JP2581204B2 - 3-group zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a small three-unit zoom lens suitable for a lens shutter camera, a video camera, and the like. The present invention relates to a small three-unit zoom lens for reducing the distance to the image plane).

(Prior Art) In recent years, in a lens shutter camera, a video camera, and the like, a small zoom lens having a short overall lens length has been demanded with the downsizing of the camera. Among them, the applicant of the present invention has proposed a relatively small zoom lens including a standard angle of view (a focal angle of about 50 mm when converted to a 35 mm still camera at a shooting angle of view of 2ω = 47 degrees), for example, in Japanese Patent Laid-Open No. 63-271214. ing.

The publication has three lens groups, a first group having a negative refractive power, a second group having a positive refractive power, and a third group having a negative refractive power, in that order from the object side. Discloses a so-called three-unit zoom lens in which the zoom lens is moved from the wide-angle end to the telephoto end by moving the zoom lens toward the object side under certain conditions.

In general, if the refractive power of each lens unit in a zoom lens is increased, the amount of movement of each lens unit to obtain a predetermined zoom ratio is reduced, and the overall length of the lens can be reduced. However, simply increasing the refractive power of each lens group increases aberration fluctuations associated with zooming, and in particular, it is difficult to obtain good optical performance over the entire zooming range, especially when aiming for high zooming. There is a point.

(Problems to be Solved by the Invention) The present invention utilizes the refractive power arrangement of a zoom lens proposed in Japanese Patent Application Laid-Open No. 63-271214 by the present applicant, and further improves the lens configuration of each lens group. In particular, it is an object of the present invention to provide a three-unit zoom lens having high optical performance over the entire zoom range, with a zoom ratio of about 3 to 4 while shortening the entire lens length.

(Means for Solving the Problems) The three-unit zoom lens according to the present invention includes, in order from the object side, a first group having a negative refractive power, a second group having a positive refractive power, and a third group having a negative refractive power. The zoom lens system includes three lens groups, and performs zooming from the wide-angle end to the telephoto end by moving each of the first, second, and third groups toward the object side, and focuses on the second and third groups. The distances are f2 and f3, the focal length of the entire system at the wide-angle end is fW, the principal point interval between the first and second groups at the wide-angle end, and the principal point interval between the second and third groups and E1W, respectively. When E2W, 1.0 <| f3 / f2 | <1.4 (1) 0.7 <E1W / fW <1.5 (2) 0.83 <E2W / fW <1.50 (3) And

(Example) FIGS. 1 to 9 are lens cross-sectional views of Numerical Examples 1 to 9 of the present invention, respectively.

In the figure, I is a first group having a negative refractive power, II is a second group having a positive refractive power, III is a third group having a negative refractive power, and arrows are used when zooming from the wide-angle side to the telephoto side. The moving direction of each lens group is shown.

When zooming from the wide-angle end to the telephoto end, the zoom lens according to this embodiment moves the first to third units independently in the object side direction as shown in each drawing. As described above, at the time of zooming from the wide-angle end to the telephoto end, the first lens unit is moved to the object side, thereby effectively reducing the overall length of the lens at the wide-angle end. That is, a refractive power arrangement is adopted in which the entire length of the lens is short on the wide-angle side and long on the telephoto side.

When the zoom lens is composed of the three lens units having the above-described refractive power, the refractive power ratio of the second unit and the third unit is conditional expression (1), and the first unit and the second unit at the wide-angle end are different from each other. Is set so that the principal point interval E1W of the second lens group and the principal point interval E2W of the second and third lens units satisfy the conditional expressions (2) and (3).

As a result, it is possible to easily obtain a predetermined zoom ratio, shorten the back focus at the wide-angle end while maintaining good optical performance over the entire zoom range, and significantly shorten the overall length of the lens at the wide-angle end. I have.

 Next, the technical meaning of each of the above conditional expressions will be described.

Conditional expression (1) relates to the ratio of the focal lengths of the second and third units that contribute to zooming of the three-unit zoom lens of the present invention.
Beyond the upper limit of conditional expression (1),
In order to obtain a predetermined zoom ratio, the refractive power of the group becomes too weak, and the amount of movement of the third group must be increased, which is not suitable because the entire lens system becomes large. Alternatively, the refracting power of the second lens unit becomes too strong, and the Pesshibar sum becomes large in the positive direction. If the lower limit of the conditional expression (1) is exceeded, the refractive power of the second lens unit becomes so weak that the movable unit necessary for zooming, that is, the distance between the first lens unit and the distance between the third lens unit and the lens unit are secured to be large. This is not appropriate because the necessity arises and the entire lens system becomes large.

If the upper limit of conditional expression (2) is exceeded, the overall length of the lens will increase, making it difficult to achieve a compact overall lens system. If the lower limit is exceeded, it is difficult to obtain a large zoom ratio, which is not appropriate.

The conditional expression (3) is the same as the conditional expression (2). If the upper limit of the conditional expression (3) is exceeded, the overall length of the lens becomes large, making it difficult to achieve a compact lens system, and exceeding the lower limit. It becomes difficult to increase the zoom ratio. Equations (2) and (3) indicate that the first, second, and third lens units move toward the object side from the wide-angle end to the telephoto end. It is an indispensable requirement for downsizing and downsizing.

In the present invention, in order to obtain particularly good optical performance over the entire zoom range, it is preferable that the second unit has at least two negative lenses. At this time, the second group is composed of two lens groups, a 21st group and a 22nd group, with a diaphragm interposed therebetween.
It is preferable that the 21st and 22nd units have at least one negative lens.

With such a configuration, the first lens unit has a negative refractive power, the third lens unit has a negative refractive power, and the second lens unit is divided into two groups with a diaphragm interposed therebetween. A lens is provided, and the entire lens system is a substantially symmetrical lens system, thereby reducing the occurrence of distortion mainly in the entire zoom range.

In order to satisfactorily correct barrel distortion, particularly at the wide-angle side, at least one lens surface of the negative lens of the 21st group should have a concave surface facing the object side and at least one lens surface of the negative lens of the 22nd group should be used.
The two lens surfaces are preferably configured so that the concave surface faces the image surface side.

In addition, in the present invention, the lens configuration may be set as follows.

(A) The first is to reduce the diameter of the diaphragm in the second group as much as possible and to make the 21st group in the second group more positive than the lens group having a positive refractive power in order to satisfactorily correct mainly the overcorrection of spherical aberration. It is good to configure.

(B) The second is the principal point interval E1 between the first and second groups in zooming.
In order to increase the distance E2 between the principal points of the second and third lens groups and to secure a sufficiently long moving distance during zooming between the second and third lens groups and to easily obtain a predetermined zoom ratio, When the distance from the principal point on the image plane side of the first lens unit to the last lens surface of the second lens unit is O ₁ ′, and the length on the optical axis of the first lens unit is L1, 0.6 <| O ₁ ′ / L1 | < 1.5 (4) It is better to satisfy the following condition.

If the upper limit of conditional expression (4) is exceeded, the total lens length will increase.
If the lower limit is exceeded, the amount of movement of each lens group is limited, making it difficult to obtain a predetermined zoom ratio.

(C) Third, when the distance between the last lens surface of the second unit and the first lens surface of the third unit at the wide-angle end is B2W, 3 <E2W / B2W <5 (5) .

As a result, the zooming effect of the third lens unit is sufficiently increased, that is, the amount of movement accompanying zooming of the third lens unit is sufficiently ensured. If the upper limit of conditional expression (5) is exceeded, the moving space of the third lens unit becomes small, and it becomes difficult to secure a predetermined zoom ratio.

If the lower limit is exceeded, the overall length of the lens at the wide-angle end increases, or the negative refractive power of the third lens unit must be increased in order to move the principal point of the third lens unit toward the object side. This is not suitable because the image surface becomes too large and the image surface becomes over-finished.

(D) Fourth constitute from at least one negative lens and at least one positive lens of the first group, the refractive index of the material of the positive lens or a negative lens in the first group in this case was a n ₁ 1.55 <n ₁ <1.8 It is desirable to satisfy (6). It is desirable that the difference in the refractive index between the materials of the at least one positive lens and the negative lens is small.
For this reason, it is desirable that the refractive index of the material of an arbitrary lens is within the conditional expression (6). Deviating from the upper limit results in insufficient correction of spherical aberration, and deviating from the lower limit results in excessive correction, which is not appropriate.

(E) Fifth, when the Abbe number of the material of the positive lens included in the 21st group of the second group is _ν2P , it is desirable that _ν2P > 40 (7) be satisfied. Outside of this range, axial chromatic aberration on the telephoto side is particularly poor, which is not good. Especially the 21st
When a positive lens is disposed closest to the object side in the group, and when the Abbe number of the material of the positive lens is ν _21P , it is more desirable from the viewpoint of chromatic aberration that ν _21P > 55 (8).

(F) Sixth, in order to remove inward coma flare and barrel distortion due to underlines on the wide-angle side, positive refractive power becomes stronger toward any lens surface in the first group toward the periphery. Alternatively, it is preferable to use an aspheric surface having a shape in which the negative refractive power becomes weak.

Further, in order to eliminate the barrel-shaped distortion at the wide-angle end and the inward coma caused by the upper line at the telephoto end, the positive refractive power becomes weaker toward one of the lens surfaces of the third group toward the periphery. Alternatively, it is preferable to use an aspherical surface having a shape in which the negative refractive power becomes strong.

(G) It is preferable to move the 21st and 22nd units differently when zooming from the wide-angle end to the telephoto end in order to reduce aberration fluctuations due to zooming and further increase the zoom ratio.

In the present invention, focusing (distance adjustment) is desirably performed in the first lens group, but may be performed in the third lens group. Alternatively, only a part of the zoom region or only a part of the object distance region may be performed in the third lens unit. You may go in.

Next, numerical examples of the present invention will be described. In numerical examples
Ri is the radius of curvature of the i-th lens surface in order from the object side,
Is the i-th lens thickness and air gap from the object side, Ni and ν
i is the refractive index and Abbe number of the glass of the i-th lens in order from the object side.

Table 1 shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples.

(Effects of the Invention) According to the present invention, by specifying the refractive power and the lens configuration of the three lens groups as described above, the entire lens system can be reduced in size and the zooming ratio can be as high as about 3; Moreover, it is possible to achieve a three-unit zoom lens having excellent optical performance over the entire zoom range.

[Brief description of the drawings]

1 to 9 are lens sectional views of Numerical Examples 1 to 9 of the present invention, respectively, and FIGS. 10 to 18 are various aberration diagrams of Numerical Examples 1 to 9 of the present invention, respectively. I, II, I
II indicates the first, second, and third groups in order, arrows indicate the directions of movement of the respective lens groups during zooming from the wide-angle end to the telephoto end, and (A) indicates a wide-angle end and (B) indicates telephoto in the lens cross-sectional views. Show the edge.
In the aberration diagrams, (A), (B), and (C) denote aberrations at the wide-angle end, the middle, and the telephoto end, respectively, d denotes the d-line, g denotes the g-line, SC
Is a sine condition, ΔS is a sagittal image plane, and ΔM is a meridional image plane.

Claims (4)

(57) [Claims] 1. A lens system comprising: a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power. Change the magnification to 1st.2nd.
The third group is moved to the object side, and the second and third groups are moved.
The focal lengths of the groups are f2 and f3, respectively, the focal length of the entire system at the wide-angle end is fW, and the principal point distance between the first and second groups at the wide-angle end and the principal point distance between the second and third groups are A three-unit zoom lens, which satisfies the condition of 1.0 <| f3 / f2 | <1.4 0.7 <E1W / fW <1.5 0.83 <E2W / fW <1.50 when E1W and E2W, respectively. 2. The second group has two lens groups, a 21st group and a 22nd group, with a diaphragm interposed therebetween, and the 21st and 22nd groups have at least one negative lens. The three-unit zoom lens according to claim 1, wherein: 3. The negative lens according to claim 2, wherein the negative lens of the 21st group has a concave surface facing the object side, and the negative lens of the 22nd group has a concave surface facing the image plane side. Group zoom lens. 4. The three-unit zoom lens according to claim 2, wherein said twenty-first unit has a positive refractive power.