JPH06300968A - Zoom lens - Google Patents

Abstract

PURPOSE:To provide a zoom lens which has excellent optical performance from a wide-angle end to a telephoto end and is low in cost and compact as to a large-aperture zoom lens which is used for a video camera and has about X10-12 power and F/1.4-1.6. CONSTITUTION:The zoom lens consists of a 1st group which has positive refracting power, a 2nd group which functions to vary the power and has negative refracting power, a 3rd group which has positive refracting power, and a 4th group which functions to correct image plane variation due to power variation or variation in object distance and has positive refracting power. Then this lens is constituted as a telephoto type which features the arrangement of an aperture stop on the object side of the positive lens in the 3rd group, the increased air gap between the positive lens and negative lens of the 3rd group, and the presence of the front principal point and rear principal point of the 3rd group in the positive lens in the 3rd group or in the object-side space.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens having a high zoom ratio and a large aperture used in a video camera.

[0002]

2. Description of the Related Art Currently, lenses for video cameras are 8 to 1
Zoom lenses having a high zoom ratio of 0 or more are becoming mainstream, and there is a strong demand for reduction in size and weight. Although the 1/3 inch size image pickup device is mainly used, the size of the device is being further reduced. As a result, the size of one pixel itself becomes very small, so that the lens system is required to have a large aperture and high-performance optical performance. further,
There is a strong demand for cost reduction, and in particular for zoom lenses, where the cost of lens parts is a major factor, there is a strong demand for the realization of a zoom lens that does not increase in the number of components due to the expansion of functions with a large aperture and high zoom ratio. .

Conventionally, in a zoom lens for a video camera, as seen in Japanese Patent Laid-Open No. 2-55308, a first lens unit having a positive refracting power and being fixed in order from the object side,
The second lens group having a negative refractive power and movable for zooming, the third lens group having a positive refractive power and fixed, and the positive lens group having a positive refractive power and correcting the image plane variation due to zooming and focusing. For this reason, a zoom lens having a four-group configuration including a movable fourth group is well known. In the above publication, the variable power ratio is 8 times, F
/1.4 is introduced that consists of 12 spherical lenses.

In Japanese Laid-Open Patent Publication No. 4-310910, only 11 spherical lenses, which is one less, is used, and the zoom ratio is 8 to 1.
A bright, high-power zoom lens with 0x and F / 1.4 has been proposed. This is composed of a first group of three, a second group of three, a third group of two, and a fourth group of three. In order to achieve a high zoom ratio of 10 times or more while maintaining compactness, it is necessary to increase the refracting power of the second lens unit and shorten the zoom stroke. For this reason, spherical aberration that is undercorrected occurs in the third lens group having a two-lens structure as in the above publication, and a high-performance lens cannot be obtained using only a spherical lens.

Therefore, it is becoming common practice to employ an aspherical lens in the third lens group. For example, JP-A-4-361
Japanese Patent Publication No. 214-214 proposes a compact zoom lens having an 11-lens configuration in which one aspherical lens is used in the third lens group with a zoom ratio of about 10 times, but the maximum aperture ratio is F.
It remains at about 1.8.

[0006]

SUMMARY OF THE INVENTION The present invention is to realize a high zoom ratio and a compact size while maintaining the large aperture of the zoom lens disclosed in Japanese Patent Laid-Open No. 4-310910. Adopted a unique lens type that includes one aspherical surface in the group, and a zoom ratio of 10 times F / 1.4, 12
Provides a low-cost and compact zoom lens with a large aperture of F / 1.6 at double magnification, consisting of 11 elements, and having good optical performance over the entire zoom range from the wide-angle end to the telephoto end. The purpose is to do.

[0007]

A zoom lens system according to the present invention comprises, in order from the object side, a first lens unit having a positive refractive power composed of a cemented positive lens and a positive meniscus lens, and a negative meniscus lens. A second lens unit having a negative refracting power, which is composed of three lenses including a cemented negative lens and has a function of varying the magnification, a positive lens having an aspherical surface on the biconvex object side and a meniscus negative lens having a convex image side. A third group of positive refracting power composed of two lenses and
Then, it is composed of a cemented positive lens and a positive lens, and is composed of a fourth lens unit having a positive refracting power and having a function of correcting an image plane variation due to zooming or a change in object distance. It is arranged on the object side of the positive lens of the third group, and the air gap between the positive lens and the negative lens of the third group is d13,
When the focal length of the group is f3, the refractive index of the third group negative lens at the d-line is n3n, and the Abbe number is ν3n, the above objects can be achieved by satisfying (Equation 1) and (Equation 2). It is what

[0008]

In the above construction, a telephoto type of two lenses in the second lens group, which is unique to the third lens group, is formed, and the principal point position of the third lens group is arranged on the object side to achieve compactness. Also, the third
By introducing an aspherical surface into the positive lens of the group and devising the glass material of the negative lens of the third group, 11
High performance is achieved with a single-piece configuration.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 3 is a block diagram of the first embodiment of the zoom lens according to the present invention. The first lens group having a positive refractive power, the second lens group having a negative refractive power for performing zooming, the aperture stop S, the third lens group having a positive refractive power, and an image plane variation due to zooming are corrected and focusing is performed. In order to realize compactness and high zoom ratio in the zoom lens composed of the fourth lens unit having a positive refractive power and the equivalent glass T, is it necessary to reduce the air space between the lens units constituting this zoom lens? , Or it is necessary to increase the refractive power of each lens group.

In the former case, it is important to shorten the moving area of the second lens unit for zooming and the air gap between the third lens unit and the fourth lens unit. By the way, when the aperture stop S is arranged in front of the third lens unit, the air distance between the third lens unit and the fourth lens unit is for determining the condition of the telecentric system. If it is too close to the position, the angle of the chief ray that exits the fourth lens unit becomes large, and an undesirable phenomenon such as poor color reproducibility occurs.

Therefore, in order to solve this problem, the third
The air gap between the positive and negative lenses of the group,

[0012]

[Equation 7]

Within the range of, that is, as shown in FIG.
The air distance between the positive lens X and the negative lens Y of the third group is increased so that the front principal point H and the rear principal point H ′ of the third group are located inside the third group positive lens or in the object side space. It is a telephoto type that has the characteristics of this.

As a result, the air gap between the third lens unit and the fourth lens unit becomes small, and it is possible to prevent an increase in the size from the third lens unit to the image plane. Since the principal point interval e between the second lens unit and the third lens unit on the telephoto end side can be reduced while providing a space for arranging the aperture stop S, the movement area of the second lens unit for zooming is large. Can be prevented.

If the lower limit of (Equation 1) is exceeded, the principal point position of the third lens unit will not be located in front of the third lens unit, and it will not contribute to downsizing of the lens system. If the upper limit is exceeded, the refractive power of the positive lens in the third group will be weak and the refractive power of the negative lens will be strong, so that the Petzval sum will be large on the negative side, and good image surface characteristics cannot be obtained.

In the latter case, the Petzval sum becomes large on the minus side, and good image plane characteristics cannot be obtained. In order to solve this, it is desirable to use a glass material having a low refractive index for the positive lenses of the first, third and fourth groups, and a glass material having a high refractive index for the negative lenses of the second group.

The first group is composed of a cemented positive lens composed of a negative meniscus lens and a biconvex positive lens, and a meniscus positive lens having a convex surface facing the object side. A glass material having a low refractive index is used for the biconvex positive lens and the meniscus positive lens. ing. The cemented positive lens reduces the occurrence of axial chromatic aberration, and the positive meniscus lens is the first
The power of the group is strengthened and the fluctuation of spherical aberration and coma is suppressed to a small level. In addition, the positive meniscus lens also plays a role of correcting a large negative distortion aberration generated on the wide-angle end side.

The second lens group is composed of a meniscus negative lens and a cemented negative lens of a biconcave negative lens and a meniscus positive lens, and a glass material having a high refractive index is used for the meniscus negative lens. With the cemented negative lens, the variation of chromatic aberration due to zooming is suppressed to a small extent, and chromatic aberration of magnification is corrected well.

The third lens group is composed of a biconvex positive lens and a meniscus negative lens having a convex surface facing the image side. The biconvex positive lens uses a glass material having a low refractive index. The divergent light flux from the second group is made afocal to correct the spherical aberration small.

The fourth group consists of a cemented positive lens having a meniscus negative lens and a biconvex positive lens, and a positive lens having a strong convex surface facing the object side. The biconvex positive lens and the positive lens are made of a glass material having a low refractive index. I am using. The cemented positive lens corrects a small amount of axial chromatic aberration, the positive lens strengthens the power of the fourth lens group, and corrects the image plane variation due to zooming or suppresses the spherical aberration variation due to movement for focusing.

However, when a glass material having a low refractive index is used for the positive lens group positive lens, the curvature of the surface becomes strong and a high-order coma aberration occurs with a small number of constituents, which is not preferable. Japanese Patent Laid-Open No. 4-
In Japanese Patent No. 361214, the maximum aperture ratio remains at about F / 1.8, and if an even larger aperture is attempted, high performance cannot be realized for the above reason.

[0022]

[Equation 8]

The third lens group is composed of two lenses, a positive lens element and a negative lens element, for achieving a large aperture. It is necessary to use a low refractive index glass material for the third lens group negative lens. As described above, a glass material having a low refractive index is used for the third lens group positive lens, but the curvature of the lens surface becomes strong due to the low refractive index, and the incident angle of the axial ray incident on the third lens group becomes compact. Because of the large size, large negative spherical aberration occurs in the third lens group.
Therefore, the burden of aberration correction on the third group becomes large, and a large number of lenses is required for the third group.

In order to compose the third lens group with two lens groups, two lenses are provided.
An aspherical surface is introduced on the object side of the group positive lens. In this case, the aspherical shape has a curvature that becomes weaker from the optical axis toward the lens outer diameter, and a large negative spherical aberration due to the spherical surface can be favorably corrected. Japanese Unexamined Patent Publication No. 4-310910 describes that it is appropriate to use a high refractive index glass material for the third lens group positive lens. However, in the case where the third lens group is composed of only two lens groups and two spherical lens systems. By using a high refractive index glass material for the third lens group positive lens, it is possible to weaken the curvature of the surface and suppress the occurrence of spherical aberration, so that spherical aberration in the entire system is well corrected.

Although it is stated that the aspherical lens is not satisfactory in terms of cost, low-cost glass mold lenses are being put to practical use thereafter, and it is rational by satisfying the following processing restriction conditions. Design can be realized.

(1) Limited to a glass material having a relatively low refractive index.
(2) The shape of the lens itself and the assembling eccentricity are made into a convenient shape. (3) The aspherical shape is made to have no inflection point.
(4) The ratio of the central thickness to the peripheral thickness is not large.
(5) Keep the amount of aspherical surface small.

At this time, the radius of curvature of the aspherical reference spherical surface is r
12, in the F number ray height on the short focus side in the third group,
When r12 is a reference spherical surface, Δ is the aspherical amount of the aspherical surface with respect to the reference spherical surface in the optical axis direction, and NA is the image-side numerical aperture on the short focus side.

[0028]

[Equation 9]

It is desirable to satisfy the following condition. If the lower limit of (Equation 9) is exceeded, the eccentricity accuracy and manufacturing error deteriorate, which is not preferable. If the upper limit is exceeded, the residual amount of undercorrected spherical aberration is too large. (1) coincides with the improvement of Petzval sum, and in order to satisfy (2), it is desirable that the aspherical lens has a single-sided aspherical surface.

At this time, it is desirable that the negative lens of the third lens group be a meniscus lens convex on the image side. In order to achieve a large aperture in the four-group zoom lens of the present embodiment, FIG.
As shown in the optical path diagram of (3), coma flare due to a light beam below the principal ray L becomes a problem when opening near the short focal point side, which causes deterioration of image quality. In the concave surface of the third lens group negative lens, high-order overcorrected spherical aberration is generated, and the correction amount of the spherical aberration due to the aspherical amount of the third lens group positive lens is well balanced between the on-axis aberration curve and the off-axis aberration curve. This coma flare is also kept small, and it also fulfills the role of satisfying the condition (5).

When a glass material having a high refractive index exceeding 1.65 is used for the third lens group negative lens, the positive lens surface has a large curvature because the third lens group positive lens has a low refractive index. As a result, too much high-order coma aberration occurs, which makes it difficult to correct the residual aberration. In addition, a large amount of uncorrected spherical aberration is generated, and the shape also violates the conditions (3) and (4), which is not preferable. Abbe number of the third group negative lens is 50
With a low-dispersion glass exceeding the above range, the difference in dispersion between the positive lens and the negative lens becomes small, so that the residual component of the axial chromatic aberration in the third lens group becomes large, and it becomes difficult to correct the axial chromatic aberration of the entire system. .

As described above, by using the low refractive index glass for the third group negative lens, it is possible to excellently correct various aberrations, so that a large aperture lens is realized. Further, by disposing the aperture stop on the object side of the third lens unit, the height of the off-axis light beam in the first lens unit becomes low, which contributes to the reduction of the lens diameter of the first lens unit and the telecentric system condition. You can get closer. The above (Equation 7) and (Equation 8) are conditions for realizing a compact and large-diameter zoom lens.

[0033]

[Equation 10]

And

[0035]

[Equation 11]

Is for defining the front principal point position H and the rear principal point position H'of the third group shown in FIG. 1, f31 is the focal length of the third group positive lens X, and f32 is the negative lens. The focal length of Y is shown, and e is the principal point distance e between the positive lens and the negative lens. Exceeding the lower limits of (Equation 10) and (Equation 11) means that the air gap between the third lens group positive lens X and the negative lens Y becomes large, and the height of the axial ray incident on the negative lens Y is high. The effect of the concave surface is weakened due to the decrease of the light intensity, and a coma flare due to a light beam below the principal ray L shown in FIG. Further, since the third lens group positive lens has a weak refractive power and the negative lens element has a strong refractive power, the Petzval sum becomes large on the negative side, which is not preferable. When the upper limit is exceeded, the principal point position of the third lens unit is not located in front of the third lens unit, which does not contribute to downsizing of the lens system.

[0037]

[Equation 12]

Is a conditional expression relating to the air gap between the third group and the fourth group, and Lw is the third group at the wide-angle end when the object point is at infinity.
The air distance between the fourth lens unit and the fourth lens unit, and f4 represents the focal length of the fourth lens unit. When the value goes below the lower limit, the moving space of the fourth lens unit for focusing becomes insufficient, so that the off-axis ray height incident on the fourth lens unit becomes low against the shortened close-up distance of the photographing and the conditions of the telecentric system. Correction of chromatic aberration is also disadvantageous. If the upper limit is exceeded, the off-axis rays entering the fourth lens group will become high, and the outer diameter of the fourth lens group will be increased in order to secure the amount of light, which is against compactness.

[0039]

[Equation 13]

Represents the position sensitivity on the image plane with respect to the minute movement amount of the second lens unit at the telephoto end, and β2 and β3
And β4 are the second values at infinity at the telephoto end.
The lateral magnifications of the group, the third group, and the fourth group are shown, respectively.
In the method of correcting the image plane variation due to the magnification change or the change of the object distance by the fourth lens group, the fourth lens group moves along a convex locus toward the object side together with the movement of the second lens group having the variable magnification function. Therefore, it is necessary to determine the movement curve of the fourth group so that control by a motor or the like does not become difficult during autofocus.

When the value goes below the lower limit, the movement amount of the second lens unit becomes large, which is against the compactness. If the upper limit is exceeded, the positional sensitivity and eccentricity sensitivity of the second group will become severe, which will interfere with the assembly of the lens system and the amount of movement of the fourth group with respect to the second group near the telephoto end at infinity. The change in the value becomes severe, and it becomes difficult to control the autofocus by the motor or the like.
In JP-A-4-310910, this range is 1.2.
~ 1.6. Due to the high zoom ratio and compactness, the range of (Equation 13) is shifted in the direction in which focus control becomes difficult. However, mechanical improvements can be made to improve control technology. For example, by increasing the resolution of the zoom encoder that controls the position of the second lens unit, the amount of movement of the fourth lens unit is reduced, and autofocus control becomes possible. Therefore, this range is also practical.

Examples 1 to 4 satisfying these conditions are shown in the following table. In the table, r1, r2, ··· are the radii of curvature of each lens surface counted from the object side, d1, d2, · · · are the wall thickness and air gap of each lens, and n1, n2, · · · are each lens , D1, and ν1, ν2, ··· are Abbe numbers based on the d line. r21 and r22 are refracting surfaces of an equivalent parallel plane glass corresponding to a crystal filter or a glass plate of an image pickup device.
Further, the aberration performance diagrams of the respective examples are shown in order from FIG. 4 to FIG. 15 at the wide-angle end, the middle and the telephoto end, and the focal length of the entire system is f, the F number is F /, and the angle of view is 2ω. Shows. The aspherical coefficients A, B, C, D are calculated by

[0043]

[Equation 14]

Shall be given in. However, the optical axis direction is x
The distance from the axis and the optical axis is h, and r is the radius of curvature of the reference spherical surface.

(Example 1)

[0046]

[Table 1]

(Example 2)

[0048]

[Table 2]

(Example 3)

[0050]

[Table 3]

(Example 4)

[0052]

[Table 4]

[0053]

As is apparent from the above description, under the lens structure and conditions of the present invention, the F number is 1.4 to 1.
6. It is possible to realize a compact and high-performance zoom lens for a video camera having a zoom ratio of about 10 to 12 with 11 constituent elements including one aspherical surface.

[Brief description of drawings]

FIG. 1 is a diagram showing a principal point position of a third lens group.

FIG. 2 is an on-axis and off-axis optical path diagram in the present invention.

FIG. 3 is a configuration diagram of an embodiment of the present invention.

FIG. 4 is a diagram showing aberration performance at the wide-angle end in Embodiment 1.

FIG. 5 is a diagram showing aberration performance at an intermediate position in Example 1.

6 is a diagram showing aberration performance at the telephoto end in Embodiment 1. FIG.

7 is a diagram showing aberration performance at the wide-angle end in Embodiment 2. FIG.

FIG. 8 is a diagram showing aberration performance at an intermediate position in Example 2.

9 is a diagram showing aberration performance at the telephoto end in Embodiment 2. FIG.

FIG. 10 is a diagram showing aberration performance at the wide-angle end in Embodiment 3.

FIG. 11 is a diagram showing aberration performance at an intermediate position in Example 3;

FIG. 12 is a diagram showing aberration performance at the telephoto end in Embodiment 3.

FIG. 13 is a diagram showing aberration performance at the wide-angle end in Embodiment 4.

FIG. 14 is a diagram showing aberration performance at an intermediate position in Example 4.

FIG. 15 is a diagram showing aberration performance at the telephoto end in Embodiment 4.

Claims (2)

[Claims] 1. A first lens unit having a positive refracting power composed of a cemented positive lens and a meniscus positive lens in order from the object side, and a meniscus negative lens and a cemented negative lens. A second lens unit having a negative refracting power, which has a function of changing the magnification, a positive lens having a biconvex positive lens having an aspherical surface on the object side and a negative meniscus lens having an image side convex surface. A third lens unit having a refractive power, and a fourth lens unit having a positive refractive power having a function of correcting an image plane variation due to zooming or a change in object distance, which is composed of three cemented positive lenses and a positive lens. And an aperture stop is arranged on the object side of the positive lens of the third group, and the air gap between the positive lens and the negative lens of the third group is d.
13, the focal length of the third lens unit is f3, the refractive index of the third lens unit negative lens at the d-line is n3n, and the Abbe number is ν3n. [Equation 2] A zoom lens characterized by satisfying. 2. A focal length of the third lens group positive lens is f31, a focal length of the third lens group negative lens is f32, a principal point distance between the positive lens and the negative lens is e, and an object point at infinity at the wide angle end is used. The air distance between the third group and the fourth group is Lw, and the focal length of the fourth group is f
4 and the second group at infinity at the telephoto end,
The lateral magnifications of the third group and the fourth group are β2, β3, and β, respectively.
When set to 4, [Equation 4] [Equation 5] [Equation 6] The zoom lens according to claim 1, wherein