JPH0750251B2 - Small zoom lens - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact zoom lens, and more particularly to a compact zoom lens having a simple structure for reducing the weight of the entire lens system suitable for video cameras, photographic cameras and the like. It is about lenses.

(Prior Art) Conventionally, in a zoom lens for photographing such as a video camera or a photographic camera that uses a luminous flux over substantially the entire visible region, it is possible to obtain excellent optical performance by correcting chromatic aberration particularly well among various aberrations. Has become important.

Of these, fluctuations in chromatic aberration due to focusing and zooming cause a large decrease in optical performance as the fluctuation amount increases.
For this reason, it is particularly important for a zoom lens for photographing to satisfactorily correct variations in these chromatic aberrations.

Generally, in a lens system having a plurality of lens groups that are moved on the optical axis to perform focusing, zooming, and the like, chromatic aberration is satisfactorily corrected in each lens group to be moved. As a result, the variation of chromatic aberration due to the movement of the lens group is minimized.

In order to satisfactorily correct chromatic aberration in the lens group that is moved for focusing and zooming, about 3 to 5 lenses including positive and negative refractive power lenses made of high dispersion and low dispersion materials. Need more configuration.

However, in a so-called 4-group zoom lens in which focusing is performed in the first group on the object side, zooming is performed in the second and third groups, and image formation is performed in the fourth group, all 3 to 5 moving lens groups are used. If the lens is composed of a certain number of lenses, the total lens length becomes long, the lens diameter of the first lens group becomes large, and the weight of the entire lens system increases.

In many zoom lenses, the lens weight of the first group occupies 50% or more of the whole zoom lens. Therefore, in order to reduce the weight of the entire zoom lens, the first group should be as small as possible, for example, one or two lenses. It is preferable to configure the number of lenses.

However, if the first group is composed of about 1 to 2 lenses, it becomes difficult to satisfactorily correct chromatic aberration. Especially, if the first group is composed of one lens, it is easy to reduce the weight of the zoom lens system, but a lot of chromatic aberration occurs. However, it becomes difficult to satisfactorily correct this chromatic aberration.

Japanese Patent Publication No. 57-12967 discloses a so-called four-group zoom lens having a simple structure in which the first group is composed of a single lens having a positive refractive power and a diaphragm is arranged between the third group and the fourth group. A lens system is disclosed. In the zoom lens of the publication, since the first group is composed of a single lens, many chromatic aberrations occur from the first group. Therefore, in this publication, by appropriately setting the lens configurations of the second group and the third group, it is possible to satisfactorily correct variations in chromatic aberration due to focusing and zooming, and to reduce the weight of the entire lens system.

(Problems to be Solved by the Invention) The present invention is a so-called four-group zoom lens in which the first group is composed of a single lens, and the chromatic aberration generated from the first group at this time is a lens of the second group and the third group. By properly setting the configuration, the video lens and photographic camera with a simple configuration aiming to correct the variation of chromatic aberration during focusing and zooming to reduce the overall weight of the lens system and shorten the overall lens length. An object of the present invention is to provide a small-sized zoom lens suitable for the above.

(Means for Solving Problems) First group of positive refractive power for focusing, sequentially from the object side, second group of negative refractive power that moves monotonically during zooming, second group during zooming The third lens unit has a positive refractive power that moves in the opposite direction to the moving direction of the lens unit, and the fourth lens unit that has a positive refractive power and has a fixed image forming action. A fixed diaphragm is arranged between the first lens unit and the second lens unit, the first lens unit is composed of a single lens having a positive refractive power, and the glass lenses of the second lens unit and the third lens group have an equivalent Abbe number of 2, respectively. 3. When the focal length of the i-th group is fi and the focal length at the wide-angle end of the entire system is fw 60 ≦ 2 ≦ 200 (1) −550 ≦ 3 ≦ 0 (2) 3.8 <f1 / fw <5.0 …… (3) 0.5 <| f2 / f3 | <0.7 …… (4) To satisfy the condition.

Other features of the present invention are described in the embodiments.

(Embodiment) FIGS. 1, 2 and 3 are numerical embodiments 1 to 1 described later of the present invention.
3 is a lens cross-sectional view of FIG. In the figure, reference numeral I denotes the first lens unit having a positive refractive power for focusing, which is composed of a single positive lens. II
Is the second group of negative refracting power that monotonically moves to the image side during zooming,
III is a third group of positive refractive power which moves in the opposite direction to the moving direction of the second group during zooming, IV is a fourth group of fixed positive refractive power,
SP is a fixed diaphragm arranged between the second group and the third group.

In this embodiment, the first group is composed of a single positive lens,
The lens weight of the group is reduced. Further, a fixed diaphragm is arranged at a substantially intermediate position of the lens system between the second group and the third group, which is close to the first group, and the second group and the third group are sandwiched with the diaphragm in one direction during zooming. By adopting a zoom type that moves to, the space between the second group and the third group is effectively used, and the total lens length is shortened. Further, the height of incidence of the off-axis chief ray on the first lens unit is reduced to reduce the effective diameter of the first lens unit to reduce the weight of the entire lens system and to reduce the amount of lateral chromatic aberration generated from the first lens unit. There is.

For example, if the volume of the lens is considered proportionally, the volume is proportional to the cube of the lens diameter, so if the lens diameter can be reduced by 10%, the volume can be reduced by about 27% from (0.9) ³ .

In this embodiment, since the first group does not perform achromatization, the refractive power of the single positive lens forming the first group is changed by using a plurality of lenses including conventional lenses having positive and negative refractive powers. It is possible to reduce the refracting power of the positive lens that constitutes the first group of the erased zoom lens.

As a result, in this embodiment, the curvature of the lens surface of the positive lens of the first group can be made small and the lens thickness can be made thin, and glass having a large specific gravity such as heavy flint glass is used for the positive lens for achromatization. There is no need to improve the weight reduction of the first group.

Since the first lens unit is composed of a single positive lens, the chromatic aberration generated from the first lens unit, in particular, the variation of the chromatic aberration during focusing and zooming, is corrected by the above-mentioned conditional expression regarding the lens structure of the second lens unit and the third lens unit. It is corrected well by configuring so as to satisfy.

The equivalent Abbe number of the glass constituting one lens group in the above conditional expression is the total refracting power of one lens group Φ, and the refracting power and Abbe number of the i-th single lens in the lens group are respectively Φi. , νi The value obtained by the formula

The conditional expressions (1) and (2) relate to the equivalent Abbe numbers of the glasses of the lenses constituting the second and third groups, and both of them satisfactorily correct the chromatic aberration generated in the first group and the variation of the chromatic aberration due to zooming. It is for doing.

If the upper limit of conditional expression (1) is exceeded and the equivalent Abbe number of the second group becomes too large, correction of axial chromatic aberration that occurs in the first group will be insufficient, and if the lower limit is exceeded, the equivalent Abbe number will be small. If it becomes too large, the chromatic aberration of magnification generated from the second lens group becomes large, and the variation of the chromatic aberration of magnification at the time of zooming increases, and it becomes difficult to satisfactorily correct this.

If the upper limit of conditional expression (2) is exceeded and the equivalent Abbe number of the third lens group becomes too large, it becomes difficult to satisfactorily correct fluctuations in lateral chromatic aberration that occur in the first lens group. If the Abbe's number becomes too small, the effect of correcting the chromatic aberration of the third lens group becomes insufficient, and it becomes difficult to satisfactorily correct the fluctuation of the chromatic aberration of magnification due to zooming in particular.

Conditional expression (3) relates to the positive refracting power of the first lens group, and is because the amount of chromatic aberration generated when the first lens group is composed of a single positive lens is appropriately maintained and the focusing action is performed efficiently. .

If the upper limit of conditional expression (3) is exceeded and the refractive power of the first lens unit becomes too weak, the amount of movement during focusing becomes too large and the total lens length increases, which is not desirable. If the refractive power of the first group becomes too strong below the lower limit, the amount of chromatic aberration generated from the first group becomes too large, and it becomes difficult to correct the chromatic aberration at this time by the second group and the third group. come.

Conditional expression (4) relates to the refractive power ratio between the second lens unit and the third lens unit, and is for properly correcting the variation of chromatic aberration due to zooming under the conditional expressions (1) and (2). If the upper limit of conditional expression (4) is exceeded and the refracting power of the second lens unit becomes too small as compared with the refracting power of the third lens unit, the amount of movement of the second lens unit must be increased to obtain a predetermined zoom ratio. It is not good because the total length of the lens will increase. If the lower limit value is exceeded and the refracting power of the second lens unit becomes too large as compared with the refracting power of the third lens unit, various aberrations, especially off-axis aberrations, generated from the second lens unit due to zooming will increase, resulting in a total change. It becomes difficult to satisfactorily correct over the double range.

In this embodiment, the single positive lens of the first group has an Abbe number
It is preferable to use a lens made of 60 or more materials because chromatic aberration generated from the first group can be corrected in a well-balanced manner between the second group and the third group. Next, numerical examples of the present invention will be shown. In the numerical example, Ri is the i-th order from the object side.
The radius of curvature of the th lens surface, Di is the i-th lens thickness and air distance from the object side, and Ni and νi are the refractive index and Abbe number of the glass of the i-th lens in order from the object side, respectively.

(Effect of the Invention) According to the present invention, the first group is composed of a single positive lens, and the second group
By specifying the lens configurations of the third group and the third group as described above, and disposing the diaphragm between the second group and the third group, while maintaining good correction of chromatic aberration over the entire zoom range, It is possible to achieve a small-sized zoom lens having a simple structure in which the overall length of the lens is shortened and the effective diameter of the first lens group is reduced to reduce the weight of the entire lens system.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2, and 3 are lens cross-sectional views of Numerical Embodiments 1 to 3 of the present invention, and FIGS. 4, 5, and 6 are Numerical Embodiment 1 of the present invention.
FIG. 4 is a diagram of various types of aberrations of FIG. In the aberration diagram (A),
(B) and (C) are aberration diagrams at the wide-angle end, the middle, and the telephoto end, respectively. In the figure, I, II, III and IV are the first, second, third and fourth groups,
The arrow indicates the moving direction of the lens group when zooming from the wide-angle side to the telephoto side. ΔM is a meridional image plane, ΔS is a sagittal image plane, d is a d-line, and g is a g-line.

Claims (1)

[Claims] 1. A first positive refractive power for focusing in order from the object side.
A group, a second group having a negative refractive power that moves monotonically during zooming, a third group having a positive refractive power that moves in a direction opposite to the moving direction of the second group during zooming, and a fixed imaging action. Positive refracting power fourth
A group of four lens groups, a fixed diaphragm is arranged between the second group and the third group, the first group is composed of a single lens having a positive refractive power, and When the equivalent Abbe numbers of the glasses of the lenses constituting the third group are 2, 3 respectively, the focal length of the i-th group is fi, and the focal length at the wide-angle end of the entire system is fw 60 ≦ 2 ≦ 200 −550 ≦ 3 Small zoom lens characterized by satisfying the condition of ≦ 0 3.8 <f1 / fw <5.0 0.5 <| f2 / f3 | <0.7.