JPH11223771A - Vari-focal lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized vari-focal lens system suitable for a wide angle and a high aperture ratio by moving a first lens group and a third lens group to the object side as one body at the time of the change of the focal length from the wide angle end state to the telephoto end state. SOLUTION: When the focal length is changed from the wide angle end state to the telephoto end state, a first lens group G1 and a third lens group G3 are moved as one body so that the distance between a first lens group G1 and a second lens group G2 is increased and that between the second lens group G2 and a third lens group G3 is reduced. They satisfy condition formulas I: 0.9<D/(ft-fw)<1.5 and II: 0.5<Cave.ft/FNO<0.8 where D, fw, ft, Cave, and FNO are the distance along the optical axis from the lens face of the first lens group to that of the third lens group, the focal length in the wide angle end state, the focal length in the telephoto end state, the average value of absolute values of paraxial curvatures of individual lens faces constituting the second lens group, and the aperture ratio in the telephoto end state respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact variable focal length lens system, and more particularly to a variable focal length lens system having a small F-number in a telephoto end state and a large aperture ratio covering a wide angle range.

[0002]

2. Description of the Related Art In recent years, in a lens shutter type camera,
Cameras with a zoom lens are common. The portability of the lens shutter type camera is emphasized, and the size and weight of the taking lens have been reduced in order to reduce the size and weight.

In particular, since the zoom lens has a large focal length at the telephoto end, the portability is improved when the camera is not in use, so that the distance between adjacent lens groups is minimized in the camera body. Stored. The larger the F-number, the more advantageous for miniaturization, and the larger the focal length, the more powerful the photo can be taken closer to the subject. Therefore, a zoom lens with a large focal length at the telephoto end and a large F-number is provided. Cameras were mainstream.

[0004]

However, since the shutter speed becomes slower as the F-number becomes larger, camera shake due to camera shake or the like is likely to occur, and as the focal length becomes longer, minute camera shake becomes more pronounced. There is a problem that the image is recorded as large image blur. In addition, a smaller focal length (that is, a wider angle) facilitates downsizing of the lens system, and is suitable for downsizing the camera body.

[0005] However, if both wide angle and large diameter are compatible,
It is difficult to correct coma aberration and positive distortion generated at the peripheral portion of the screen, and wide angle has not been sufficiently achieved. An object of the present invention is to solve the above problems and to provide a small variable focal length lens system suitable for widening the angle of view and increasing the aperture ratio.

[0006]

In order to achieve the above object, a variable focal length lens system according to a first aspect of the present invention comprises, in order from the object side, a first lens group having a positive refractive power, An aperture stop is provided between the first lens group and the second lens group. The second lens group has a refractive power and the third lens group has a negative refractive power. When the focal length changes to the end state, the distance between the first lens group and the second lens group increases, and the distance between the second lens group and the third lens group decreases. The first and third lens groups move integrally to the object side, and the second lens group moves to the object side, thereby satisfying the following conditional expressions (1) and (2). (1) 0.9 <D / (ft−fw) <1.5 (2) 0.5 <Cave. · Ft / FNO <0.8, where D: closest to the object side in the first lens group Distance along the optical axis from the lens surface located to the lens surface located closest to the image side of the third lens group, fw: focal length in the wide-angle end state, ft: focal length in the telephoto end state, CAve .: The average value of the absolute value of the paraxial curvature of each lens surface constituting the second lens group, FNO: aperture ratio in the telephoto end state.

In order to achieve the above object, a variable focal length lens system according to a second aspect of the present invention comprises, in order from the object side, a first lens group having a positive refractive power and a first lens group having a positive refractive power. When the focal length changes from the wide-angle end state to the telephoto end state, the distance between the first lens group and the second lens group increases. Then, all the lens groups move toward the object side so that the distance between the second lens group and the third lens group decreases, and the first lens group and the third lens group are integrated. Moving to dispose an aperture stop between the first lens group and the second lens group so that the first lens group and the third lens group are integrally positioned on the object side during short-distance focusing. To the distance between the second lens group and the first lens group. It is to move toward the object side while reduction.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the configuration of a variable focal length lens system according to the present invention will be described. The variable focal length lens system according to the present invention includes, as shown in FIG. 1, for example, a first lens unit having a positive refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power in order from the object side. When the lens position changes from the wide-angle end state to the telephoto end state, the distance between the first lens group and the second lens group increases, and the distance between the second lens group and the third lens group decreases. All three lens groups move to the object side. Here, the aperture stop is arranged between the first lens group and the second lens group and near the second lens group.

In general, the aperture stop is often arranged near the center of the optical system, and is disposed between the positive lens unit and the negative lens unit in a positive / negative two-unit zoom lens, and is disposed in the second position in a positive / negative three-unit zoom lens. It is arranged adjacent to the lens group. In the conventional positive / negative two-unit zoom, the first lens unit includes a negative subunit and a positive / negative subunit disposed on the image side thereof in order to secure a sufficient back focus in the wide-angle end state. For this reason, when the aperture is increased at the telephoto end, it is necessary to increase the aperture diameter extremely. However, in the above-described positive / negative three-unit zoom lens according to the present invention, the distance between the first lens unit and the second lens unit is widened at the telephoto end, so that the light flux by the first lens unit is converged. Therefore, the aperture diameter can be reduced as compared with the conventional positive / negative two-unit zoom.

In a so-called multi-unit zoom lens having three or more movable lens groups, when the focal length changes from the wide-angle end state to the telephoto end state, the zoom trajectory of each lens group increases as the number of movable lens groups increases. The freedom to choose is increased. By increasing the number of movable lens groups, it is possible to achieve a high zoom ratio and a large aperture ratio, but there is a problem that the fluctuation of the image plane position due to the lens position stop accuracy generated during manufacturing becomes large. Particularly, when the F-number in the telephoto end state is reduced (when the aperture ratio is increased), a higher lens position stop accuracy is required because the depth of field is narrow.

On the other hand, in the present invention, when the focal length state is changed, the zoom trajectory is selected so that the first lens group and the third lens group move integrally. Variations in the image plane position (in the direction of the optical axis) due to accuracy can be suppressed, and excellent optical performance can be achieved. Next, the function of each lens group will be described.

The first lens group has a converging function. In the telephoto end state, the overall length of the lens is shortened by increasing the distance between the first lens group and the second lens group. In the wide-angle end state, the overall length of the lens is shortened, and an off-axis light beam passing through the first lens group is brought closer to the optical axis, thereby suppressing the occurrence of coma. In the second lens group, the aperture stop is arranged near, and the off-axis light beam passes near the optical axis. Accordingly, axial aberration is mainly corrected. The second lens group also has a function of maintaining the back focus value in the wide-angle end state of the variable focal length lens system in an appropriate range.

If the back focus is extremely short in the wide-angle end state, there is a problem that a shadow of dust adhering to the lens surface closest to the image in the third lens unit is recorded on the film surface. Alternatively, to secure a predetermined amount of peripheral light,
Problems such as an increase in the lens diameter also occur. Conversely, if the back focus becomes too large, the off-axis light beam passing through the third lens group approaches the optical axis, and the fluctuation of coma due to the change in the angle of view cannot be corrected well. Therefore, it is important to set the back focus in the wide-angle end state to an appropriate value.

The third lens group enlarges the subject image formed by the first lens group and the second lens group. As the lens position changes from the wide-angle end state to the telephoto end state, the magnification (ie, the lateral magnification) increases. Magnification). In the wide-angle end state, by setting the back focus to an appropriate value as described above,
An off-axis light beam passing through the third lens group is separated from the optical axis, and as a result, the third lens group can easily correct off-axis aberrations. Further, as the back focus becomes larger (that is, the lens position changes from the wide-angle end state to the telephoto end state), the height of the off-axis light beam passing through the third lens group is closer to the optical axis, so that the magnification is changed. The accompanying fluctuation of off-axis aberration can be easily corrected.

In the first aspect of the present invention, with the above configuration, the variable focal length lens system is configured to satisfy the following conditions and so as to achieve a large aperture at the telephoto end. , Miniaturization of the lens system has been achieved. The distance between the most object-side lens surface of the first lens group and the most image-side lens surface of the third lens group is appropriately set; the average curvature of the lenses constituting the second lens group is relaxed; In the variable focal length lens system according to the present invention, the first lens unit and the second lens unit approach each other in the wide-angle end state, and the second lens unit and the third lens unit close in the telephoto end state. The lens group approaches. Accordingly, when the distance between the most object side lens surface of the first lens group and the most image side lens surface of the third lens group becomes large, the off-axis light flux passing through the third lens group at the wide angle end state is shifted from the optical axis. An off-axis light beam that passes through the first lens group in the far and telephoto end state is separated from the optical axis, resulting in an increase in the lens diameter.

Conversely, when the distance between the lens surface closest to the object in the first lens unit and the lens surface closest to the image in the third lens unit is reduced, the refracting power of each lens unit is increased, and the accuracy of the lens position is reduced. As a result, the fluctuation of the image plane position becomes large (that is, the displacement of the image plane position generated when each lens group is displaced by a minute amount in the optical axis direction becomes large), and a predetermined optical performance cannot be obtained. .

Therefore, it is desirable to appropriately set the distance between the lens surface closest to the object in the first lens unit and the lens surface closest to the image in the third lens unit, and conditions are required. In order to increase the aperture ratio, if the radius of curvature of each lens surface constituting the lens system is small, high-order spherical aberration is likely to occur, and it is difficult to achieve predetermined optical performance, and performance due to mutual eccentricity of each lens There is a problem that the deterioration becomes very large.

In the variable focal length lens system according to the first aspect of the present invention, since the second lens group is arranged near the aperture stop and mainly corrects the axial aberration, each of the second lens group is constituted. It is important to relax the curvature of the lens surface, and conditions are required. In a positive / negative three-unit zoom lens, the second lens is required to secure a sufficient back focus at the wide-angle end.
In many cases, the lens unit is composed of a negative sub-unit and a positive sub-unit disposed on the image side of the negative sub-unit.

Here, the arrangement of the aperture stop in the positive / negative / negative three-unit zoom lens includes (1) when the aperture stop is disposed between the first lens unit and the second lens unit, and (2) the aperture stop. When the aperture stop is disposed in the second lens group, there are three cases: (3) when the aperture stop is disposed between the second lens group and the third lens group.

In the above cases (2) and (3), it is difficult to achieve both sufficient back focus in the wide-angle end state and satisfactory aberration correction after downsizing. For example, in order to secure sufficient back focus at the wide-angle end state, if the lens surface closest to the object side of the negative sub-group has a shape facing a strongly concave surface toward the object side, the amount of off-axis aberration that occurs at the wide-angle end state is reduced. It becomes difficult to achieve a wide angle because of the large size. Also, if an attempt is made to reduce the amount of off-axis aberration in the wide-angle end state, it becomes impossible to sufficiently secure the back focus in the wide-angle end state.

Therefore, in the present invention, by arranging the aperture stop on the object side of the second lens group as described in (1) above, off-axis generated on the lens surface of the second lens group located closest to the object side is obtained. Suppress aberration, and achieve both large diameter and small size. Therefore, conditions are required. In the first embodiment of the present invention, the conditional expression (1) is defined to satisfy the above, and the conditional expression (2) is defined to satisfy the above.

Hereinafter, each conditional expression will be described. Conditional expression (1) is a conditional expression that defines the distance between the most object side surface of the first lens group and the most image side surface of the third lens group. When the value exceeds the upper limit value of the conditional expression (1), the lens diameter is increased as described above. Conversely, when the value goes below the lower limit value of the conditional expression (1), the fluctuation of the image plane position due to the variation of the lens stop position is caused. Becomes large, and as a result, predetermined optical performance cannot be obtained.

Conditional expression (2) is a conditional expression that defines the average curvature of the second lens group. When the value exceeds the upper limit of conditional expression (2), as described above, performance degradation due to mutual eccentricity of each lens occurring at the time of manufacturing is caused, and predetermined optical performance cannot be secured. Conversely, if the lower limit of conditional expression (2) is not reached, sufficient back focus cannot be secured in the wide-angle end state, and the lens diameter of the third lens group will be increased.

The FNO used in the conditional expression (2) indicates the aperture ratio in the state where the aperture ratio becomes the smallest at the telephoto end in terms of optical design. In the first aspect of the present invention, the second lens group includes only a negative meniscus lens having a concave surface facing the aperture stop, and a biconvex positive lens disposed on the image side of the negative lens. Preferably, it is configured.

Here, the concave surface of the meniscus-shaped negative lens has a function of securing a sufficient back focus in the wide-angle end state and correcting negative spherical aberration generated in the telephoto end state. On the other hand, if the lens surface closest to the object side of the second lens group becomes convex toward the object side, if the focal length at the wide-angle end is shortened, sufficient back focus cannot be secured, causing an increase in the lens diameter. There is a risk that it will.

Further, in the variable focal length lens system according to the first aspect of the present invention, it is preferable that the second lens group is composed of two pieces as described above. On the other hand, if the number of lenses constituting the second lens group is too large, the off-axis light beam passing through the third lens group will be extremely separated from the optical axis when the angle of view is widened. This can be difficult to achieve.

In the variable focal length lens system according to the first aspect of the present invention, the second lens group having a positive refractive power preferably has a meniscus-shaped negative lens and a positive lens. As a result, the second
Performance degradation due to mutual decentration between the negative lens and the positive lens in the lens group can be suppressed. In this configuration, the most object-side lens surface of the second lens group (the object-side lens surface of the negative meniscus lens) is an aspheric surface, and the most image-side surface (the image-side lens surface of the positive lens). Is preferably an aspherical surface. In the present invention, since the aperture stop is arranged on the object side of the second lens group, a light beam having a large angle of view enters the second lens group in the wide-angle end state. Therefore, the off-axis light flux passing through the positive lens of the second lens group passes through a position away from the optical axis in the wide-angle end state. here,
Higher optical performance can be obtained by making the lens surface on the image side of the positive lens in the second lens group an aspherical surface and satisfactorily correcting the variation of coma due to the angle of view.

Further, if the object side of the meniscus-shaped negative lens is made aspherical, it is possible to satisfactorily correct negative spherical aberration generated by the second lens unit alone, thereby achieving a large aperture ratio. Can be. In the variable focal length lens system according to the first aspect of the present invention, it is preferable that each of the first to third lens groups includes two negative lenses and two positive lenses. As a result, it is possible to satisfactorily correct at least axial aberration generated in each lens group, and to achieve desired optical performance at an arbitrary focal length from the wide-angle end state to the telephoto end state.

The variable focal length lens system according to the second aspect of the present invention comprises, as shown in FIG. 1, for example, a first lens unit having a positive refractive power and a second lens unit having a positive refractive power in order from the object side. It has a lens group and a third lens group having negative refracting power, and is basically configured such that an aperture stop is arranged between the first lens group and the second lens group. When the focal length changes from the wide-angle end state to the telephoto end state, the distance between the first lens group and the second lens group increases, and the distance between the second lens group and the third lens group decreases. Then, the first lens group and the third lens group move integrally to the object side, and the second lens group moves to the object side.

In focusing on a short distance, the first lens unit and the third lens unit move integrally to the object side, and
The lens group moves toward the object side while changing the distance from the first lens group. With this configuration, it is possible to achieve good imaging performance even when a short distance focus is set on an arbitrary subject position from an infinity position to a short distance position.

In focusing on a short distance, there are (A) a method of moving one lens group and (B) a method of moving a plurality of lens groups. (A) When one lens group is moved, it is necessary to satisfactorily correct the fluctuation of off-axis aberration that occurs at the time of short-distance focusing. In order to achieve a predetermined optical performance in an arbitrary focal length state, it is necessary to satisfactorily correct the axial aberration generated in each lens group. The change in aberration can be easily corrected, but it has been difficult to satisfactorily correct the change in off-axis aberration. (B) In the case of moving a plurality of lens groups, by changing the distance between the lens groups, it is possible to cancel out the fluctuation of off-axis aberration that occurs at the time of focusing on a close distance, so that the optical design The degree of freedom is increased, and as a result, downsizing, large diameter, and the like can be achieved.

Based on the above (A) and (B), the second embodiment of the present invention is configured to move a plurality of lens groups. In the second aspect of the present invention, the first lens group and the third lens group move integrally according to the change of the focal length state. It is desirable that the first lens group and the third lens group move integrally.

At this time, (a) a method in which the first lens group and the third lens group move to the object side, and the second lens group moves so as to change the distance from the first lens group; b)
A method is considered in which the first lens group and the third lens group move toward the image side, and the second lens group moves so as to change the distance between the first lens group and the first lens group. In the case of (b), since the angle of view is widened as the subject becomes closer, it is difficult to improve the performance. Therefore, in the second aspect of the present invention, it is desirable that the first lens group and the third lens group move to the object side as described in (a) above.

In the first and second aspects of the present invention,
In order to achieve a balance between high performance and miniaturization, it is desirable to satisfy the following conditional expression (3). (3) 0.3 <| f3 | / f1 <0.45 When the value exceeds the upper limit of conditional expression (3), the off-axis light flux passing through the first lens unit to the third lens unit in the wide-angle end state becomes light. Since it is separated from the axis, it is difficult to reduce the lens diameter.

Conversely, if the lower limit of conditional expression (3) is not reached, the convergence effect of the first lens unit is weakened and the divergence effect of the third lens unit is increased, so that the total lens length at the telephoto end is increased. Would. In addition, the first aspect of the present invention
In the second aspect, it is desirable that the following conditional expression (4) is satisfied in order to achieve high performance. (4) 0.7 <f2 / fw <0.85 Conditional expression (4) is a conditional expression defining the focal length of the second lens group.

When the value exceeds the upper limit of conditional expression (4), the convergence effect of the second lens unit is weakened, so that the off-axis light beam passing through the third lens unit in the wide-angle end state separates from the optical axis and becomes positive. Distortion cannot be properly corrected. When the value goes below the lower limit of conditional expression (4), a large amount of negative spherical aberration generated by the second lens unit alone occurs, so that predetermined optical performance cannot be obtained.

Further, in the first and second aspects of the present invention, in addition to the configuration of the claims, the following configuration is also possible. (I) In the variable focal length lens system according to any one of claims 1 to 7, the first to third lens groups include two lenses, a positive lens and a negative lens, and the second lens group. The negative lens in the middle has a meniscus lens having an aspherical surface, and the positive lens in the third lens group is
A variable focal length lens system comprising a meniscus lens having an aspheric surface.

According to the above configuration (I), at least the axial aberration among the aberrations generated in each lens group can be satisfactorily corrected, and a predetermined focal length can be set at an arbitrary focal length from the wide-angle end state to the telephoto end state. Optical performance can be obtained. In addition, the negative lens in the lens group having a positive refractive power has a meniscus shape, and the positive lens in the lens group having a negative refractive power has a meniscus shape. It is possible to prevent the performance deterioration due to mutual eccentricity of the positive lens and the negative lens constituting each lens group which sometimes occurs. Note that, in addition to the configuration (I), an aspheric surface may be provided for the positive lens in the first lens group having a positive refractive power.

(II) The variable focal length lens system according to (I), wherein the lens having the aspherical surface is made of a plastic material. With the configuration (II), the cost of the variable focal length lens system can be reduced. The plastic material has a large change in the refractive index due to environmental changes. However, since the aspherical lens has a meniscus shape, the refractive power is relatively weak, and the negative lens in the positive lens group and the positive lens in the negative lens group have plastic materials. Is used, it is possible to satisfactorily correct a change in the image plane position caused by a change in the refractive index due to an environmental change.

(III) In the variable focal length lens system according to any one of claims 1 to 7, at least one of the plurality of lenses constituting the variable focal length lens system is configured to be eccentric. A blur detection system that detects blur of the variable focal length lens system, and a driving unit that shifts an image by eccentrically driving the eccentric lens so as to correct the blur detected by the blur detection system. A variable focal length lens system further provided.

In the configuration of the above (III), the image shift can be executed so as to substantially cancel the image shift on the image plane due to camera shake or the like. It is possible to prevent failure due to image blur caused by hand shake or the like which tends to occur in the above. Here, the lens driven eccentrically is one lens group as a whole or a part of the lens groups constituting the variable focal length lens system.

In the following embodiments, the aperture stop diameter changes when the lens position changes, but there is no problem if the aperture stop diameter is not variable.

[0043]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the present invention will be described below. FIG. 1 shows a refractive power distribution of a variable focal length lens system according to each embodiment of the present invention. In order from the object side, a first lens group G1 having a positive refractive power and a second lens group G having a positive refractive power are shown.
2 and a third lens group G3 having a negative refractive power. When the focal length changes from the wide-angle end state to the telephoto end state, the first lens group G1 and the second lens group G2 The distance increases, and the second lens group G2 and the third lens group G3
All the lens units move toward the object side so that the distance between the lens groups decreases. At this time, the first lens group G1 and the third lens group G3 move integrally. Thus, in each embodiment of the present invention, the first lens group G1 and the third lens group G3 function as a lead group, and the second lens group G2 functions as a compensator group.

In each embodiment, the aspheric surface is represented by the following equation.

[0045]

(Equation 1)

(First Embodiment) FIG. 2 shows a lens configuration according to a first embodiment of the present invention.
Is a meniscus-shaped negative lens L1 having a concave surface facing the object side.
1. The second lens group G2 includes a biconvex lens L12, and a meniscus negative lens L21 having a concave surface facing the object side.
The third lens group G3 includes a positive meniscus lens L31 having a convex surface facing the image side and a negative meniscus lens L32 having a concave surface facing the object side. The aperture stop is arranged on the object side of the negative lens L21, and when the lens position changes, the second lens group G
2 and move together.

Table 1 below shows values of specifications of the first embodiment of the present invention. In the specifications of the embodiment, f is the focal length, FNO is the F number, 2ω is the angle of view, and the refractive index is d.
The value is for a line (λ = 587.6 nm). In Table 1, a lens surface having a radius of curvature of 0 represents a flat surface.

[0048]

[Table 1] The sixth, ninth, and tenth surfaces are aspherical, and the aspherical surface coefficients are as shown in Table 2 below.

[0049]

[Table 2]

Table 3 below shows the variable intervals at which the focal length is changed and the amounts of movement of the respective lens units at the time of short-distance focusing.
Shown in

[0051]

[Table 3] (Variable interval table) f 23.1000 33.2710 43.6482 D4 2.0000 4.7054 6.3325 D9 5.5325 2.8271 1.2000 BF 7.3258 14.9759 22.6259 (Moving amount of each lens group during short-distance focusing) f 23.1000 33.2710 43.6482 First lens group 1.9125 1.6991 1.9125 Second lens group 1.9125 2.1501 1.9125 Third lens group 1.9125 2.3940 1.9125 However, this is the amount of movement when focusing on a shooting distance of 0.45 m. Table 4 below shows values corresponding to the conditional expressions of the first embodiment.

[0052]

F1 = 35.1483 f2 = 18.1132 f3 = -13.6121 (1) D / (ft−fw) = 1.155 (2) CAve. · Ft / FNO = 0.667 (3) | f3 | / f1 = 0.387 (4) f2 / fw = 0.784 FIG. 5 shows various aberration diagrams of the first embodiment of the present invention, and FIG.
3 to FIG. 5 show various aberration diagrams in an infinity in-focus state, respectively. FIGS. 3, 4, and 5 show a wide-angle end state (f = 23.1), an intermediate focal length state (f = 33.3), and a telephoto state, respectively. The various aberration figures in an end state (f = 43.65) are shown.

3 to 5, the solid line in the spherical aberration diagram indicates the spherical aberration, the dotted line indicates the sine condition, y indicates the image height, and the solid line in the astigmatism diagram indicates the sagittal image plane. , The dashed line indicates the meridional image plane, and d
Indicates an aberration with respect to the d-line. The coma diagram shows the image height y =
Coma at 0, 4.3, 8.6, 12.04, 17.2, A indicates the angle of view, and H indicates the object height.

From each aberration diagram, it is clear that this embodiment has excellent correction of various aberrations and excellent imaging performance. (Second Embodiment) FIG. 6 shows a lens configuration according to a second embodiment of the present invention. The first lens group G1 includes a negative meniscus lens L11 having a concave surface facing the object side, and a biconvex lens L12. The second lens group G2 includes a negative meniscus lens L21 having a concave surface facing the object side and a biconvex lens L22. The third lens group G3 has a positive meniscus lens L31 having a convex surface facing the image side. It is composed of a meniscus-shaped negative lens L32 with the concave surface facing the object side.
The aperture stop is arranged on the object side of the negative lens L21, and moves integrally with the second lens group G2 when the lens position changes.

Table 5 below summarizes data values of the second embodiment of the present invention. In the specifications of the embodiment, f is the focal length, FNO is the F number, 2ω is the angle of view, and the refractive index is d.
The value is for a line (λ = 587.6 nm). In Table 5, the lens surface having a value of the radius of curvature of 0 represents a flat surface.

[0056]

[Table 5] The sixth, ninth, and tenth surfaces are aspherical surfaces, and the aspherical surface coefficients are as shown in Table 6 below.

[0057]

[Table 6]

Table 7 below shows the variable intervals when the focal length is changed and the amount of movement of each lens group when focusing on a short distance.
Shown in

[0059]

[Table 7] (Variable interval table) f 23.1000 33.2778 43.6500 D4 1.5779 4.2175 5.7986 D9 5.4206 2.7810 1.2000 BF 7.3000 14.9112 22.5224 (Moving amount of each lens group at close distance focusing) f 23.1000 33.2710 43.6482 First lens group 1.9028 1.6971 1.9028 Second lens group 1.9028 2.1378 1.9028 Third lens group 1.9028 2.3741 1.9028 However, this is the amount of movement when focusing on a shooting distance of 0.45 m. Table 8 below shows values corresponding to the conditional expressions of the second example. (Values corresponding to conditional expressions) f1 = 35.2654 f2 = 17.9273 f3 = 13.3634 (1) D / (ft−fw) = 1.148 (2) CAve. · Ft / FNO = 0.617 (3) | f3 | / f1 = 0.379 (4) f2 / fw = 0.772 FIG. 9 shows various aberration diagrams of the second embodiment of the present invention.
FIGS. 7 to 9 show various aberration diagrams in the infinity in-focus state, respectively. FIGS. 7, 8, and 9 show the wide-angle end state (f = 23.1), the intermediate focal length state (f = 33.3), and the telephoto state, respectively. The various aberration figures in an end state (f = 43.65) are shown.

7 to 9, the solid line in the spherical aberration diagram indicates the spherical aberration, the dotted line indicates the sine condition, y indicates the image height, and the solid line in the astigmatism diagram indicates the sagittal image plane. , The dashed line indicates the meridional image plane, and d
Indicates an aberration with respect to the d-line. The coma diagram shows the image height y =
Coma at 0, 4.3, 8.6, 12.04, 17.2, A indicates the angle of view, and H indicates the object height.

From the aberration diagrams, it is clear that the present embodiment has excellent correction of various aberrations and excellent imaging performance.

[0062]

According to the present invention, a small variable focal length lens system suitable for widening the angle and increasing the aperture ratio can be achieved.

[Brief description of the drawings]

FIG. 1 is a refractive power layout of a variable focal length lens system according to the present invention.

FIG. 2 is a cross-sectional view illustrating a configuration of a zoom lens according to a first embodiment.

FIG. 3 is an aberration diagram of the first embodiment in a wide-angle end state.

FIG. 4 is an aberration diagram of the first embodiment in an intermediate focal length state.

FIG. 5 is an aberration diagram of the first embodiment in a telephoto end state.

FIG. 6 is a sectional view showing a configuration of a zoom lens according to a second embodiment.

FIG. 7 is an aberration diagram of the second embodiment in a wide-angle end state.

FIG. 8 is an aberration diagram of the second embodiment in an intermediate focal length state.

FIG. 9 is an aberration diagram of the second embodiment in a telephoto end state.

[Explanation of symbols]

 G1: First lens group G2: Second lens group G3: Third lens group

Claims (7)

[Claims] A first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power. An aperture stop is arranged between the lens group and the second lens group, and when the focal length changes from the wide-angle end state to the telephoto end state, the distance between the first lens group and the second lens group increases. The first and third lens groups move integrally to the object side, and the second lens group moves to the object side so that the distance between the second lens group and the third lens group decreases. And a variable focal length lens system satisfying the following conditional expressions (1) and (2). (1) 0.9 <D / (ft−fw) <1.5 (2) 0.5 <Cave. · Ft / FNO <0.8, where D: closest to the object side in the first lens group Distance along the optical axis from the lens surface located to the lens surface located closest to the image side of the third lens group, fw: focal length in the wide-angle end state, ft: focal length in the telephoto end state, CAve .: The average value of the absolute value of the paraxial curvature of each lens surface constituting the second lens group, FNO: aperture ratio in the telephoto end state. 2. The variable focal length optical system according to claim 1, wherein the second lens group is disposed on the image side of the negative lens having a meniscus shape with a concave surface facing the aperture stop. A variable focal length lens system comprising only a biconvex positive lens. 3. The variable focal length lens system according to claim 2, wherein the most object side lens surface of said negative lens in said second lens group and the most image side of said positive lens in said second lens group. Is a variable focal length lens system, each of which is constituted by an aspherical surface. 4. The variable focal length lens system according to claim 3, wherein each of the first lens group and the third lens group is 2
A variable focal length lens system comprising a plurality of lenses. 5. A lens system comprising, in order from the object side, a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power. Arranging an aperture stop between the second lens group, when the focal length changes from the wide-angle end state to the telephoto end state, the distance between the first lens group and the second lens group increases, All the lens groups move toward the object side so that the distance between the second lens group and the third lens group decreases,
And the first lens group and the third lens group move integrally, and when focusing on a short distance, the first lens group and the third lens group move integrally to the object side, A variable focal length lens system, wherein the second lens group moves toward the object side while changing the distance from the first lens group. 6. The variable focal length lens system according to claim 1, wherein the following conditional expression (3) is satisfied. (3) 0.3 <| f3 | / f1 <0.45, where f3 is the focal length of the third lens group, and f1: the focal length of the first lens group. 7. The variable focal length lens system according to claim 6, wherein the following conditional expression (4) is satisfied. (4) 0.7 <f2 / fw <0.85, where f2 is the focal length of the second lens group.