JPH0532807Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to a zoom lens, and more particularly to a zoom lens that performs focusing by moving some of the lens groups that move integrally with the variable power lens group. Conventionally, as a focusing method for a zoom lens, a so-called front lens extension method, which is performed by extending the front lens group, is generally well known. For example, starting from the object side, the first lens group moves during focus operation but is fixed during zooming, the second lens group moves to perform magnification change, and the second lens group moves to maintain the image point at a fixed position. 3
In a so-called four-component mechanically corrected zoom lens consisting of a fourth lens group forming an imaging system that is fixed during zooming, when focusing from an object point at infinity to a near object point, the first lens group is I'm going back and forth. In this case, since the relationship between the object point and image point of the first lens group is unrelated to the zoom operation for a certain short-distance object point, the amount of extension of the first lens group depends on the zoom position. It remains constant even if These are the most important advantages of the front lens focusing method. However, it also has the following drawbacks. In so-called telephoto zoom lenses that have a high magnification ratio and a relatively long focal length on the telephoto side, the overall length of the lens tends to be long to correct aberrations, and the diameter of the front lens of the lens also tends to be large. Moreover, in order to minimize aberration fluctuations due to focusing, the lens configuration of the focusing lens group becomes complicated. For this reason, the weight of the lens group tends to become significantly heavier. In such a zoom lens, when focusing by extending the front lens, a very large driving force is required, and in order to perform the focusing operation smoothly, the lens barrel structure must have the precision and durability to satisfy these needs. becomes necessary. Conventionally, as a means to improve this drawback, various methods have been devised in which the first lens group, which is a focusing lens group, is divided into a plurality of lens groups, and focusing is performed with some of the lens groups. By this method, it is relatively easy to reduce the amount of extension and the lens weight of the movable lens group in the focusing lens group. However, when used as a camera lens for automatic focusing, for example, it has the following drawbacks. When focusing is performed using a driving force controlled by an electrical signal, such as a motor inside the camera, for automatic focusing, the lens group driven as in the front lens extension method is placed on the image side, that is, the camera. If the position is farthest from the internal driving force source, the structure for transmitting the driving force will be complicated. Therefore, as a lens system for an automatic focusing camera, (a) the weight of the focusing lens group is light; (b) The focusing lens group must be close to the image plane. It is desirable that With a focusing method other than the front lens feeding method,
In order to satisfy the above conditions, for example, in the method of moving the imaging system lens group of the four-component mechanically corrected zoom lens described above or a part thereof, and focusing from an object point at infinity to a near object point, There is a problem in that the amount of extension of the focus lens group changes depending on the zoom position. Generally, the amount of extension is small on the wide-angle side and large on the telephoto side, and the amount is approximately proportional to the square of the zoom ratio. A zoom lens in which the amount of extension of the focusing lens group differs depending on the focal length, for example, when used as a lens for an automatic focusing camera, can be used with high accuracy on the telephoto side, where the amount of extension is large. In order to perform quick focusing operations, it is necessary to drive the lens, and on the wide-angle side where the amount of movement is small,
It is necessary to slow down the lens group drive speed. It is not preferable that the amount of movement of the focus lens group differs depending on the zoom position as described above because it complicates the mechanism. Therefore, in order to alleviate the difference in the amount of extension of the focusing lens group between the wide-angle side and the telephoto side, it is conceivable to make the focusing lens group movable during zooming. The methods are roughly divided into (i) The focusing lens group is movable during zooming, and the imaging magnification of the focusing lens group is changed by changing the relative position of the focusing lens group, and the imaging magnification is changed between wide-angle and telephoto. Alleviates the difference in the amount of feed on both sides. (ii) Consisting of multiple focusing lens groups, these lens groups are movable separately during zooming, and by changing the refractive power of the focusing lens group itself, it can be adjusted to wide-angle and telephoto positions. Alleviates the difference in the amount of feed on both sides. (Tokukai Akira
56−165107). However, in both cases, the focusing lens group moves during zooming, so the zoom locus of the other moving lens groups changes, which is different from when the focusing lens group was fixed, and the image point remains constant. There are many cases where there is no solution to the trajectory of the lens group to correct the position, or where the refractive power of the variable magnification lens group must be further strengthened in order to obtain the desired zoom ratio. . In general, if the focusing lens group is driven in response to the zoom operation in order to alleviate the difference in the amount of extension of the focus lens group between the wide-angle and telephoto zoom positions, the reverse magnification change due to the movement of the focusing group This has a large effect and is not preferable in achieving a high magnification ratio. An object of the present invention is to provide a zoom lens that has a focusing method suitable for, for example, an automatic focusing camera, in which the effect of reverse magnification change due to movement of a focusing lens group is small. In order to achieve this objective, in a zoom lens having at least two lens groups that move along the optical axis in order to change the focal length of the entire system while keeping the image point position constant, one of the lens groups or A compound lens group F consisting of a plurality of lens groups that moves integrally in conjunction with the zooming operation is arranged behind both, and the compound lens group F
Focusing is performed by moving some of the lens groups. In the zoom lens according to the present invention, the composite refractive power of the focusing compound lens group F is made small, so even if the compound lens group F is moved with the zoom operation, there is almost no effect of inverse magnification, and there is no aberration. The occurrence of this can also be extremely reduced. Focusing is performed by moving a part of the compound lens group F that has a certain degree of refractive power, so there is an advantage that focusing can be achieved with a small amount of movement. Furthermore, since the lens group relatively close to the image plane side is moved, the mechanism for driving it from the camera body side is simplified, which is preferable for application to cameras with automatic focus adjustment. Furthermore, in order to perform focusing with some lens groups of compound lens group F, the relative position of the movable focusing lens group changes, and by utilizing the difference in imaging magnification, it is possible to move between the wide-angle side and the telephoto side. It becomes possible to alleviate the difference in quantity. In this case, on the wide-angle side, focusing is performed using the lens group with a small refractive power in the compound lens group F, and on the telephoto side, focusing is performed with a lens group with a large refractive power among the compound lens group F, and the wide-angle It is also possible to alleviate the difference in the amount of extension between the side and telephoto side. Although the objective of the present invention is achieved by the lens configuration described above, specifically, it is preferable to satisfy the following conditions. When the focal length of the compound lens group F with small combined refractive power is f _F , and the focal length of the entire system at the telephoto end zoom position is f _T , (1) −1/3f _T <1/f _F <1/3f Satisfy the condition _T. By setting the focal length within such a range, even if the compound lens group F is moved together with the zoom operation, the effect of reverse magnification change is small, and the amount of aberrations generated can be reduced. If the upper limit or lower limit of conditional expression (1) is exceeded, the effect of inverse magnification will appear, which is not desirable for increasing the magnification of the zoom lens, and the amount of aberrations will increase, making it difficult to obtain good optical performance. It becomes difficult. Further, it is preferable that the compound lens group F according to the present invention is composed of two lens groups having positive and negative refractive powers, and focusing is performed by moving one of the lens groups. Since it is necessary to make the combined refractive power as small as possible and to move a lens group with a certain degree of refractive power during focusing, the compound lens group F should be composed of two lens groups with positive and negative refractive powers. This simplifies the lens configuration and also simplifies the mechanism for moving the lens group. Furthermore, in the present invention, it is also possible to focus by moving any plurality of lens groups from the compound lens group F, either integrally or independently. Such a method is preferable for improving the performance of a zoom lens because it enables focusing while suppressing the occurrence of aberrations as much as possible. Next, embodiments of the zoom lens according to the present invention will be described with reference to each drawing. 1 and 2 are explanatory diagrams of optical systems showing focusing methods of zoom lenses according to embodiments 1 and 2 of the present invention, respectively. In Figure 1, V is a lens group for variable magnification, C is a lens group for image plane correction, and F is a compound lens group, consisting of a lens group F ₁ with positive refractive power and a lens group F ₂ with negative refractive power.
have. Composite lens group F along with zoom operation
When focusing, move the lens group.
Only F ₁ or lens group F ₂ is moved. P is a lens group arranged in front of lens group V, R is an imaging lens group, and lens group P and lens group R may be omitted. In FIG. 2, V ₁ and V ₂ are lens groups for varying magnification, and the other lens groups are the same as in FIG. 1. FIG. 3 shows an example in which the function of the lens group for image plane correction is carried out by two lens groups C ₁ and C ₂ , and there is no lens group for image formation. In addition, in FIG. 4, a compound lens group F for focusing is arranged between the variable power lens group R and the image plane correction lens group C, and the other lens groups are the same as in FIG. 1. . In addition, when changing the magnification of the lens group for variable magnification or the lens group for image plane correction,
Corrective action is not strictly distinguished, and as commonly used, refers to dominant action. Focusing is done using lens group F ₁ as in Figure 1.
Alternatively, this can be done by moving the lens group _F2 . In Examples 1 and 2, the case where the composite lens group F is composed of two lens groups is shown, but it goes without saying that it may be composed of two or more lens groups. Next, numerical examples of zoom lenses using the focusing methods of the first and second embodiments of the present invention will be shown.
In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air gap of the i-th lens from the object side, and Ni and υi are the i-th lens surface from the object side, respectively. are the refractive index and Atsube number of the glass. In Numerical Examples 1 and 2, R14 is the aperture surface. Lens sectional views of Numerical Example 1 and Numerical Example 2 are shown in FIGS. 3 and 4, respectively. Further, the numerical values of 1/3f _T and 1/f _F corresponding to the conditional expression of the present invention are shown below.

【table】

【table】

【table】

【table】

[Table] In Numerical Example 1, lens groups with positive and negative refracting powers with small combined refractive power are inserted into the imaging system lens group (fourth group) of the four-component mechanically corrected zoom lens described above. As an example, when using a positive lens group consisting of R15 to R19 for focusing, the amount of extension for a short distance object point (3 m from the image plane) is:
It is 0.46mm at the wide-angle end and 2.99mm at the telephoto end.
The ratio of 6.5 is smaller than the square of the zoom ratio of 7.5, and the difference in the amount of extension between the wide-angle side and the telephoto side is alleviated. Numerical Example 2 has a first lens group V ₁ with a positive refractive power and a second lens group with a negative refractive power for zooming.
Lens group V ₂ and the third lens with positive refractive power for image plane correction.
This is an example in which the lens group C, the fourth lens group F ₁ , and the fifth lens group F ₂ are movable, and focusing is performed with the fourth lens group for a close object point. In Numerical Example 2, focusing is performed by moving forward a fourth lens group consisting of R15 to R19 and having positive refractive power. Near object point (1.5
m) is 0.96mm at the wide-angle end and 6.26mm at the telephoto end, and the ratio of 6.5 is smaller than the square of the zoom ratio of 7.5, reducing the difference in the amount of extension between the wide-angle end and the telephoto end. ing. Further, cross-sectional views of lenses of Numerical Example 3 and Numerical Example 4 are shown in FIG. 11 and FIG. 12, respectively.

【table】

【table】

【table】

【table】 [Brief explanation of the drawing]

Figures 1 and 2 are explanatory diagrams of the optical arrangement and zoom locus by zooming in Example 1 and Example 2 of the present invention, respectively. Figures 3 and 4 are numerical values of Example 1 and Figure 4 of the present invention, respectively. A cross-sectional view of the lens of Example 2, FIG.
6 is a diagram showing various aberrations for an object point at infinity and an object point 3 m from the image plane in numerical example 2. are explanatory diagrams of the optical arrangement and zoom trajectory of the third and fourth embodiments of the present invention and the eleventh embodiment, respectively.
Figures 12 and 12 are lens cross-sectional views of numerical embodiment 3 and numerical embodiment 4 of the present invention, respectively. Figure 13 is numerical embodiment 3, and Figure 14 is numerical embodiment 4 of the infinite distance object point and near distance. It is a diagram of various aberrations for an object point. In the figure, a,
b and c indicate the zoom position at the wide-angle end, middle, and telephoto end, M indicates the meridional image plane, and S indicates the sagittal image plane.

Claims (1)

[Claims for Utility Model Registration] A zoom lens having at least two lens groups that move along the optical axis in order to change the focal length of the entire system while keeping the image point position constant; A compound lens group F consisting of a plurality of lens groups that moves integrally in conjunction with the zooming operation is arranged behind both, and focusing is performed by moving some of the lens groups of the compound lens group F. The compound lens group F
A zoom that satisfies the condition -1/3f _T < 1/f _F < 1/3f _T , where f _F is the focal length of F and f _T is the focal length of the entire system at the zoom position at the telephoto end. lens.