JPH0534597A - Zoom lens - Google Patents

Abstract

PURPOSE:To make the zoom lens small in size and light in weight by allowing it to satisfy a specific condition and using effectively an aspherical lens, in the zoom lens of a rear focus system constituted of four groups. CONSTITUTION:A first lens group 1 is constituted by joining or separating a negative meniscus lens 11 and a positive lens 12 whose convex surfaces are turned to an object side in order from the object side. At least one surface of the nearest surface 11a to the object side or the nearest surface 12a to an image side in a first lens group 1 is formed to the aspherical surface. Also, it is allowed to satisfy a condition of an expression I. In the expression I, RF, KF and ADF denote a paraxial radius of curvature of nearest surface 11a to the object side, a cone constant, and a biquadratic aspherical coefficient related to height from an optical axis, respectively. Also RR, KR and ADR denote a paraxial radius of curvature of the nearest surface 12a to the image side, a con constant, and a biquadratic aspherical coefficient related to height from the optical axis, respectively. In such a way, a correction of a spherical aberration by the aspherical surface of a first lens group 1 works on positive as a tertiary spherical aberration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens of a rear focus type having a high zoom ratio, which is effective for use in, for example, a still camera or a video camera.

[0002]

2. Description of the Related Art Currently, the specifications of zoom lenses used in consumer video cameras include a zoom ratio of 6 times or more and an aperture ratio of F.
It is brighter than /2.0. In addition to such specifications, further miniaturization and weight reduction of the lens system are required. Therefore, the imager size of the charge-coupled image pickup device (CCD) used as an image pickup device is becoming smaller. Due to the idea of scaling (SCALING), the smaller the imager size, the smaller the lens itself.

[0003]

However, as the size of the lens becomes smaller, the required accuracy also becomes higher due to the scaling, and there is a disadvantage that a high processing accuracy is required for each part and the productivity deteriorates. In particular, a lens element finished into a spherical surface by polishing not only has a high processing accuracy, but also has a small radius of curvature, which makes it difficult to process the lens element, which may significantly increase the manufacturing cost.

In recent years, a method of manufacturing a lens by molding plastic, glass or the like has been established. In the case of mass production of lenses by integral molding, there is a possibility that the manufacturing cost can be reduced in the future, in addition to satisfying the required processing accuracy, as compared with lenses by the polishing method. Further, in the case of molding, it is easy to make the lens surface aspherical, and by adopting an aspherical lens, it is possible to reduce the number of lens elements that make up the zoom lens and to make the zoom lens smaller and lighter. There is a nature.

In this regard, as a zoom lens having a relatively high zoom ratio and a large aperture ratio, a first lens group having a positive refractive power, a second lens group having a negative refractive power, in order from the object side,
It has a third lens group having a positive refracting power and a fourth lens group having a positive refracting power, the first lens group and the third lens group are fixed, and the second lens group is moved to change the magnification. There is known a rear focus type zoom lens having a four-group configuration in which the fourth lens group is moved to perform image plane movement compensation and focusing in accordance with zooming (for example, Japanese Patent Laid-Open Publication No. 6-58242).
2-215225 gazette). However, such a conventional rear focus type zoom lens is composed of, for example, 10 or more spherical lenses, and there is a limit to the reduction in size and weight.

In view of the above point, the present invention has an object to reduce the size and weight of such a rear focus type zoom lens by effectively using an aspherical lens.

[0007]

A zoom lens according to the present invention comprises, for example, as shown in FIG. 1, a first lens group (1) having a positive refractive power and a second lens having a negative refractive power in order from the object side. A group (2), a third lens group (3) having a positive refractive power, and a fourth lens group (4) having a positive refractive power, the first lens group and the third lens group are fixed, A rear-focus type zoom lens having a four-group configuration in which the second lens group is moved to perform zooming, and the fourth lens group is moved to perform image plane movement compensation and focusing in accordance with zooming. And the first lens group (1) is composed of two lenses, which are cemented or separated from each other, consisting of a negative meniscus lens (11) and a positive lens (12) having convex surfaces facing the object side in order from the object side. , The most object side in the first lens group (1) (11a) or the most image side surface (1
At least one surface of 2a) is formed into an aspherical surface, and a fourth-order aspherical coefficient relating to the paraxial radius of curvature, conic constant and height from the optical axis of the surface (11a) closest to the object is respectively RF, As KF and ADF, the paraxial radius of curvature of the most image-side surface (12a), the conic constant, and the fourth-order aspherical coefficients relating to the height from the optical axis are RR, KR, and AD, respectively.
When R is set, the condition of the following expression 1 is satisfied.

[Number 1] ^{KF / (8RF 3) + ADF} -KR / (8RR 3) -ADR <0

In the present invention, the surface closest to the object (11
a) or one of the most image-side surfaces (12a) is a spherical surface, in which case the corresponding conic constant and the fourth-order aspherical coefficient relating to the height from the optical axis are 0 and 0, respectively. Is.

[0009]

According to the present invention, since the first lens group (1) is composed of two lenses by utilizing the aspherical surface, the first lens group (1) and hence the entire zoom lens can be made compact. The weight can be reduced. Furthermore, spherical aberration can be effectively corrected by the aspherical surface. In this case, the aspherical surface itself does not contribute to the correction of chromatic aberration, but since the present invention has two lenses, the axial chromatic aberration can be effectively corrected by the dispersion of these two lenses and the distribution of the refractive power. You can In addition, a lens having a two-element configuration in which axial chromatic aberration is controlled generally tends to cause negative spherical aberration to an unacceptable level, and as a method for controlling spherical aberration, overcorrection (excessive correction) is not performed. It is desired to approach 0 within the range, that is, the polarity remains negative. Therefore, in the present invention, by imposing the condition of the above expression 1, the correction of the spherical aberration due to the aspherical surface acts positively as the third-order spherical aberration.

Further, when the first lens group (1) is composed of a negative meniscus lens (11) and a positive lens (12), when the two lenses are cemented, the cemented surface is an aspherical surface. Even so, the effect of the aspherical surface is small because the difference in refractive index on both sides of the aspherical surface is small. On the other hand, those two
When the opposing surfaces of the lenses are made aspherical when the lenses are separated, it is difficult to control the air gap. Therefore, in order to form an aspherical surface on the two lenses, it is most effective to make one or both of the most object-side surface and the most image-side surface an aspherical surface.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the zoom lens according to the present invention will be described below with reference to the drawings. FIG. 1 shows a basic configuration of an optical system of a zoom lens according to an embodiment of the present invention. In FIG. 1, a fixed first lens group 1 having a positive refractive power in order from an object side to an image side, a negative lens group A second lens group 2 as a variator having a refractive power, a fixed third lens group 3 having a positive refractive power, and a fourth lens group 4 as a compensator having a positive refractive power are arranged. Assuming that the components perpendicular to and parallel to the optical axis of the zoom lens of the arrows A1 to A3 correspond to the difference in magnification and the movement of the lens group, respectively, the second lens group 2 moves corresponding to the arrow A1. As a result, zooming is performed, and the fourth lens group 4 moves in accordance with the arrows A2 and A3, so that the image plane variation associated with zooming and the focusing are performed. That is, in this example, positive, negative, positive, positive 4
This is a rear focus type zoom lens with a group configuration.

Further, a diaphragm 5 is provided between the second lens group 2 and the third lens group 3 (however, the position of the diaphragm 5 may be other than this), and a distance between the fourth lens group 4 and the image plane is set. A glass block 6 serving as an optical low-pass filter is arranged in. Further, the first lens group 1 is composed of a negative meniscus lens 11 having a convex surface facing the object side and a positive lens 12 which is cemented or separated from the negative meniscus lens 11, and the second lens group 2 has a convex surface facing the object side. Negative meniscus lens 21 and negative lens 2 aimed at
2 and a positive lens 23 cemented to it, the third lens group 3 thereof is composed of a positive lens 31 and a negative lens 32, and the fourth lens group 4 thereof is a negative meniscus lens having a convex surface facing the object side. 41 and a positive lens 42 cemented to it.

Further, although an aspherical surface is used in this example,
K is the conic constant, y is the height from the optical axis, Z is the distance from the tangent plane of the aspherical vertex of the aspherical surface at the height y from the optical axis, R is the paraxial radius of curvature, AD, AE , AF, and AG are aspherical coefficients of 4th to 10th, respectively, the aspherical shape is expressed as follows.

[Equation 2]

Then, in the first lens group 1 in FIG.
The paraxial radius of curvature of the surface 11a closest to the object, the conic constant, and the fourth-order aspherical coefficients relating to the height from the optical axis are RF and K, respectively.
Assuming that F and ADF are RR, KR, and ADR, respectively, the paraxial curvature radius of the most image-side surface 12a, the conic constant, and the fourth-order aspherical coefficients related to the height from the optical axis are RR, KR, and ADR, respectively. (3) is satisfied.

Equation 3] ^{KF / (8RF 3) + ADF} -KR / (8RR 3) -ADR <0 This surface 12 of the surface 11a or the most image side of the most object side
It means that at least one of a is an aspherical surface, and this aspherical surface acts as a third-order spherical aberration in the positive direction.

Generally, when designing a zoom lens having a plurality of lens groups, it is necessary to distribute the various aberrations generated by the respective groups during zooming in a well-balanced manner. The distribution of the aberration amount becomes difficult as the aperture ratio becomes large and the zoom ratio becomes large, and the number of lenses used tends to increase accordingly. In particular, in the first lens group, the axial marginal ray height changes largely at the telephoto end and the wide-angle end due to zooming, but the axial chromatic aberration and the spherical aberration are proportional to the square and the fourth power of the axial marginal ray height, respectively. Since it changes, it is difficult to control both the axial chromatic aberration and the spherical aberration at the same time. Therefore, for example, in a zoom lens for a video camera having a diameter ratio of more than about F / 2.5 and a zoom ratio of more than about 5 times, three lenses have been used in the first lens group.

On the other hand, in this example, the first lens group 1 uses an aspherical surface to correct the spherical aberration first. Further, since axial chromatic aberration cannot be corrected on an aspherical surface, the first lens group 1 is composed of a negative meniscus lens 11 and a positive lens 12 in this example. These two lenses are made of glass types having different dispersions, and the axial chromatic aberration is controlled by adjusting the distribution of the respective refractive powers. Further, when controlling the spherical aberration with the first lens group, it is desired to approach 0 within a range where overcorrection (excessive correction) is not performed, that is, the polarity remains negative. In this regard, a lens system having a two-lens structure in which axial chromatic aberration is controlled as in this example generally tends to cause a negative spherical aberration amount to exceed an allowable value. Therefore, the correction of the spherical aberration by the aspherical surface in the first lens group 1 needs to act in the positive direction as the third-order spherical aberration in order to cancel the negative spherical aberration amount. Therefore, in this example, the condition of the equation 3 is satisfied.

As described above, in this example, the first lens group 1 is composed of two lenses, so that the number of lenses is reduced as a whole, and the zoom lens can be made compact and lightweight. In this case, both the axial chromatic aberration and the spherical aberration can be satisfactorily corrected by adopting the two-lens structure and by imposing the condition of the expression 3 in which at least one surface is an aspherical surface. If at least one of the two lenses is an aspherical surface,
Even if the joining surface is an aspherical surface when the sheets are joined, the effect is small because the difference in the refractive index on both sides is small.
Further, when the two lenses are separated and the opposing surfaces of the two lenses are aspherical surfaces, it is difficult to control the air gap and the like. Therefore, when forming an aspherical surface on these two lenses, it is most effective to make one or both of the most object-side surface 11a and the most image-side surface 12a an aspherical surface. However, it goes without saying that the joining surface or the facing surface may be made aspherical together.

Next, lens data, aspherical surface data and variable data of a plurality of numerical embodiments of the present invention will be shown respectively. In the lens data of each numerical example, the curvature radius of the i-th (i = 1, 2, 3, ...) Surface and the i-th surface and the (i + 1) -th surface are sequentially arranged from the object side to the image plane side. The interplanar spacings are Ri and Di, respectively. The i-th surface and (i +
1) The refractive index and Abbe number of the medium between the first surface and the second surface are Ni and νi, respectively, and N when the medium is air
i and νi are blank. In the lens data, the surface marked with * is an aspherical surface, and the radius of curvature R of this surface means the paraxial radius of curvature. On the other hand, in the aspherical surface data of each numerical example, K is the conical constant of the surface described in the left column, AD,
AE, AF, and AG are fourth-order to tenth-order aspherical surface coefficients of the surfaces described in the left column, and the aspherical surface shape is determined by substituting these conical constants and aspherical surface coefficients into the equation 2.

Further, common to all numerical embodiments,
The overall focal length f of the zoom lens is 10 to 60 mm, F
The value is 2 to 2.8 and the half angle of view ω is 23.2 to 4.1 °. In addition, a wide-angle position (f = 10.0), an intermediate position (f = 30.3) and a telephoto position (f = 6).
The value of the variable interval Di in 0.0) is shown as variable data.

[First Embodiment] In this embodiment, in the first lens group 1 of FIG. 1, a negative meniscus lens 11 and a positive lens 12 are used.
And the surface 12a closest to the image side is aspherical. A lens configuration diagram of this example is shown in FIG. 2, in which a wide-angle position (f = 10.0) and an intermediate position (f = 30.
3) and longitudinal aberration diagrams at the telephoto position (f = 60.0) are shown in FIGS. 3 to 5, respectively.

A. Lens data i Ri Di Ni νi 1 27.99 1.71 1.805 25.5 2 19.62 6.86 1.589 61.3 * 3 -173.25 2.16 (variable) 4 32.12 1.14 1.834 37.2 5 10.67 3.92 6 -13.13 1.14 1.697 48.5 7 14.34 3.29 1.847 23.8 8 -112.88 23.03 (variable) ) 9 ∞ (aperture) 0.57 * 10 16.53 4.29 1.589 61.3 * 11 -26.78 4.75 12 -28.28 1.14 1.847 23.8 13 100.00 10.39 (variable) 14 18.66 1.14 1.785 25.7 15 11.76 5.71 1.694 53.3 * 16 -30.87 8.99 (variable) 17 ∞ 8.29 1.517 64.2 18 ∞ 0

B. Aspherical surface data K AD AE AF AG 3 surface 0 0.376 × 10 ^-5 -0.102 × 10 ^-8 -0.107 × 10 ^-10 0.226 × 10 ^-13 10 surface 0 -0.299 × 10 ^-5 -0.259 × 10 ^-6 0.829 × 10 ^-9 0.112 x 10 ^-9 11 faces 0 0.544 x 10 ^-4 -0.197 x 10 ^-6 -0.215 x 10 ^-8 0.164 x 10 ^-9 16 faces 0 0.699 x 10 ^-4 -0.419 x 10 ^-7 -0.116 x 10 ^-8 0.133 x 10 ^-10

C. Variable data f = 10.0 f = 30.3 f = 60.0 D3 2.16 17.69 24.76 D8 23.03 7.50 0.43 D13 10.39 7.22 10.55 D16 8. 99 12.16 8.83

[Second Embodiment] In this embodiment, in the first lens group 1 of FIG. 1, a negative meniscus lens 11 and a positive lens 12 are used.
And the surface 11a closest to the object side is aspherical. A lens configuration diagram of this example is shown in FIG. 6, in which a wide-angle position (f = 10.0) and an intermediate position (f = 30.
3) and longitudinal aberration diagrams at the telephoto position (f = 60.0) are shown in FIGS. 7 to 9, respectively.

A. Lens data i Ri Di Ni νi * 1 27.99 1.71 1.805 25.5 2 19.62 6.86 1.589 61.3 3 -173.25 2.16 (variable) 4 32.12 1.14 1.834 37.2 5 10.67 3.92 6 -13.13 1.14 1.697 48.5 7 14.34 3.29 1.847 23.8 8 -112.88 23.03 (variable) ) 9 ∞ (aperture) 0.57 * 10 16.53 4.29 1.589 61.3 * 11 -26.78 4.75 12 -28.28 1.14 1.847 23.8 13 100.00 10.39 (variable) 14 18.66 1.14 1.785 25.7 15 11.76 5.71 1.694 53.3 * 16 -30.87 8.99 (variable) 17 ∞ 8.29 1.517 64.2 18 ∞ 0

B. Aspherical data K AD AE AF AG 1 surface 0 -0.167 × 10 ^-5 -0.238 × 10 ^-8 0.341 × 10 ^-11 -0.736 × 10 ^-14 10 surface 0 -0.299 × 10 ^-5 -0.259 × 10 ^-6 0.829 × 10 ^-9 0.112 × 10 ^-9 11 surfaces 0 0.544 × 10 ^-4 -0.197 × 10 ^-6 -0.215 × 10 ^-8 0.164 × 10 ^-9 16 surfaces 0 0.699 × 10 ^-4 -0.419 × 10 ^-7 -0.116 × 10 ^-8 0.133 × 10 ^-10

C. Variable data f = 10.0 f = 30.3 f = 60.0 D3 2.16 17.69 24.76 D8 23.03 7.50 0.43 D13 10.39 7.22 10.55 D16 8. 99 12.16 8.83

[Third Embodiment] In this embodiment, in the first lens group 1 of FIG. 1, a negative meniscus lens 11 and a positive lens 12 are used.
Is separated, and the surface 12a closest to the image side is aspherical. A lens configuration diagram of this example is shown in FIG. 10, in which a wide-angle position (f = 10.0) and an intermediate position (f = 30.
3) and longitudinal aberration diagrams at the telephoto position (f = 60.0) are shown in FIGS. 11 to 13, respectively.

A. Lens data i Ri Di Ni νi 1 27.35 1.71 1.805 25.5 2 19.39 0.14 3 19.64 6.86 1.589 61.3 * 4 -186.45 2.16 (variable) 5 32.12 1.14 1.834 37.2 6 10.67 3.92 7 -13.13 1.14 1.697 48.5 8 14.34 3.29 1.847 23.8 9 -112.88 23.03 (variable) 10 ∞ (aperture) 0.57 * 11 16.53 4.29 1.589 61.3 * 12 -26.78 4.75 13 -28.28 1.14 1.847 23.8 14 100.00 10.39 (variable) 15 18.66 1.14 1.785 25.7 16 11.76 5.71 1.694 53.3 * 17 -30.87 8.99 (variable) ) 18 ∞ 8.29 1.517 64.2 19 ∞ 0

B. Aspheric surface data K AD AE AF AG 4 surfaces 0 0.335 × 10 ^-5 -0.148 × 10 ^-8 -0.112 × 10 ^-10 0.964 × 10 ^-14 11 surfaces 0 -0.299 × 10 ^-5 -0.259 × 10 ^-6 0.829 × 10 ^-9 0.112 x 10 ^-9 12 faces 0 0.544 x 10 ^-4 -0.197 x 10 ^-6 -0.215 x 10 ^-8 0.164 x 10 ^-9 17 faces 0 0.699 x 10 ^-4 -0.419 x 10 ^-7 -0.116 x 10 ^-8 0.133 x 10 ^-10

C. Variable data f = 10.0 f = 30.3 f = 60.0 D4 2.16 17.69 24.76 D9 23.03 7.50 0.43 D14 10.39 7.22 10.55 D17 8. 99 12.16 8.83

The present invention is not limited to the above-mentioned embodiments, and it goes without saying that various configurations can be adopted without departing from the gist of the present invention.

[0033]

According to the present invention, the first lens group is made up of two lenses, and the aspherical surface is made to act positively as the third-order spherical aberration, so that a negative lens system is likely to occur in the two-lens system. Since the amount of spherical aberration is canceled out, there is an advantage that the zoom lens can be made smaller and lighter while properly correcting various aberrations. Further, since the number of lenses constituting the first lens group is reduced from three to two in the related art, there is an advantage that the lens barrel structure for fixing the lenses is simplified and the manufacturing cost can be further reduced.

[Brief description of drawings]

FIG. 1 is a lens sectional view showing a basic configuration of an optical system of an embodiment of a zoom lens according to the present invention.

FIG. 2 is a lens configuration diagram showing a configuration of a first example.

FIG. 3 is a longitudinal aberration diagram at the wide-angle position of the first embodiment, where d is d line, e is e line, F is F line, S is sagittal image plane, and T is tangential image plane.

FIG. 4 is a longitudinal aberration diagram at the intermediate position of Example 1.

FIG. 5 is a longitudinal aberration diagram for Example 1 at a telephoto position.

FIG. 6 is a lens configuration diagram showing a configuration of a second example.

FIG. 7 is a longitudinal aberration diagram at the wide-angle position of the second example.

FIG. 8 is a longitudinal aberration diagram at the intermediate position of Example 2.

FIG. 9 is a longitudinal aberration diagram for Example 2 at the telephoto position.

FIG. 10 is a lens configuration diagram showing a configuration of a third example.

FIG. 11 is a longitudinal aberration diagram at the wide-angle position of the third example.

FIG. 12 is a longitudinal aberration diagram at the intermediate position of the third example.

FIG. 13 is a longitudinal aberration diagram for Example 3 at the telephoto position.

[Explanation of symbols]

 1 1st lens group 2 2nd lens group 3 3rd lens group 4 4th lens group 11 Negative meniscus lens 11a The most object side surface 12 Positive lens 12a The most image side surface

Claims (1)

1. A first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a positive lens group in order from the object side. A fourth lens group having a refractive power, the first lens group and the third lens group are fixed,
A rear-focus type zoom lens having a four-group configuration in which the second lens group is moved to perform zooming, and the fourth lens group is moved to perform image plane movement compensation and focusing in accordance with zooming. The first lens group is composed of two lenses which are cemented or separated from each other and are composed of a negative meniscus lens having a convex surface facing the object side and a positive lens in this order from the object side. At least one of the object-side surface or the most image-side surface is formed as an aspherical surface, and a fourth-order aspherical surface relating to the paraxial radius of curvature, conic constant, and height from the optical axis of the most object-side surface. Coefficients are RF and KF respectively
And ADF, the paraxial radius of curvature of the most image-side surface,
When the fourth-order aspherical coefficients relating to the conic constant and the height from the optical axis are RR, KR, and ADR, respectively, the condition of KF / (8RF ³ ) + ADF-KR / (8RR ³ ) -ADR <0 is satisfied. A zoom lens that is characterized by doing so.