JPH11258502A - Zoom lens and optical unit using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a zoom lens which is miniaturized and whose optical ability is improved. SOLUTION: Relating to a zoom lens, four lens groups constituted of the first group 1 of positive refractive force, the second group 2 of negative refractive force, the third group 3 of positive refractive force and the fourth group 4 of positive refractive force are installed in order from an object-side, the second group 2 is moved !o a two image surface-side and power from a wide angle end to a telephoto end is varied, and the fluctuation of an image surface with power varying is corrected by moving the fourth group 4. The second group 2 is constituted of the four single lenses of three negative lenses and one positive lens, and at least one aspherical face is given.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens, and more particularly to a zoom lens suitably used for a video camera and the like, and a rear focus type zoom lens having a high zoom ratio of 20 times and a small number of constituent lenses while having a high zoom ratio. It is about a lens.

[0002]

2. Description of the Related Art Various types of zoom lenses conventionally used in photographic cameras and video cameras adopt a so-called rear focus system in which a lens group behind the first group on the object side is moved to perform focusing. It has been proposed. This is because the rear focus method moves a relatively small and light lens group during focusing, so that the driving force is small and quick focusing is possible, so that it is compatible with the auto focus system. Because there is.

As such a rear focus type zoom lens, for example, in Japanese Patent Application Laid-Open No. 63-44614, a first lens unit having a positive refractive power and a negative lens having a negative refractive power for zooming are sequentially arranged from the object side. A second lens unit, a third lens unit having a negative refractive power for correcting an image plane variation caused by zooming, and a fourth lens unit having a positive refractive power.
In a so-called four-unit zoom lens composed of four lens units, the third unit is moved to perform focusing. However, in such a configuration, the moving space of the third lens group must be secured, and the overall length of the lens tends to increase. JP-A-63-278013 also discloses a positive first
A zoom lens composed of a group, a negative second group, a negative third group, and a positive fourth group is disclosed. The second group performs zooming, and the fourth group performs image plane correction and focusing. However, in these zoom methods using the negative third lens group, the divergent light from the second lens group is further diverged by the third lens group. In addition, the variation in aberration due to focus tends to increase.

As a zoom type in which this is improved, Japanese Patent Application Laid-Open Nos. Sho 62-24213 and 63-247316 are disclosed.
In this publication, there are four lens groups, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power, in that order from the object side. Disclosed zoom lens,
The zooming is performed by moving the second lens unit, and the image plane fluctuation caused by the zooming is corrected by moving the fourth lens unit and focusing is performed.

JP-A-63-29718 discloses a first lens unit having a positive refractive power, a negative lens, a negative lens,
A second lens unit composed of three positive lenses and movable as a whole with a negative refractive power during zooming and mainly controlling zooming; and a third lens unit having a positive refractive power and including an aspheric surface. Discloses a zoom lens composed of a fourth lens unit which has a positive refracting power with a slightly large air gap, corrects an image plane fluctuation accompanying zooming, and moves for focusing.

Japanese Patent Application Laid-Open No. 5-72472 discloses a first lens unit having a positive refractive power and a fixed first lens unit in order from the object side, a second lens unit having a negative refractive power and moving on the optical axis for zooming, and a positive lens unit. An aspherical zoom lens having a third lens unit having a refractive power and a fixed focusing function and a fourth lens unit having a positive refractive power and moving on the optical axis to maintain the image plane position is disclosed. In the zoom lens disclosed in this publication, the second group includes a meniscus negative lens, a biconcave lens, and a positive lens, the third group includes a single lens having one or more aspheric surfaces, and the fourth group includes one lens. It is composed of a lens having an aspherical surface or more.

[0007] However, the above three publications disclose the second
A zoom lens having an aspherical surface in the group is not disclosed.

On the other hand, in US Pat. No. 4,299,454, at least the first positive lens unit, the second negative lens unit, and the rear positive lens unit having a positive refractive power are arranged in order from the object side, and at least including the negative lens unit. A zoom lens is disclosed in which zooming is performed by moving two lens groups, and a second negative lens group includes first and second negative lenses and a positive doublet from the object side. U.S. Pat. No. Re. 32,923 discloses a first positive lens group, a second negative lens group,
An aperture, a third positive lens group, and a fourth positive lens group,
The first and fourth lens groups move in the same direction during zooming, and a fixed zoom lens with a diaphragm during zooming is disclosed.

However, the above two US patents also disclose:
Again, no example is disclosed in which an aspherical surface is arranged in the second lens unit.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a zoom lens having a novel construction which has not been achieved in the past, and has a high overall optical performance despite the fact that the entire lens system is reduced in size and has a high zoom ratio. It is another object of the present invention to provide a zoom lens having a simple configuration in which the number of lenses is reduced.

[0011]

In order to achieve the above object, the present invention provides, in order from the object side, a first group of positive refractive power,
The zoom lens has four lens groups, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power. A zoom lens that performs zooming to a telephoto end and corrects an image plane variation caused by zooming by moving the fourth group, wherein the second group includes three negative lenses and one positive lens. And four single lenses,
It has at least one aspherical surface.

[0012]

Next, embodiments of the present invention will be specifically described with reference to the drawings.

FIGS. 1 to 3 are lens cross-sectional views of Numerical Examples 1 to 3 to be described later, and FIGS. 4 to 6 are aberration diagrams of each Numerical Example. 4A to 6, (a) is an aberration diagram at a wide-angle end, (b) is an aberration diagram at an intermediate focal length, and (c).
9 is an aberration diagram at a telephoto end. In the aberration diagram, d,
g is the d-line and g-line, ΔM and ΔS are the meridional image plane, the sagittal image plane, and the chromatic aberration of magnification is the g-line.

In the lens sectional views of FIGS.
Denotes a first to fourth lens units, SP denotes an aperture, P denotes a glass block such as a CCD phase plate or a low-pass filter, and I denotes an image plane. In the present embodiment, at the time of zooming from the wide-angle end to the telephoto end, the second lens unit 2 is moved to the image plane side as indicated by an arrow in the figure, and the image plane fluctuation due to zooming is changed by the fourth lens unit 4. It is moved and corrected. The first group 1, the third group 3, and the stop SP are fixed during zooming. The fourth group 4
The moving trajectory accompanying zooming varies depending on the object distance, and the moving trajectory represented by a solid line in the figure represents an infinitely distant object, and the trajectory represented by a broken line represents a case of a close object. Further, the zoom lens of the present embodiment employs a rear focus method in which the fourth unit 4 is moved on the optical axis to perform focusing.

The most characteristic point of the present invention is as follows.
The second group 2 is composed of four single lenses, three negative lenses and one positive lens, and at least one of the lens surfaces constituting the second group 2 is aspheric.

That is, in a zoom lens of the type according to the present invention, the second group 2, which greatly contributes to zooming, is configured as described above, thereby reducing the power sharing of each lens and reducing the Petzval sum. Can be achieved. As a result, even at a high zoom ratio, fluctuations in the image plane due to zooming can be reduced. Further, by arranging one or more aspheric surfaces in the second group 2, the optical performance can be improved. In the present embodiment, the object-side surface of the positive lens that forms the second group 2 is an aspheric surface.

The present invention is achieved by the above-described configuration, but in order to perform better aberration correction, the second lens unit 2 includes, in order from the object side, the image plane as compared to the object side. The negative 21st lens with a strong concave surface facing the side, the negative 22nd lens with a concave concave surface on both sides, the 23rd positive lens with a strong convex surface facing the object side compared to the object side, and both negative negative lenses Second
It is desirable to configure with four lenses.

When the zoom ratio is increased in the zoom type as in the present invention, it is necessary to increase the amount of movement of the second unit 2 which greatly contributes to the zooming function, or to shorten the focal length of the second unit 2. . The former method is not preferable because it causes an increase in the size of the zoom lens, and the latter method does not increase the size of the zoom lens.
The load on the group 2 is large, and it becomes difficult to maintain good optical performance. Therefore, by configuring the second lens unit 2 as described above, it is possible to keep the optical performance good while preventing the entire system from being enlarged. In particular, with respect to the second group 2, chromatic aberration is favorably corrected by arranging the negative lens, the negative lens, the positive lens, and the negative lens substantially symmetrically from the object side. That is, color erasure of the principal point is favorably performed. The off-axis optical performance is improved by disposing an aspherical surface.

Further, by arranging the aspherical surface in the second group on the positive twenty-third lens, it becomes possible to correct aberration more effectively. In particular, off-axis flare can be corrected well. At this time, it is desirable that the aspherical surface has a shape in which the positive refractive power becomes weaker toward the periphery of the lens.

When the focal length of the entire system at the wide-angle end and the telephoto end of the zoom lens is fw, ft, and the focal length of the second lens unit is f2, 0.25 <| f2 / fA | <0.41 (0.41) 1)

[0021]

[Outside 2] It is desirable to satisfy the following conditional expression.

This is a conditional expression for making the focal length (in other words, power) of the second lens unit appropriate. If the focal length is longer than the upper limit of conditional expression (1), it is preferable in terms of aberration correction, but in order to obtain a desired zoom ratio, the amount of movement of the second unit must be increased, and This leads to an increase in the overall size, which is not preferable. Conversely, when the value exceeds the lower limit, the Petzval sum becomes negatively large, and the image surface falls, so that it is difficult to maintain good optical performance.

When the average Abbe number of the material of the positive lens in the second group is νp, and the average Abbe number of the material of the negative lens is vn, 36 <νn <65 (2) 20 <νp <35 (3) It is desirable to satisfy the following conditional expression.

This is a conditional expression for favorably correcting chromatic aberration generated in the second lens unit. As described above, since the second lens unit greatly contributes to zooming, it is necessary to properly correct aberrations generated here. Particularly in a zoom lens having a high zoom ratio such that the zoom ratio exceeds 20 times, it is important to satisfactorily correct chromatic aberration. If the upper limit of conditional expression (2) is exceeded, axial chromatic aberration will be over and correction will be excessive. Conversely, if the value exceeds the lower limit, the axial chromatic aberration becomes under, resulting in insufficient correction. In the conditional expression (3), the occurrence of the phenomenon is opposite to that in the conditional expression (2), but if the upper limit value or the lower limit value is exceeded, it is still difficult to satisfactorily correct the chromatic aberration.

To perform better aberration correction, the second
Assuming that the average refractive index of the material of the negative lens in the group is Nn, it is desirable to satisfy the following conditional expression: 1.71 <Nn <1.95 (4)

This is related to the conditional expression (1),
Under the condition for preventing the deterioration of Petzval sum by using a high refractive index glass for the negative lens, if the value exceeds the limit value, the curvature of field becomes worse.

When the radius of curvature of the i-th lens surface from the object side of the second group is R2i, 0.79 <| R22 / f2 | <1.32 (5) 1.28 <| R24 / R25 | <3.20 (6) 0.98 <| R26 / R27 | <3.55 (7) It is desirable to satisfy at least one of the following conditional expressions.

These conditional expressions are conditions for correcting spherical aberration, coma, astigmatism, and field curvature in a well-balanced manner.

When the value exceeds the upper limit of conditional expression (5), coma aberration increases. On the other hand, when the value exceeds the lower limit, the image surface is curved so as to be concave toward the object side.

Conditional expression (6) is a conditional expression for correcting each aberration while canceling each aberration well on each other. If the value exceeds the upper limit of conditional expression (6), the spherical aberration becomes under and the correction becomes insufficient. Conversely, exceeding the lower limit is not preferable because inward coma aberration increases.

Exceeding the upper limit of conditional expression (7) is not preferable because spherical aberration will be excessive and correction will be excessive. Conversely, exceeding the lower limit is not preferable because barrel-shaped distortion at the wide-angle end increases.

With respect to the conditional expressions (5) to (7), the respective functions and effects can be obtained by satisfying the respective conditional expressions. However, it is desirable to satisfy all the conditional expressions in view of aberration correction. Needless to say here.

Hereinafter, numerical examples of the present invention will be described.

In the numerical examples, Ri is the radius of curvature of the i-th surface in order from the object side, and Di is the i-th surface and the i-th surface.
+ 1st surface spacing (lens thickness or air spacing), N
i and νi are the refractive index and Abbe number of the material of the i-th lens, respectively.

The aspherical shape has an X-axis in the optical axis direction, an H-axis perpendicular to the optical axis, a positive traveling direction of light, R is a paraxial radius of curvature, and each aspherical coefficient is K, B, C, D. , E, F,

[0036]

[Outside 3] This is represented by

For example, the display of “eZ” is “1”.
0- ^Z ".

[0038]

[Outside 4]

[0039]

[Outside 5]

[0040]

[Outside 6]

[0041]

[Table 1]

Next, an embodiment of a video camera using the zoom lens of the present invention as a photographic optical system will be described with reference to FIG.

In FIG. 7, reference numeral 10 denotes a video camera body,
Reference numeral 11 denotes a photographing optical system constituted by the zoom lens of the present invention, 12 denotes an image pickup device such as a CCD for receiving a subject image by the photographing optical system 11, 13 denotes recording means for recording the subject image received by the image pickup device 12, and 14 Is a finder for observing a subject image displayed on a display element (not shown). The display element is constituted by a liquid crystal panel or the like,
A subject image formed on the image sensor 12 is displayed.

As described above, by applying the zoom lens of the present invention to an optical device such as a video camera, a compact optical device having high optical performance can be realized.

[0045]

As described above, according to the present invention,
It is possible to realize a zoom lens that has a small lens size, a high zoom ratio, high optical performance despite a bright lens having an F-number of about 1.4, and a small number of lenses. .

[Brief description of the drawings]

FIG. 1 is a lens cross-sectional view of a zoom lens according to Numerical Example 1.

FIG. 2 is a lens cross-sectional view of a zoom lens according to Numerical Example 2.

FIG. 3 is a sectional view of a zoom lens according to a third numerical example;

FIG. 4 is an aberration diagram of the zoom lens according to Numerical Example 1.

FIG. 5 is an aberration diagram of a zoom lens according to Numerical Example 2.

FIG. 6 is an aberration diagram of the zoom lens according to Numerical Example 3.

FIG. 7 is a diagram for describing an embodiment in which the zoom lens of the present invention is applied to a video camera.

[Explanation of symbols]

 Reference Signs List 1 first group 2 second group 3 third group 4 fourth group SP stop P glass block I image plane

Claims (11)

[Claims] 1. A first group having a positive refractive power, in order from the object side,
The zoom lens has four lens groups, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power. A zoom lens that performs zooming to a telephoto end and corrects an image plane variation caused by zooming by moving the fourth group, wherein the second group includes three negative lenses and one positive lens. And four single lenses,
A zoom lens having at least one aspheric surface. 2. The second group includes, in order from the object side, a negative 21st lens having a strong concave surface facing the image surface side, a negative 22nd lens having concave surfaces on both surfaces, and a positive 21st lens having a strong convex surface facing the object side. 23rd
2. The zoom lens according to claim 1, wherein the zoom lens comprises a lens and a negative twenty-fourth lens having both concave surfaces. 3. The zoom lens according to claim 2, wherein the aspherical surface is formed on the twenty-third lens. 4. The focal lengths of the entire system at the wide-angle end and the telephoto end are respectively fw and ft, and the focal length of the second group is f2.
0.25 <| f2 / fA | <0.41 4. A conditional expression which satisfies the following conditional expression.
The zoom lens described. 5. When the average Abbe number of the material of the negative lens of the second group is vn and the Abbe number of the material of the positive lens is νp, the following conditional expression is satisfied: 36 <νn <6520 <νp <35. 5. The method according to claim 1, wherein:
The zoom lens described. 6. The optical system according to claim 1, wherein, when the average refractive index of the material of the negative lens of the second group is Nn, the following conditional expression is satisfied: 1.71 <Nn <1.95.
The zoom lens described. 7. The radius of curvature of the i-th lens surface from the object side of the second lens unit is R2i, and the focal length of the second lens unit is f2.
Wherein the conditional expression 0.79 <| R22 / f2 | <1.32 is satisfied.
The zoom lens described. 8. When the radius of curvature of the i-th lens surface from the object side of the second group is R2i, the conditional expression 1.28 <| R24 / R25 | <3.20 is satisfied. Claims 1 to 7
The zoom lens described. 9. The conditional expression 0.98 <| R26 / R27 | <3.55 is satisfied, where R2i is the radius of curvature of the i-th lens surface from the object side of the second lens unit. Claims 1 to 8
The zoom lens described. 10. The zoom lens according to claim 1, wherein the focusing is performed by moving the fourth group. 11. An optical apparatus comprising the zoom lens according to claim 1.