JPH0640171B2 - Zoom lenses - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens suitable for a photographic camera, a video camera, etc., and particularly to a lens by forming an aspherical surface on at least one lens surface of a predetermined lens group. The present invention relates to a zoom lens that maintains good optical performance while simplifying the configuration and downsizing the entire lens system.

(Prior Art) Conventionally, a zoom lens of a type in which the first lens unit on the object side has a negative refracting power and is moved together with other lens units during zooming is relatively easy to widen the angle of view for photography. It is widely used in cameras and video cameras. Among them, a zoom lens having an aspherical surface on the lens surface in the first lens group to improve the optical performance is disclosed, for example, in US Pat.
It is proposed in Japanese Patent No. 64811. Each of these zoom lenses has two lens groups, a first lens group having a negative refracting power and a second lens group having a positive refracting power, and performs zooming by changing the distance between the two lens groups. Of which, U.S. Pat.
In No. 064, the first lens group is composed of a lens having a negative refractive power and a lens having a positive refractive power, which are made of a plastic material, and an aspherical surface is provided on the lens surface on the image plane side of the lens having a negative refractive power, which is excellent. Has optical performance. However, since this zoom lens has a two-group configuration, the refracting power of the first lens group cannot be increased so much that the entire lens length at the wide-angle end (first
The length from the lens surface to the image surface) tends to be long.

On the other hand, in Japanese Patent Laid-Open No. 59-64811, the first lens group is composed of two lenses, a lens having a negative refracting power and a lens having a positive refracting power. It has an aspherical surface. This aspherical surface has a shape that further strengthens the converging action of a lens having a positive refractive power, thereby canceling various aberrations generated by the lens having a negative refractive power. For this reason, the amount of high-order aberrations tends to increase. Further, if the negative refracting power of the first lens unit is strengthened to reduce the size of the entire lens system, various aberrations occur more frequently, and it is difficult to satisfactorily correct the various aberrations at this time.

(Problems to be Solved by the Invention) The present invention is advantageous in that at least one lens surface of a predetermined lens group is effectively provided with an aspherical surface to simplify the lens configuration and downsize the entire lens system. An object of the present invention is to provide a zoom lens having optical performance.

(Means for Solving Problems) The first lens group having a negative refractive power, the second lens group having a positive refractive power, and the third lens group have three lens groups in order from the object side. In a zoom lens that performs zooming by moving a lens group and a second lens group on the optical axis, the first lens group includes two lenses, an eleventh lens having a negative refractive power and a twelfth lens having a positive refractive power. And an aspherical surface is provided on the image-side lens surface of the eleventh lens, the focal lengths of the entire system at the first lens group and the telephoto end are f _I and f _T , respectively, and the refraction of the glass of the eleventh lens is the incidence and Abbe's number, respectively N1, 0.58 ≦ when was _{ν1 | f I / f T |} ≦ 0.96 ...... (1) 1.65 ≦ N1 ...... (2) 40 ≦ ν1 ...... (3) becomes possible to satisfy the condition Is.

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

(Embodiment) FIGS. 1, 2, and 3 are numerical embodiments 1, 2, and 3 of the present invention, respectively.
3 is a lens cross-sectional view of FIG. In the figure, I is a first lens group having a negative refractive power, II is a second lens group having a positive refractive power, and III is a third lens group having a positive or negative refractive power. The first lens group I and the second lens group II are moved as shown by the arrows to perform zooming from the wide-angle end to the telephoto end.

As described above, the zoom lens of the present embodiment is composed of three lens groups, and when the refractive power of the first lens group or the second lens group is strengthened by providing a fixed third lens group especially during zooming. Various aberrations that occur are satisfactorily corrected, the overall length of the lens is shortened, and the overall size of the lens system is reduced. For example, when the positive refracting power of the second lens group is increased in order to obtain a predetermined zoom ratio with a small amount of movement during zooming, that is, in order to increase the zooming efficiency, It has a reverse refractive power corresponding to the power, that is, a negative refractive power. As a result, various aberrations produced by the second lens group are corrected in a well-balanced manner.

Generally, in order to satisfactorily correct various aberrations that occur when the refracting power of the lens group is increased, the number of lenses may be increased. However, in this method, the lens structure becomes complicated, and conversely the total lens length may increase. Therefore, in the present embodiment, the first lens group is made up of the eleventh lens having negative refractive power and the first lens group having positive refractive power.
Consists of two lenses of 12 lenses, and various aberrations that occur when the lens configuration is simplified and the negative refractive power of the first lens group is strengthened to shorten the total lens length are described. A good correction is made by applying an aspherical surface to the lens surface.

In particular, the image side lens surface of the 11th lens is a concave lens surface,
The concave lens surface is provided with an aspherical surface having a shape in which the negative refracting power becomes weaker as the distance from the optical axis increases, thereby performing well-balanced aberration correction.

For example, in Numerical Example 1 described later, the paraxial radius of curvature of the lens surface having an aspherical surface is 19.2533, but the height from the optical axis is 14.
The radius of curvature of the reference sphere at position 04 is 19.4674. That is, the curvature is weakened to weaken the negative refracting power. By thus weakening the negative refractive power around the lens surface by the aspherical surface in the first lens group having negative refractive power, it becomes possible to correct various aberrations generated by the first lens group in a well-balanced manner.

That is, in the wide-angle type zoom lens as in the present embodiment, a large amount of barrel distortion occurs on the wide-angle side and overcorrected spherical aberration occurs on the telephoto side. In the present embodiment, these various aberrations are corrected by using the aspherical surface described above so as to have a smooth shape.

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

Conditional expression (1) is for setting the refracting power of the first lens group appropriately, and for efficiently reducing the total lens length while reducing the amount of aberration generation. First value exceeds the upper limit of conditional expression (1)
When the negative refracting power of the lens group becomes weaker, the amount of movement of the first lens group must be increased due to zooming, and as a result, the total lens length at the wide-angle end increases. If the lower limit is exceeded and the negative refractive power of the first lens group becomes strong, it becomes difficult to form the first lens group with two lenses even if an aspherical surface is used, and the number of lenses must be increased. As a result, the total lens length increases, which is not preferable.

Conditional expressions (2) and (3) relate to the refractive index and the dispersion of the glass of the 11th lens having the aspherical surface of the first lens group,
This is for performing good aberration correction while being composed of two lenses. If conditional expressions (2) and (3) are deviated and the refractive index becomes low or the dispersion becomes large, fluctuations in spherical aberration, chromatic aberration, etc. due to zooming when the first lens group is composed of two lenses It becomes difficult to correct it well.

In this embodiment, in order to reduce the fluctuation of aberrations during zooming and maintain good optical performance of the entire screen, both the eleventh lens and the twelfth lens of the first lens group have a meniscus with a convex surface facing the object side. The second lens group is a positive lens whose lens surface on the object side has a stronger curvature, a positive meniscus lens whose convex surface faces the object side, a negative lens whose lens surface on the image side has a stronger curvature, and It is preferable that both lens surfaces are made up of positive lenses, and the third lens group is made up of a meniscus negative lens whose convex surface faces the object side.

Here, when the paraxial radius of curvature of the image side surface of the negative lens of the first lens group is R _A and the radius of curvature of the object side surface of the positive lens is R _B , it is preferable that the relationship R _A <R _B holds.

This is because the negative lens generally has a shape in which the curvature of the image side surface is as strong as possible in order to improve the off-axis characteristic on the wide-angle side, but improves the axial characteristic on the telephoto side. Therefore, it is preferable that the both surfaces have the same curvature.

Although the above shapes are contradictory shapes, by making the image side surface of the negative lens an aspherical surface of the above shape, even if the paraxial curvature on the image side is increased, the axial characteristics on the telephoto side are also corrected well. It becomes possible.

In this embodiment, focusing can be performed by the first lens group so that variation in aberration is small, but it may be performed by the second lens group or the third lens group.

Next, numerical examples of the present invention will be shown. Ri in the numerical examples
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, Ni and νi
Are the refractive index and the Abbe number of the glass of the i-th lens in order from the object side.

The aspherical shape is the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis,
Let R be the paraxial radius of curvature, A, B, C,
When D and E are aspherical coefficients respectively It is expressed by

(Effect of the Invention) According to the present invention, by configuring each lens group as described above, it is possible to achieve a zoom lens having good optical performance that simplifies the lens configuration and downsizes the entire lens system. You can

[Brief description of drawings]

FIGS. 1, 2, and 3 are numerical embodiments 1, 2 and 3 of the present invention, respectively.
A lens cross-sectional view of No. 3 and FIGS. 4, 5, and 6 are aberration diagrams of Numerical Examples 1, 2, and 3 of the present invention, respectively. In the aberration diagrams, (A), (B), and (C) are aberrations at the wide-angle end, the middle, and the telephoto end, respectively. In the figure, I, II, and III are the first, second, and third lens groups, the arrow indicates the moving direction during zooming, ΔM is the meridional image plane, and ΔS is the sagittal image plane.

Claims (1)

[Claims] 1. A first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group in order from the object side. In a zoom lens that performs zooming by moving two lens groups on the optical axis, the first lens group includes two lenses, an eleventh lens having a negative refractive power and a twelfth lens having a positive refractive power. 11
The lens surface on the image side of the lens is composed of an aspherical surface whose negative refracting power becomes weaker toward the peripheral portion of the lens, and the second lens group has a positive lens whose object side lens surface has a stronger curvature, and an object side. A meniscus-shaped positive lens having a convex surface facing the lens, a negative lens having a stronger curvature on the image-side lens surface, and a positive lens having a convex surface on both lens surfaces. The third lens group has a convex surface directed toward the object side. The first lens group and the focal length of the entire system at the telephoto end are fI and fT, respectively, and the refractive index and Abbe number of the glass of the eleventh lens are N1 and ν1, respectively. When the paraxial radius of curvature of the image side lens surface of the 11th lens is RA and the radius of curvature of the object side lens surface of the 12th lens is RB, 0.58 ≦ | fI / fT | ≦ 0.96 1.65 Satisfies the condition of ≦ N1 40 ≦ ν1 RA <RB Zoom lens according to claim Rukoto.