JPS6358326B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a zoom lens, and in particular, the focal length suitable for a zoom lens for a 35 mm camera is that zooming starts from a slightly shorter focal length than a standard lens.
This relates to a zoom lens with a zoom ratio of approximately 2x to 3x. Conventionally, it consists of three lens groups in order from the object side: a first lens group with negative refractive power, a second lens group with positive refractive power, and a third lens group with positive refractive power, and each of these three lens groups Various three-group type zoom lenses that perform zooming by moving independently have been proposed. For example, Japanese Patent Laid-Open No. 158316/1983 proposes a three-group type zoom lens in which the second lens group and the third lens group are close to each other at the wide-angle end. Since the positive refractive power of the third lens group of this three-group type zoom lens is relatively weak, the amount of zoom movement must be increased in order to obtain a large zoom ratio. This is a so-called two-group type zoom lens, which consists of a front group with negative refractive power and a rear group with positive refractive power, and zooms by changing the distance between both lens groups. This is in contrast to the case where, because the positive refractive power of the rear group is relatively large, a certain degree of zoom ratio can be obtained even if the amount of movement due to zooming is small. In the three-group type zoom lens mentioned above, since it is necessary to secure movement space for the lens groups during zooming, the total lens length (distance from the first lens surface to the image plane) becomes long, especially at the wide-angle end zoom position. This tends to lead to an increase in the total length of the lens and an increase in the aforementioned lens diameter. Generally speaking, in order to reduce the overall length of the lens and the diameter of the front lens of the three-group type zoom lens mentioned above, it is necessary to strengthen the refractive power of the first lens group with negative refractive power and the second lens group with positive refractive power. However, simply increasing the refractive power increases the amount of aberrations that occur, making it difficult to properly correct these aberrations. SUMMARY OF THE INVENTION An object of the present invention is to provide a compact zoom lens having a short overall lens length and a small front lens diameter, and which achieves good aberration correction. The features of the lens configuration of the zoom lens for achieving the object of the present invention are, in order from the object side, a first lens group with negative refractive power, a second lens group with positive refractive power, and a third lens group with positive refractive power. and a fourth lens group with negative refractive power, and zooming is performed by independently moving the first lens group, the second lens group, and the third lens group. be. The first to third lens groups collectively constitute a front group with positive refractive power, the fourth lens group with negative refractive power constitutes a rear group, and the front and rear groups form a telephoto type lens. It constitutes a system. In this way, by making the entire zoom lens a telephoto type lens configuration including a front group with positive refractive power and a rear group with negative refractive power, the overall length of the lens can be shortened. During zooming, the focal length of the front group is always smaller than the focal length of the entire zoom lens system at that zoom position, that is, its refractive power is stronger. For this reason, even if the amount of movement of each lens group in the front group that performs the zoom lens's magnification change function is smaller than that of a conventional zoom lens without a fourth lens group with negative refractive power, the predetermined zoom ratio can still be maintained. It becomes possible to obtain. Therefore, a predetermined zoom ratio can be obtained with a small amount of movement, making it possible to make the zoom lens more compact. In particular, the fourth lens group with negative refractive power allows the second
Since the positive refractive power of the lens group can be strengthened, a high zoom ratio can be obtained even if the amount of movement of the second lens group is reduced. The positive refractive power of the second lens group can be increased, and the negative refractive power of the first lens group can also be increased accordingly. As a result, the movement locus of the first lens group during zooming can be made into a shape that is close to reciprocating motion, and the total lens length at the zoom position at the wide-angle end and the telephoto end can be approximately changed.
It is possible to make the zoom lens more compact while achieving a predetermined variable power ratio. That is, in general, the first lens group often takes a zoom locus located in the object side direction at the wide-angle end compared to the telephoto end. Therefore, the maximum overall length of the lens is the zoom position at the wide-angle end. As the refractive power of the first lens group is strengthened, the first lens group will take a zoom locus that is located closer to the object side at the telephoto end than at the wide-angle end. However, if the negative refractive power of the first lens group is strengthened, many aberrations will occur, and it will be difficult to correct these aberrations, so it is not possible to simply increase the negative refractive power. In the present invention, when the configuration is made up of four lens groups, the fourth and final lens group has a negative refractive power, and in accordance with the negative refractive power of the fourth lens group, the second lens group has a positive refractive power, and By giving a predetermined negative refractive power to the first lens group according to the refractive power of the second lens group, it is possible to cause the first lens group to move almost reciprocatingly as described above, so it is possible to zoom This makes it possible to make the lens more compact. Furthermore, it is possible to simultaneously reduce the diameter of the front lens of the zoom lens. Furthermore, by making the third lens group have a positive refractive power and moving it during zooming, the entire zoom lens system can be made into an appropriate telephoto type, and the zoom lens can be made compact and aberrations can be corrected well.
In other words, the telephoto ratio can be controlled by the positional relationship between the third lens group and the fourth lens group, and good aberration correction can be achieved by the positive refractive power of the third lens group. With the lens configuration described above, it is possible to achieve a compact zoom lens with good aberration correction.However, in order to achieve the object of the present invention more effectively, each lens group in the front group must be zoomed to It is preferable to move it as follows. In other words, the first and second lens groups are closest to each other at the telephoto end zoom position, and the third and fourth lens groups are closest to each other at the telephoto end zoom position.
The lens groups should be brought closest to each other at the wide-angle end zoom position. By making the first lens group and the second lens group the closest to each other at the telephoto end zoom position, the open aperture diameter can be made smaller when an aperture stop is placed after the second lens group. The lens can be made compact, and aberrations at the telephoto end zoom position can be effectively reduced. Also the third
By using a zoom type in which the lens group and the fourth lens group are brought closest to each other at the wide-angle end zoom position, sufficient back focus can be obtained. Furthermore, in the zoom lens according to the present invention, the focal lengths of the first lens group and the fourth lens group are f ₁ and f ₄ , respectively, and the focal length of the zoom lens system at the telephoto end is
When f _T , it is preferable to satisfy the following conditions: (1) |f ₄ |>f _T (2) |f ₁ |<f _T. Here, conditional expression (1) is intended to control the refractive power of the front group of the zoom lens system to suppress aberrations generated from the front group as much as possible, and to lengthen the back focus at the wide-angle end zoom position. If deviating from 1), there will be more aberrations generated from the front group, making it difficult to correct aberrations well, and the back focus will become shorter.
When used with mmTTL single-lens reflex cameras, it will not be sufficient to rotate the mirror. Conditional expression (2) is intended to shorten the overall lens length and further reduce the outer diameter of the front lens.If conditional expression (2) is not satisfied, the overall lens length will become longer at the wide-angle end zoom position, and the front lens will become shorter. The lens diameter becomes large, which is not preferable. In the zoom lens of the present invention, the focal lengths of the first lens group and the fourth lens group are determined by the above-mentioned conditional expressions.
It is preferable to satisfy (1) and (2), but
More preferably, the focal lengths f ₁ and f ₄ of the first lens group and the fourth lens group are (3) 1.4f _T ≦｜f ₄ ｜≦9f _T (4) 0.4f _T ≦｜f ₁ ｜≦0.9f _T It is best to keep it within this range. |f ₄ | in conditional expression (1)
＞f _T , but |f ₄ | in conditional expression (3)
By setting ≧1.4f _T , sufficient back focus can be ensured. Also, by setting |f ₄ |≦9f _T , the negative refractive power of the fourth lens group can be prevented from weakening, and the zoom lens can be made more compact. By setting |f ₁ |≧0.4f _T in conditional expression (4), it is possible to maintain a good balance between the total lens length at the wide-angle end and the telephoto end, and to correct the large aperture ratio at the telephoto end and the accompanying spherical aberration. It becomes easier to perform well.
Also, by setting |f ₁ |≦0.9f _T , the total length of the lens at the wide-angle end zoom position can be made smaller. Further, in the zoom lens according to the present invention, the first
A lens with a positive refractive power is placed on the closest to the image side of the lens group, and the second lens group includes, in order from the object side, two or more lenses with a positive refractive power, a lens with a negative refractive power, and a lens with a positive refractive power. In order to achieve good aberration correction, it is preferable that the third lens group be composed of at least one lens with negative refractive power and one lens with positive refractive power. In the numerical examples of the present invention described later, the fourth lens group is shown as being composed of one lens in order to make the zoom lens as compact as possible, but of course it can also be composed of two or more lenses. It is preferable that the fourth lens group is composed of a plurality of lenses because focusing can be performed by moving the entire fourth lens group or a part of the lens group. Next, numerical examples 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 i-th lens thickness and air gap from the object side, and Ni and νi are the i-th lens surface from the object side, respectively. These are the refractive index and Atsube number of the lens glass. R13 in Numerical Example 1 and R9 in Numerical Example 2
is the aperture. Numerical Example 1 has a zoom ratio of 2.75 and an angle of view w = 72.8° ~
The specifications are 30 degrees, F number F No. = 4 to 4.5, and the aperture diaphragm is R13. Numerical Example 2 has a zoom ratio of 1.89 and an angle of view w = 61.3° ~
34.9°, F number FNo. = 4 to 4.5 specifications,
The aperture diaphragm is R9. As described above, in this embodiment, the open F No. is 4 to 4.5, but since there is a margin in particular for the spherical aberration with respect to the allowable width, it is extremely easy to achieve a large aperture ratio.

【table】

【table】

【table】

[Table] Focusing in the zoom lens according to the present invention may be performed by moving the entire zoom lens, or may be performed by moving only one lens group, for example, the fourth lens group, or even two or more lens groups. For example, the third lens group and the fourth lens group may be moved integrally or independently. It is preferable to adopt a focusing method in which two or more lens groups are moved, since this is advantageous in correcting aberrations. In the above numerical examples 1 and 2, if only one lens group is moved non-linearly and the remaining lens groups are moved linearly, it becomes easy to manufacture the cam groove to be provided in the lens barrel. . On the other hand, if two or more lens groups are moved non-linearly, the movement mechanism becomes somewhat complicated, but the degree of freedom in movement increases, which is advantageous in correcting aberrations. Next, Table 1 shows the third-order aberration coefficients of Numerical Example 2. As is clear from Table 1, the spherical aberration (SA) of the fourth lens group remains constant during zooming, but both the astigmatism (AS) and distortion aberration (DS) act in the direction of overcorrection. The effect of aberration AS is stronger at the wide-angle end and weaker at the telephoto end. In the zoom lens according to the present invention, the refractive power of the first and second lens groups is made slightly stronger, so that astigmatism due to insufficient correction can be seen from the front group of the first to third lens groups, especially on the wide-angle side. Since aberrations are more likely to occur, the action of the fourth lens group becomes extremely effective in correcting them. Additionally, since barrel-shaped distortion is more likely to occur on the wide-angle side, the fourth lens group has an extremely effective effect in correcting this distortion. As described above, in the zoom lens according to the present invention, the fourth lens group has the effect of canceling various aberrations generated from the front group of the first to third lens groups.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 and FIG. 2 are numerical example 1 of the present invention,
A cross-sectional view of the zoom lens of Numerical Example 2, FIG.
FIG. 4 is a diagram of various aberrations of Numerical Example 1 and Numerical Example 2 of the present invention, respectively. In the figure, DM is the meridional image plane and DS is the sagittal image plane. , , indicate the first, second, third, and fourth lens groups, respectively, and the arrow indicates the direction of movement by zooming.

Claims (1)

[Claims] 1. A first lens group having negative refractive power in order from the object side,
It has a second lens group with positive refractive power, a third lens group with positive refractive power, and a fourth lens group with negative refractive power,
The first lens group and the second lens group are arranged so that the first lens group and the second lens group are closest to each other at the telephoto end zoom position, and the third lens group and the fourth lens group are the closest to each other at the wide-angle end zoom position. , move the second lens group and the third lens group independently, and change the focal lengths of the first lens group and the fourth lens group, respectively.
When f ₁ , f ₄ , and the focal length of the entire system at the telephoto end are f _T , the following conditions are satisfied: 1.4f _T ≦｜f ₄ ｜≦9f _T 0.4f _T ≦｜f ₁ ｜≦0.9f _T Features a zoom lens.