ₂₂ <1.65 is satisfied, The optical performance can be further improved.

In the above-mentioned conditional expression (3), and at the same time a negative refractive power weakens angle of the lower limit value below the first lens group G ₁ becomes difficult, because the positive refractive power of the second lens group G ₂ becomes strong, Spherical aberration is overcorrected, making it difficult to achieve a large aperture ratio. Also, the positive refractive power of the value exceeds the upper limit value when the second lens group G ₂ is weakened, compact becomes difficult. A first lens group G _1, by defining the power distribution between the second lens group G _2, large aperture ratio, it is possible to angle and compactness are a well-balanced the achieved variable magnification optical system. Conversely, in an optical system that does not satisfy this conditional expression (3), the application is different from that of the present invention, or even if the optical system is used for the same application, the overall size and focal length of the optical system are It is considered to be significantly different and cannot be compared with the present invention.

  Further satisfactory optical performance can be obtained by satisfying the following conditional expressions (4) and (5) in addition to the above three conditional expressions.

νd ₁₃ <27 (4)
νd ₂₃ <27 (5)
However,
[nu] d _13: Abbe number at the d-line of the third lens L2-3 of the second lens group _{G 2:} first lens group _{G 1} of the Abbe number [nu] d ₂₃ with respect to the d-line of the third lens L1-3

In the above conditional expressions (4) and (5), if this upper limit is exceeded, it is difficult to correct axial chromatic aberration, and optical performance cannot be maintained. In order to further improve the axial chromatic aberration, it is desirable that νd ₁₃ = νd ₂₃ .

In addition to the above conditional expressions (1) to (3) or (1) to (5), by satisfying the following conditional expression (6), a variable magnification optical system more advantageous in manufacturing can be obtained.
_{| Rf 22 / rb 22 | <} 1.0 ... (6)
However,
rf _22: curvature of the object side surface of the second lens L2-2 of the second lens group _{G 2} radius rb _22: curvature of the surface of the image plane side of the second lens L2-2 of the second lens group _{G 2} radius

  In the conditional expression (6), when the value exceeds the upper limit, the curvature of the image-side surface is larger than the object-side surface, and the ray angle on the axis is large, so the second lens L2-2. The optical performance is greatly deteriorated when the arrangement position of is shifted in the direction perpendicular to the optical axis. When the eccentricity sensitivity is increased, it is necessary to increase the component accuracy, which is not preferable in manufacturing.

Specific examples of the variable magnification optical system according to the present invention will be described below.
<Example 1>
The configuration of the variable magnification optical system according to Example 1 is as shown in FIG.

In the upper part of Table 1 below, the radius of curvature R (mm) of each lens surface of Example 1 and the axial top surface spacing of each lens (center thickness of each lens and the air spacing between each lens; the same in Tables 2 and 3) ) D (mm), the refractive index N _d and the Abbe number ν _d of each lens at the d-line. In Table 1 and Tables 2 and 3 below, the numbers corresponding to the respective symbols are sequentially increased from the object side, and the surface with * on the left side of the surface number is an aspherical surface. Represents. The radius of curvature R of these aspheric surfaces is shown as the value of the radius of curvature R on the optical axis in each table, but in the corresponding lens configuration diagram, the leader line is not necessarily the optical axis. Some are not drawn from the intersection with.

In the middle of Table 1 below, the focal length f (mm), the F value, the angle of view 2ω (degrees), and the variable value 1 to variable 3 of the shaft upper surface distance at the wide angle end (WIDE) and the telephoto end (TELE) are shown. Indicates.
Further, the lower part of Table 1 below shows the eccentricity K and third to _tenth aspheric coefficients A _{3 to} A _{10 related} to the aspheric surface.

  FIG. 2 is an aberration diagram showing various aberrations (spherical aberration, astigmatism, distortion) at the wide-angle end (WIDE) and the telephoto end (TELE) of the variable magnification optical system according to Example 1. In these aberration diagrams and FIGS. 6 and 8 below, ω represents a half angle of view. Each aberration diagram of astigmatism shows aberrations on the sagittal image surface and the tangential image surface (the same applies to FIGS. 6 and 8).

  As shown in Table 1 and FIG. 2, the variable magnification optical system of Example 1 has a bright F value of 0.99 to 1.59, a wide angle of view angle 2ω of about 138 to about 49 °, and each aberration is excellent. It is a high-performance variable magnification optical system that can be corrected to

In the variable magnification optical system according to the present invention, such as those of Example 1, the first lens L2-1 of the second lens group G ₂ (fourth lens L ₄ in FIG. _1), the aspherical By using the biconvex lens, a higher aberration correction effect can be obtained.

Here, by comparing the first example with the conventional example, the effect of reducing the decentering sensitivity by the variable power optical system of the present invention in which the first lens L2-1 is an aspherical lens will be specifically shown.
FIG. 3 shows a case where the first lens L2-1, which is an aspherical lens, is disposed at the design position (before decentering) at the wide-angle end of the variable magnification optical system according to Example 1, and is perpendicular to the optical axis. It is the figure which showed the change of an aberration at the time of decentering 0.05mm (after decentering). FIG. 4 is a diagram showing changes in aberrations before and after decentration at the telephoto end of the variable magnification optical system according to Example 1 as in FIG.

  As a comparative example, FIG. 9 shows the second lens L2-2, which is an aspherical lens, at the design position at the wide-angle end of the wide-angle zoom lens according to Example 1 described in Japanese Patent Laid-Open No. 2003-287777, which is a conventional example. It is the figure which showed the change of an aberration when arrange | positioning (before decentration) and when decentering 0.05 mm perpendicularly | vertically with respect to an optical axis (after decentration). FIG. 10 is a diagram showing the change in aberrations before and after decentration at the telephoto end of the wide-angle zoom lens according to Example 1 of the conventional example, as in FIG.

  However, since the image size of Example 1 of the present invention is different from that of Example 1 of the conventional example, the scale of the vertical axis of the lateral aberration diagram is adjusted so that the following relational expression is satisfied. , Shown to be comparable.

SCAb = (Db / Da) × SCAa
However,
Da: Image size of the variable magnification optical system according to Example 1 of the present invention SCAa: Maximum value on the vertical axis of the aberration diagram of the variable magnification optical system according to Example 1 of the present invention Db: According to Example 1 of the conventional example Image size of wide-angle zoom lens SCAb: Maximum value on the vertical axis of the aberration diagram of the wide-angle zoom lens according to Example 1 of the conventional example

In the present invention and the conventional example, both achieve a small size with a large aperture ratio. However, in FIGS. 9 and 10 corresponding to FIGS. 3 and 4, the change in aberration after decentering becomes large. cage, in an aspheric lens or a subsequent stage of the lens or the most object side of the second lens group G _2, it is apparent that eccentric sensitivity significantly different.

<Example 2>
The configuration of the zoom optical system according to Example 2 is as shown in FIG.
In the upper part of Table 2 below, the curvature radius R (mm) of each lens surface of Example 2, the axial distance D (mm) of each lens, the refractive index N _d and the Abbe number ν _d of each lens at the d-line are shown. Show.

In the middle of Table 2 below, the focal length f (mm), F value, angle of view 2ω (degrees), and variable values 1 to 3 of the shaft upper surface distance at the wide angle end (WIDE) and the telephoto end (TELE) are shown. Indicates.
Furthermore, the lower part of the following Table 2 shows the eccentricity K and the third to _tenth aspheric coefficients A _{3 to} A _{10 related} to the aspheric surface.

FIG. 6 is an aberration diagram showing various aberrations (spherical aberration, astigmatism, distortion) at the wide-angle end (WIDE) and the telephoto end (TELE) of the variable magnification optical system according to Example 2.
As shown in Table 2 and FIG. 6, the variable magnification optical system of Example 2 has a bright F value of 0.99 to 1.58, a wide angle of view 2ω of about 138 to about 49 °, and each aberration is excellent. It is a high-performance variable magnification optical system that can be corrected to

<Example 3>
The configuration of the zoom optical system according to Example 3 is as shown in FIG.
In the upper part of Table 3 below, the curvature radius R (mm) of each lens surface of Example 3, the axial distance D (mm) of each lens, the refractive index N _d and the Abbe number ν _d of each lens at the d-line are shown. Show.

In the middle of Table 3 below, the focal length f (mm), the F value, the angle of view 2ω (degrees), and the variable of the shaft upper surface distance at the wide angle end (WIDE) and the telephoto end (TELE) are variable 1 to variable 3 values. Indicates.
Further, the lower part of Table 3 below shows the eccentricity K and third to _tenth aspheric coefficients A _{3 to} A _{10 related} to the aspheric surface.

  FIG. 8 is an aberration diagram showing various aberrations (spherical aberration, astigmatism, distortion) at the wide-angle end (WIDE) and the telephoto end (TELE) of the variable magnification optical system according to Example 3.

  As shown in Table 3 and FIG. 8, the variable power optical system of Example 3 has a bright F value of 0.99 to 1.71, a wide angle of view angle 2ω of about 129 to about 44 °, and each aberration is excellent. It is a high-performance variable magnification optical system that can be corrected to

  Table 4 below shows values corresponding to the conditional expressions (1) to (6) for Example 1, Example 2, and Example 3. Example 1, Example 2, and Example 3 all satisfy all conditional expressions (1) to (6).

  The variable magnification optical system according to the present invention is not limited to the above-described embodiments, and various other modifications can be made. For example, the shape of the lenses constituting each lens group can be changed as appropriate. In the second lens group, the number of lenses closer to the image plane than the second lens L2-2 can be changed as appropriate.

  In the variable power optical system of the present invention, in addition to the first lens L2-1 of the second lens group, an aspherical lens can be further arranged to further improve the optical performance. For example, the second lens L2-2 of the second lens group may also be configured as an aspheric lens.

  In addition, each lens constituting the variable magnification optical system of the above embodiment is formed of a glass glass material. In particular, a lens having an aspheric surface can be formed of a plastic material advantageous in terms of cost instead of the glass glass material. Is possible.

FIG. 3 is a diagram illustrating a configuration of a variable magnification optical system according to the first embodiment. Aberration diagrams showing various aberrations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 1 The figure which shows the change of the aberration accompanying the axial shift | offset | difference eccentricity in the wide angle end of Example 1. FIG. The figure which shows the change of the aberration accompanying axial deviation eccentricity in the telephoto end of Example 1. FIG. 5 is a diagram illustrating a configuration of a variable magnification optical system according to the second embodiment. Aberration diagrams showing various aberrations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 2. FIG. 10 is a diagram illustrating a configuration of a variable magnification optical system according to the third embodiment. Aberration diagrams showing various aberrations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 3 The figure which shows the change of the aberration accompanying the axial shift | offset | difference eccentricity in the wide angle end of a prior art example. The figure which shows the change of the aberration accompanying axial deviation eccentricity in the telephoto end of a conventional example

Explanation of symbols

L ₁ ~L ₇ lens G _{1, G 2} lens groups X optical axis 1 stop 2 filter unit 3 a solid-state imaging device (CCD) imaging surface

Claims (4)

In order from the object side, a first lens group having a negative refractive power, a stop, and a second lens group having a positive refractive power are arranged.
As the zoom lens moves from the wide-angle end to the telephoto end, zooming is performed by moving the second lens group to the object side on the optical axis, and the first lens group is moved to the imaging plane side. Is configured to correct the movement of the image plane due to this scaling operation,
The first lens group includes, in order from the object side, a first lens L1-1 having a negative refractive power, a second lens L1-2 having a negative refractive power, and a third lens L1-3 having a positive refractive power. Consisting of three single lenses
The second lens group includes a first lens L2-1 having a positive refractive power and at least one aspherical surface in order from the object side to the most object side, and a biconvex lens having a positive refractive power. Two single lenses of the second lens L2-2 are arranged,
A variable magnification optical system characterized by satisfying the following conditional expressions (1) to (3):
nd ₂₁ > 1.55 (1)
nd ₂₂ <1.75 (2)
_{1.0 <| f 2 / f 1} | <1.8 ... (3)
However,
nd _21: refractive index nd ₂₂ at the d-line of the first lens L2-1 of the second lens group refractive index at the d-line of second lens L2-2 of the second lens group f _1: focal length of the first lens group f ₂ : Focal length of the second lens group In the first lens group, the first lens L1-1 is a meniscus lens, the second lens L1-2 is a biconcave lens,
The second lens group has, on the image plane side of the second lens L2-2, a third lens L2-3 composed of a meniscus lens having a negative refractive power in order from the object side, and a positive refractive power. And a fourth lens L2-4 made of a biconvex lens is arranged, and is composed of four single lenses,
The zoom lens system according to claim 1, wherein the following conditional expressions (4) and (5) are satisfied.
νd ₁₃ <27 (4)
νd ₂₃ <27 (5)
However,
[nu] d _13: Abbe number [nu] d ₂₃ for the d-line of the third lens L1-3 of the first lens group: Abbe number at the d-line of the third lens L2-3 of the second lens group The zoom lens system according to claim 1 or 2, wherein the following conditional expression (6) is satisfied.
_{| Rf 22 / rb 22 | <} 1.0 ... (6)
However,
rf _22: curvature of the object side surface of the second lens L2-2 of the second lens group rb _22: curvature of the surface of the image plane side of the second lens L2-2 of the second lens group 4. The variable magnification optical system according to claim 1, wherein the first lens L <b> 2-1 of the second lens group is a biconvex lens having both aspheric surfaces.

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