JPH0588083A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To reduce the size and obtain a high power variation ratio by composing a 2nd lens group of four-group constitution of a negative lens, a biconcave lens, and a positive lens, i.e., three elements in three groups in order from an object side and properly setting the radii of curvature of the 4th and 5th elements from the object side of the 2nd lens group. CONSTITUTION:This zoom lens has a 1st lens group I with positive refracting power, the 2nd lens group II with negative refracting power, a 3rd lens group III with positive refracting power, and the 4th lens group IV with positive refracting power; while the 1st and 3rd lens groups I and III are fixed, the 2nd lens group II is moved to vary the power and the 4th lens group IV is moved so as to put the zoom lens in focus while correcting image plane variation due to the power variation of the 4th lens group IV. The 2nd lens group II has three single lenses, i.e., a negative lens which is greatly concave to the image plane side, a biconcave lens, and a positive lens which is greatly convex to the object side and satisfies an inequality I. In the inequality I, RIIi is the radius of curvature of an (i)th lens surface in the 2nd lens group.

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 an F number of about 1.6 to 1.8 used in a video camera or a photographic camera, and particularly to a rear focus type zoom lens having a small zoom ratio and a high zoom ratio. It is about.

[0002]

2. Description of the Related Art Conventionally, various zoom lenses used for video cameras, photographic cameras and the like have been proposed which employ a so-called rear focus type in which focusing is performed by moving a lens unit other than the first lens unit on the object side. There is. In general, a rear focus type zoom lens has a feature that a relatively small and lightweight lens group is moved to perform focusing, so that the driving force of the focus lens group is small and quick focusing can be performed.

For example, in Japanese Patent Laid-Open No. 62-24213, a first lens group having a positive refractive power, a second lens group having a negative refractive power for zooming, and a third lens having a positive refractive power are sequentially arranged from the object side. Group, and four lens groups of a fourth lens group having a positive refractive power, wherein the first and third lens groups are fixed, and the second lens group is fixed.
Rear focus type zoom lens for moving the fourth lens group so as to correct the image plane variation due to the zooming while moving the lens group for zooming and for focusing by moving the fourth lens group. Is proposed. As seen from the examples of this patent, the second lens group having a variable power function is composed of a negative meniscus single lens whose convex surface faces the object side in order from the object side, and a negative lens whose both lens surfaces are concave surfaces. It consists of a cemented lens cemented with a positive lens.
The cemented lens mainly corrects axial chromatic aberration, spherical aberration, and off-axis coma.

Further, if the refractive power of the second lens group is simply strengthened in order to further reduce the size and increase the magnification of such a conventional zoom lens, the aberration of the cemented surface of the second lens group can be corrected. The burden was too great, and it was difficult to achieve high performance in the entire zoom range.

[0005]

SUMMARY OF THE INVENTION The present invention is a technique relating to the improvement of the rear focus type zoom lens described above, and is intended to further improve the performance and downsize the zoom lens having a large aperture ratio and a high zoom ratio. To aim.

[0006]

A feature of the present invention is that the first lens group having a positive refractive power, the second lens group having a negative refractive power, and the first lens group having a positive refractive power are arranged in this order from the object side. The third lens group and the fourth lens group of the fourth lens group having a positive refractive power are provided, and the first and third lens groups are stationary, and the second lens group is fixed.
A zoom lens that moves a lens group to perform zooming, moves the fourth lens group to correct an image plane variation due to zooming, and moves the fourth lens group to perform focusing. The second lens group has three single lenses in order from the object side: a negative lens with a strong concave surface facing the image side, a biconcave lens, and a positive lens with a strong convex surface facing the object side. and to satisfy the _{R II5 / R II4 <0.92 (} 1) following condition. However, R _IIi is the radius of curvature of the ith lens surface counted from the object side of the second lens group.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a lens sectional view at the wide-angle end according to an embodiment of the present invention. In the figure, I is the first lens group of positive refractive power which is fixed during zooming, II is the second lens group of negative refractive power for zooming, and III is the third lens of positive refractive power which is fixed during zooming. group,
Reference numeral IV denotes a fourth lens unit having a positive refractive power that corrects an image plane variation due to zooming and focuses, and SP is a fixed diaphragm. Then, zooming from the wide-angle end to the telephoto end is performed by moving each lens group along the optical axis so as to draw a locus as shown by an arrow.

In this embodiment, in such a zoom type, the second lens unit has a negative lens having a strong concave surface facing the image side in order from the object side, a biconcave lens, and a positive lens having a strong convex surface facing the object side. It is possible to correct aberrations satisfactorily in the entire zoom range even if the zoom type is changed slightly and the magnification is increased by specifying the conditional expression (1).

Conditional expression (1) relates to the curvature of the air lens provided between the second lens and the third lens in the second lens group, and if R _II5 becomes larger _{than the} upper limit, spherical aberration at the telephoto end. Is overcorrected, and when the value goes below the lower limit, coma occurs and the performance near the wide end deteriorates.

By constructing the second lens group as described above in the present embodiment, it is possible to achieve miniaturization and high magnification while maintaining high optical performance, but to achieve further miniaturization and high performance. By configuring the fourth lens group that moves by zooming with a single lens consisting of at least one positive lens and one negative lens, it is possible to suppress the variable chromatic aberration that occurs in the fourth lens group and to introduce an aspherical surface. , Spherical aberration and distortion in the intermediate range of zooming are corrected.

Further, by constructing the third lens group by a single lens having at least one aspherical surface, the number of lens elements can be reduced to shorten the overall length, and spherical aberration at the wide end can be favorably aspherical. Correcting.

When the refracting power of the second lens group is strengthened in an attempt to reduce the size and increase the magnification, the curvature of the lenses forming the second lens group becomes small, and aberration occurs, which causes aberration fluctuation due to zooming. Since it becomes large and the optical performance deteriorates, it is desirable that the average value of the refractive index of the lenses constituting the second lens group is 1.65 or more.

Next, numerical examples of the present invention will be shown. In Numerical Examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap from the object side, and Ni and Vi are of the i-th lens in order from the object side, respectively. The refractive index of glass and the Abbe number. R ₁₉ and R ₂₀ are face plates, filters and the like.

The aspherical shape has an X axis in the optical axis direction, an H axis in the direction perpendicular to the optical axis, a positive light traveling direction, and R as a paraxial radius of curvature,
When K, B, C, D, and E are aspherical coefficients, respectively

[0015]

[Outer 1] It is expressed by

The table shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.

[0017]

[Outside 2]

[0018]

[Outside 3]

[0019]

[Outside 4]

[0020]

[Table 1]

[0021]

As described above, according to the present invention,
F-number 1.6 to 1.8 and large aperture ratio, zoom ratio 10 to 1
It is possible to achieve a rear focus type zoom lens having a high zoom ratio of about 2, a small size, a small number of components, and excellent aberration correction with excellent optical performance over the entire zoom range.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of a zoom lens according to the present invention.

FIG. 2 is a diagram of various types of aberration at the wide-angle end of the zoom lens of Numerical Example 1.

FIG. 3 is a diagram of various types of aberration in the intermediate portion of the zoom lens of Numerical Example 1.

FIG. 4 is a diagram of various types of aberration at the telephoto end of the zoom lens of Numerical Example 1.

FIG. 5 is a diagram of various types of aberration at the wide-angle end of the zoom lens of Numerical Example 2.

FIG. 6 is a diagram of various types of aberration in the intermediate portion of the zoom lens of Numerical Example 2.

FIG. 7 is a diagram of various types of aberration at the telephoto end of the zoom lens of Numerical Example 2.

FIG. 8 is a diagram of various types of aberration at the wide-angle end of the zoom lens of Numerical Example 3.

FIG. 9 is a diagram of various types of aberration in the middle part of the zoom lens of Numerical Example 3.

FIG. 10 is a diagram of various types of aberration at the telephoto end of the zoom lens of Numerical Example 3.

[Explanation of symbols]

 I First lens group II Second lens group III Third lens group IV Fourth lens group d d line g g line ΔM meridional image surface ΔS sagittal image surface

Claims (3)

[Claims] 1. A first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, in order from the object side. It has four lens groups, and the first and third lens groups are made stationary, and the second lens group is moved to perform zooming, so that the fourth lens group can correct the image plane variation due to zooming. In the zoom lens for moving the fourth lens group to perform focusing, the second lens group includes, in order from the object side, a negative lens having a strong concave surface toward the image side, a biconcave lens, and an object. zoom lens satisfies the _{0.38 <R II5 / R II4 <} 0.92 condition: has three single lenses of a positive lens having a strong convex surface facing the side. However, R _IIi is the i-th lens counted from the object side of the second lens group.
The radius of curvature of the th lens surface. 2. The fourth lens group has at least one single lens consisting of a positive lens and a negative lens, and at least one
The zoom lens according to claim 1, wherein the zoom lens has an aspherical surface equal to or larger than the surface. 3. The zoom lens according to claim 1, wherein the third lens group comprises a single lens having one or more aspherical surfaces.