JPH09311275A - Small-sized zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten entire length by making the diameters of a front lens and a rear lens small by providing a third lens group or a fourth lens group with an aspherical surface and satisfying a specified condition. SOLUTION: This small-sized zoom lens is constituted of seven groups being eight lenses, that is, a first group lens being a biconvex lens and a biconcave lens and having negative power, a second group lens consisting of a meniscus lens having a concave surface on an object side and having negative power, the third and the fourth group lenses consisting of the positive bicovex lenses which are a front group, a fifth lens group consisting of the positive biconvex lens and sixth and seventh group lenses consisting of the negative lens and the negative meniscus lens having strong curvature on the object side which are a rear group. At least one surface of the third or the fourth group lens possesses the aspherical surface, and the conditions of -1.77<f1.2/fw<-0.88 and 0.7<fI.G/fw<0.84 are satisfied. In this case, f1.2 is the focal distance of the first and the second group lenses, (fw) is the focal distance at a wide end, and fI.G is the focal distance of the front group.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact zoom lens used in compact cameras such as lens shutter cameras.

[0002]

2. Description of the Related Art Conventionally, as this type of zoom lens, there is one in which the first lens group is composed of a negative power lens,
For example, the first lens of the first lens group of the telephoto type two-group zoom lens is a negative lens.
There is JP-A-3-213814, in which the third lens unit and the first lens unit are composed of a lens unit having a negative power in order from the object side and a lens unit having a positive power.

[0003]

However, the former is not practical because the number of lenses is small but the material is expensive. In the latter case, the number of lenses is large, the total lens length is long, the back focus cannot be lengthened, the rear lens diameter becomes large, and it cannot be made compact.

SUMMARY OF THE INVENTION An object of the present invention is to provide a compact zoom lens having a short overall length, in which the diameters of the front lens and the rear lens are reduced in order to reduce the size of the lens itself. Furthermore,
The present invention provides a compact zoom lens which has a small zoom-out amount in spite of having a zoom ratio of about 3 in order to reduce the size of the lens barrel.

[0005]

In order to achieve the above object, the present invention has the following lens system configuration. That is, in a two-group type zoom lens including a front group having a positive power and a rear group having a negative power and changing the focal length by changing the distance between the two groups, the front group is arranged in order from the object side. A first lens group consisting of a biconvex lens and a biconcave lens cemented together with negative power, a second lens group consisting of a negative power meniscus lens having a concave surface on the object side, and a third lens group consisting of a positive biconvex lens. A fourth group lens composed of a positive biconvex lens, a fifth group lens composed of a rear group composed of a positive biconvex lens in order from the object side, and a sixth group lens composed of a negative lens having a strong curvature on the object side. ,
The seventh lens group consisting of a negative meniscus lens having a strong curvature on the object side and the seventh lens group consisting of eight lenses,
At least one of the group lens or the fourth group lens
(EN) Provided is a small-sized zoom lens having two surfaces having an aspherical surface and satisfying the following conditional expressions.

[0006]

[Equation 1]

The first group lens of the positive / negative type two-group zoom lens is usually a positive lens, but in order to reduce the front lens diameter and further lengthen the back focus at the wide end, a negative lens is used. It has a retrofocus type configuration that starts from the lens.

In the case of the retrofocus type, the total lens length tends to be longer than the focal length, but the total length is shortened by using an aspherical lens for the third lens group or the fourth lens group.

The conditional expression (1) relates to the power of the negative lens group of the front group, and if the lower limit is exceeded, aberration correction will be easy, but the back focus will become short, the rear lens diameter will become large, and the upper limit will be exceeded. And the back focus becomes too long, which becomes a factor of enlarging the lens frame.

The conditional expression (2) represents a limitation on the total length of the lens system. When the value goes below the lower limit, the back focus at the telephoto end becomes long, so that the amount of extension due to zooming becomes large and the lens frame becomes large. If the upper limit is exceeded, the distance between the front group and the rear group at the wide end becomes large, and the total lens length becomes long. Further, the back focus at the wide end becomes shorter, which becomes a factor of increasing the rear lens diameter.

In terms of aberration, if the upper limit of (1) is exceeded, coma will increase and astigmatism in the negative direction will increase.
If the lower limit of (2) is exceeded, negative spherical aberration will increase.

Further, by providing an aspherical surface to the third or fourth lens group, it is possible to shorten the total lens length of the front lens group without increasing the number of lenses despite the retrofocus type. Alternatively, the aspherical surface of the fourth lens group corrects negative spherical aberration. And
The cemented lens of the first group has an appropriate difference in refractive index to correct field curvature and astigmatism.

According to the structure of the present invention, by reducing the diameters of the front lens and the rear lens, the lens itself can be downsized, and a compact zoom lens having a short total length can be provided. Further, since the size of the lens barrel is reduced, it is possible to provide a compact zoom lens having a zoom ratio of about 3 and a small amount of extension during zooming.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. The aspherical surface used in the present invention is given by the following equation.

[0015]

[Equation 2]

[0016]

EXAMPLES Four examples of the present invention will be described in Tables 1 to 4 below. Further, sectional views and aberration diagrams of the respective examples are shown in FIGS. 1 to 5. In each example, the surface number indicates the corresponding surface number of each lens, which is counted from the object side. The intervals 10 and 16 change due to zooming. Aspherical data are shown in the bottom column along with the surface number. The surface number 10 corresponds to the diaphragm surface, and the radius of curvature 0 indicates that the radius of curvature is infinite.

The refractive index (ni) is the refractive index at the d-line (588 nm), and the dispersion is the Abbe number. The aspherical coefficient indicates the value of each coefficient shown in the mathematical formula. Also, the interval (d
i) is negative when it is on the left side of the front surface, positive when it is on the right side, and the radius of curvature (ri) is negative when it is concave on the object side and positive when it is convex.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

[Table 4]

The features of the embodiment are shown below. Example 1 is
This is a typical example of this embodiment. The second embodiment is an example in which the back focus becomes long at the wide end. Example 3
This is an example in which the distance between the front and rear groups is wide and the back focus is short at the wide end. The fourth embodiment is an example in which the feeding amount is large.

[0023]

By using a negative lens for the first lens group and using an aspherical surface for the third or fourth lens group, the lens diameter and the total lens length can be shortened, and the lens frame can be downsized. A zoom lens that can satisfactorily correct various aberrations can be provided.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a zoom lens system according to the present invention.

FIG. 2 is a sectional view on the short focus side and aberration diagrams on the short focus side and the long focus side of Example 1.

FIG. 3 is a cross-sectional view on the short focus side and aberration diagrams on the short focus side and the long focus side of Example 2.

FIG. 4 is a sectional view on the short focus side and aberration diagrams on the short focus side and the long focus side of Example 3;

FIG. 5 is a cross-sectional view on the short focus side and aberration diagrams on the short focus side and the long focus side of Example 4.

[Explanation of symbols]

di interval (negative if it is on the left of the front surface, positive if it is on the right) ri radius of curvature (negative if concave on the object side, positive if convex)

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 the two groups. From the side, a bi-convex lens and a bi-concave lens are cemented together to have a negative power, a first group lens is composed of a negative power meniscus lens having a concave surface on the object side, and a positive bi-convex lens. A third lens group including a third lens group, a fourth lens group including a positive biconvex lens element, a fifth lens group including a rear lens group including a positive biconvex lens element in order from the object side, and a negative lens element having a strong curvature on the object side. It is composed of 8 lenses in 7 groups including a group lens and a 7th group lens composed of a negative meniscus lens having a strong curvature on the object side, and at least one surface of either the 3rd group lens or the 4th group lens is Has a spherical, and compact zoom lens that satisfies the following conditional expressions. [Equation 1]