JPH10232350A - Compact zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an inexpensive high-performance compact zoom lens constituted of small number of lenses and adopting a plastic lens by constituting the zoom lens of a positive front group consisting of 1st to 4th lenses and a negative rear group consisting of 5th and 6th lenses and satisfying specified conditional expressions. SOLUTION: This zoom lens is constituted of the positive front group 1 and the negative rear group 2, and a focal distance is changed by changing an interval between the groups. The front group 1 is constituted of the 1st lens which is a negative meniscus and whose object side is a convex surface, the 2nd lens which is negative, the 3rd lens which is positive and the 4th lens which is a positive meniscus and whose object side is a concave surface from the object side. The rear group 2 is constituted of the 5th lens which is positive and whose object side is the concave surface and the 6th lens which has large curvature on the object side from the object side. The 1st group lens and the 5th group lens have at least one aspherical surface. The conditional expressions f1/5<f2<0 and |f1.2|>2.0*F1 are satisfied. In the expressions, f1 and f2 are the focal distances of the 1st and the 2nd lenses, F1 is the focal distance of the 1st group and f1.2 is the synthetic focal distance of the 1st and the 2nd lenses.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small zoom lens used for a lens shutter type compact camera or the like.

[0002]

2. Description of the Related Art As a zoom lens of this type, a lens system in which the number of lenses is particularly reduced and cost is reduced has been proposed in recent years (JP-A-3-127008, JP-A-3-3208).
-127,009, JP-A-3-158814, JP-A-3-158
168608).

[0003] In addition, a zoom lens system which actively uses a plastic material whose precision has been remarkably advanced in recent years has been proposed (Japanese Patent Laid-Open No. 3-180808).

[0004]

However, although the number of sheets has been reduced, a thin plastic aspherical layer is formed on a glass aspherical lens or a glass-based lens which is still expensive to manufacture. At present, it is necessary to use the formed hybrid lens and the like, and it cannot be said that the cost is sufficiently satisfied.

Further, a plastic zoom lens system cannot be said to have sufficient performance, and there is a problem that the number of lenses used is large.

SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive and high-performance compact zoom lens having a zoom ratio of about 2.5 times and comprising six lenses and a small number of lenses and adopting a plastic lens. Is to provide.

[0007]

In order to achieve the above object, the present invention comprises a front group having a positive power and a rear group having a negative power, and the focal length is changed by changing the distance between the two groups. In the zoom lens of the two-group system for changing, the front group includes, in order from the object side, a meniscus-shaped first lens having a negative power with a convex surface facing the object side, a second lens having a negative power, and a positive power. A third lens having a meniscus fourth lens having a positive power with a concave surface facing the object side, wherein the rear group is a fifth lens having a positive power with a concave surface facing the object side in order from the object side. A lens system having a negative sixth lens having a strong curvature on the object side, wherein both the first lens unit and the fifth lens unit are lens systems having at least one aspheric surface, and satisfy the following conditional expressions. That you have configured To provide a compact zoom lens according to symptoms.

F 1/5 <f2 <0 (1) | f1 · 2 |> 2.0 * F1 (2) where f1: focal length of first lens f2: second lens Focal length F1: Focal length of first group f1.2: Synthetic focal length of first and second lenses That is, the first and fifth lenses are composed of a total of six lenses in six groups, and both the first and fifth lenses have at least one surface. By adopting a lens system having an aspherical surface, it is possible to satisfactorily correct various aberrations in combination with the above structural features.

Here, in the two-unit zoom lens of this type, the aberration correction is characterized by the asymmetry of the power arrangement. Such an arrangement includes a so-called retrofocus zoom having a negative power in the front group and a positive power in the rear group in terms of aberration correction, and therefore, a configuration in which the arrangement of each lens is completely reversed. Many are seen (JP-A-62-220915 and JP-A-3-238413).

The main points of aberration correction are as follows.
Negative spherical aberration generated in the positive first lens group and large fluctuation of spherical aberration in the positive direction due to a change in the distance between the first and second lens groups are suppressed, and large positive distortion generated in the short focal length side is corrected. And the correction of the Petzval sum, which tends to be negative due to downsizing, and the like.

According to the above configuration, most of the power of the first lens unit is concentrated on a portion near the stop. However, in order to suppress generation of spherical aberration due to power concentration in a single lens, positive power is divided into a third lens and a fourth lens to suppress generation of spherical aberration. In addition, the first and second lenses are provided with only the weak power represented by the conditional expression (2), thereby preventing the power from being dispersed in the first lens unit and making it difficult to correct aberration.

Further, the distortion and the field curvature are corrected by employing an aspherical surface on at least one surface of the first lens. Further, this aspherical surface also has a function of alleviating fluctuation of spherical aberration due to zoom. Further, at least one surface of the fifth lens is made aspherical, thereby correcting distortion and astigmatism. With the configuration as described above, good aberration can be maintained even though the number of components is small.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. Examples of the present invention are shown in Tables 1 to 3, and sectional views and aberration diagrams showing the structure of the present invention are shown in FIGS.

The aspheric surface used in the present invention is given by the following equation.

[0015]

(Equation 1)

Where z: depth from the tangent plane to the surface vertex c: paraxial curvature of the surface h: height from the optical axis k: conical constant A: fourth order aspherical coefficient B: sixth order non-spherical surface Spherical coefficient C: 8th order aspherical coefficient D: 10th order aspherical coefficient

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the zoom lens according to the present invention will be described below. In each embodiment, the surface number indicates the corresponding surface number of each lens counted in order from the object side, and the intervals 9 and 13 are changed by zooming. The data of the aspherical surface is shown in the lowermost column together with the surface number. Also, surface number 9
Corresponds to the stop surface, and a curvature radius of 0 indicates that the curvature radius is infinite. The refractive index is d-line (588 nm)
Denotes the refractive index, and dispersion denotes the Abbe number. The aspheric coefficient indicates the value of each coefficient shown in the equation.

Here, the interval is negative when left of the front surface, positive when right, and the radius of curvature is negative when concave on the object side and positive when convex.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

As described above, according to the present invention, the sixth group and the sixth group are provided.
A small, high-performance and inexpensive small zoom lens can be provided by using plastics while having a small number of lenses.

[Brief description of the drawings]

2 to 4 show a sectional view on the short focus side and aberration diagrams on the short focus side and the long focus side with respect to the first to third embodiments of the present invention.

FIG. 1 is a sectional view showing a configuration of a small zoom lens according to the present invention.

FIG. 2 is a cross-sectional view and aberration diagrams of the first embodiment of the present invention.

FIG. 3 is a cross-sectional view and aberration diagrams of a second embodiment of the present invention.

FIG. 4 is a cross-sectional view and aberration diagrams of a third embodiment of the present invention.

[Explanation of symbols]

 1 front group 2 rear group

Claims (1)

[Claims] 1. A two-group type zoom lens comprising a front group having a positive power and a rear group having a negative power and changing a focal length by changing an interval between two groups, wherein the front group is In order from the object side, a meniscus first lens having a negative power with a convex surface facing the object side, a second lens having a negative power, a third lens having a positive power, and a positive lens having a concave surface facing the object side. The rear group includes, in order from the object side, a fifth lens having a positive power with a concave surface facing the object side, and a negative sixth lens having a strong curvature on the object side. Wherein both the first lens group and the fifth lens group have at least 1
A small zoom lens, comprising: a lens system having one surface having an aspherical surface, wherein the lens system is configured to satisfy the following conditional expressions. f 1/5 <f2 <0 | f1 · 2 |> 2.0 * F1 where f1: focal length of the first lens f2: focal length of the second lens F1: focal length of the first group f1 · 2: second Composite focal length of 1.2 lenses