JP3005037B2 - Compact high zoom lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a multi-unit moving zoom lens, which has a zoom ratio of about 6 times, a single-lens reflex camera, a video camera,
The present invention relates to a compact zoom lens suitable for an electronic still camera.

[Conventional technology]

In a zoom lens having a focal length at the wide angle end shorter than the screen diagonal line and a zoom ratio of 3 or more, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a positive lens unit. A four-unit zoom lens including a fourth lens unit having a refractive power is well known.

Such a zoom lens is disclosed in JP-B-61-55093,
This is proposed in Japanese Patent Application Laid-Open No. 58-127908.

[Problems to be solved by the invention]

However, although the zoom lenses disclosed in these publications have a relatively high zoom ratio, the overall length of the lens, the diameter of the front lens, and the amount of movement of each lens unit for zooming are large, and these are kept small to obtain high performance. I couldn't say.

An object of the present invention is to provide a compact and high-performance zoom lens having a high zoom magnification of about 6 times.

[Means for solving the problem]

In order to achieve the above object, the first lens unit having a positive refractive power, the second lens unit having a negative refractive power, the third lens unit having a positive refractive power, and the fourth lens having a positive refractive power are arranged in order from the object side. The first lens unit and the fourth lens unit move toward the object side during zooming from the wide-angle end to the telephoto end, and the air gap between the second lens unit and the first lens unit is monotonous. The third lens group moves so that the air gap between the second lens group and the second lens group decreases monotonically, and the first lens group and the second lens group during zooming from the wide-angle end to the telephoto end.
When the moving amounts of the lens units are A ₁ and A ₂ , the focal length of the entire system at the telephoto end is f _T , and the focal length of the first lens unit is f ₁ , 0.11 <A ₂ / (A ₂ − A ₁ ) <0.26 (1) 0.25 <f ₁ / f _T <0.39 (2)

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

I is a first lens group having a positive refractive power, II is a second lens group having a negative refractive power, III is a third lens group having a positive refractive power, and IV is a fourth lens having a positive refractive power. Group. During zooming from the wide-angle end to the telephoto end, each lens group is moved along the optical axis as shown by the arrow. Conditional expression (1) is satisfied in order to reduce the size of the lens system while obtaining a high zoom ratio.

Conditional expression (1) is intended to keep the total lens length compact on both sides of the short focal length side and the long focal length side, while keeping the lens diameter of the first lens group small, ie, exceeding the lower limit of conditional expression (1). Therefore, when the movement amount of the first lens group is large and the movement amount of the second lens group is small, it is advantageous for shortening the overall length of the lens on the short focal length side. Further, as a result, the distance between the first lens unit and the stop disposed in the rear lens unit (third lens unit) is shortened, which is advantageous in reducing the lens diameter of the first lens unit. However, since the telephoto type on the long focal length side is weakened, the overall length of the lens on the long focal length side becomes large, which is not desirable. Exceeding the upper limit of conditional expression (1)
When the movement amount of the lens group is small and the movement amount of the second lens group is large, a strong telephoto type is obtained on the long focal length side, which is advantageous for shortening the entire length of the lens on the long focal length side, but increases the overall length of the lens on the short focal length side. In addition, since the distance between the first lens group and the stop disposed in the rear lens group (third lens group) becomes long, the lens diameter of the first lens group becomes large, which is not good.

Further, the focal length of the first lens group is set so as to satisfy the conditional expression (2).

If the positive refractive power of the first lens unit is increased beyond the lower limit of the conditional expression (2), it is advantageous in terms of the total lens length, the front lens diameter, and the zoom stroke. Value and cannot be corrected by other lens groups. When the first lens group is a focusing lens group, particularly in the long focal length side, the displacement in the negative direction becomes remarkable, which is not good. If the positive refracting power of the first lens unit is weakened beyond the upper limit of the conditional expression (2), zooming fluctuation of spherical aberration and focus fluctuation will be advantageous, but it is not good because the entire lens system becomes large.

By the way, conventionally, in a zoom lens as in the present invention, a method of increasing the refractive power of each lens group in order to reduce the total length of the lens, the amount of movement of the lens for zooming, and the diameter of the front lens has been generally adopted. However, when the refractive power is increased in this manner, it becomes difficult to correct aberrations by each lens group alone, and a large aberration variation occurs particularly during zooming. For example, the first lens group having a relatively large zoom stroke as compared with the other lens groups has a large displacement in the negative direction of spherical aberration during zooming, and has a stronger refractive power than the other lens groups. In the second lens group, not only spherical aberration but also astigmatism in the positive direction during zooming increase. Then, in the third lens group having the second largest magnification change ratio after the second lens group, coma particularly tends to be displaced in the negative direction (introverted coma aberration) during zooming. Therefore, in a zoom lens that attempts to reduce the overall length while having a high zoom ratio as in the present invention, it is important to suppress aberrations generated in each lens group as much as possible and to eliminate aberrations particularly between the lens groups. It is becoming. That is,
In a four-unit zoom lens system according to the present invention, taking the cancellation relationship of the spherical aberration among the lens units as an example, the fluctuation of the remaining spherical aberration from the first lens unit to the third lens unit is sufficiently canceled between the lens units. However, in the present invention, the fourth lens group is moved to the object side, and the fourth lens group is configured as described later to remove the fluctuation of residual spherical aberration. are doing.

Here, the operation of the present invention will be described in more detail. As described above, the absolute value of the residual spherical aberration from the first lens unit to the third lens unit is negative, and is further displaced in the negative direction from the short focal length side to the long focal length side during zooming.
Also, the absolute value of coma is negative (introverted coma),
Similarly, it is displaced in the negative direction. The astigmatism also has a negative absolute value and is displaced in the positive direction.

Therefore, as a condition of the aberration generated in the fourth lens group,
Spherical aberration has a positive absolute value and is displaced in the positive direction when zooming from the short focus side to the long focus side, and coma also has a positive absolute value and is also displaced in the positive direction and astigmatism also has an absolute value. It is necessary to displace in the positive direction and conversely in the negative direction. However, due to the action of the positive refracting power inherently possessed by the fourth lens group, in particular, the spherical aberration tends to have a negative absolute value and promote a negative amount as a whole lens system. Therefore, in the present invention, the fourth lens group is arranged such that a strong convex surface (A surface) faces the image surface side and a concave surface (B
A) is composed of a cemented lens having a negative refractive power as a whole (the cemented surface is defined as a C surface), which is obtained by bonding a meniscus positive lens facing the surface) and a negative lens having a strongly concave surface facing the object side.
The negative spherical aberration generated on the surface is canceled by the positive spherical aberration generated on the surface C, and the spherical aberration of the entire lens system is corrected by the spherical aberration generated on the surface B.

The removal of spherical aberration fluctuation during zooming occurs on the A-plane against an increase in the incident height of the axial luminous flux even when the fourth lens group is moved to the object side during zooming from the short focus side to the long focus side. If the setting is such that the negative spherical aberration and the positive spherical aberration generated on the C plane are always cancelled, B
Positive spherical aberration generated with an increase in the incident height of the axial light beam incident on the surface can be displaced in the positive direction. or,
With respect to coma, positive coma is generated on both sides of the C-plane and the A-plane to correct the coma of the entire lens system and to remove coma aberration fluctuation during zooming, as the fourth lens group moves toward the object side. As a result, the incident height of the off-axis light beam incident on the C-plane and the A-plane decreases, so that the positive coma can be displaced in the positive direction on both sides of the C-plane and the A-plane. Further, regarding astigmatism, since relatively large positive astigmatism mainly occurs on the C plane, the astigmatism of the entire lens system is corrected by controlling the radius of curvature of the C plane, and astigmatism during zooming is obtained. The removal of the fluctuation can be displaced in the negative direction because the incident height of the off-axis light beam incident on the C-plane decreases as the fourth lens group moves toward the object side. Desirably, the surface B has an aspherical surface having a shape in which the curvature increases as the distance from the optical axis increases. In addition to promoting the positive displacement of spherical aberration in proportion to an increase in the incident height of the axial light beam incident on the surface B due to zooming, , Coma, astigmatism,
This is effective in correcting field curvature in a well-balanced manner.

Preferably, the ratio of the refractive power of the surface A to the refractive power of the surface C is set as follows. That is, the refractive power of surface A is φ
0.5 <| φ _C | / φ _A <1.5 (φ _C <0) ‥ (3) where _A and C plane refractive powers are φ _C.

When the refractive power of the surface A is increased beyond the lower limit of the expression (3), the amount of negative spherical aberration generated on the surface A increases, and the addition of the surface C and the surface B cannot be canceled and the entire lens system Is deteriorated in the negative direction, including the fluctuation.
In particular, astigmatism on the short focal length side worsens in the negative direction, which is not good.

If the absolute value of the negative refractive power of the C-plane becomes larger than the upper limit of the expression (3), the occurrence of positive spherical aberration in the fourth lens group as a whole increases, and correction on the short focal length side becomes slightly advantageous. On the long focal length side, the correction becomes excessive and the displacement is largely shifted in the positive direction. or,
Similarly, astigmatism is overcorrected, which is not good. Furthermore,
Above the upper limit or below the lower limit of the expression (3), the occurrence of positive coma increases, and in particular, strong inward coma occurs on the long focal length side, which is not good.

Embodiments 1 and 2 of the present invention have the following configurations. The first lens group includes, in order from the object side, a cemented lens having a positive refractive power as a whole, comprising a cemented negative meniscus lens having a concave surface facing the image surface and a biconvex lens, and a positive meniscus lens having a strong convex surface facing the object side. The second lens is composed of a meniscus negative lens having a strong concave surface facing the image surface side, a biconcave lens, a biconvex lens, and a biconcave lens having a strong curvature of the lens surface on the object side. The third lens group is a biconvex lens. The biconvex lens is composed of a cemented lens having a positive refracting power as a whole, which is formed by bonding a biconcave lens having a strong curvature of the lens surface on the object side, and the fourth lens group is a biconvex lens having a strong curvature of the lens surface on the image side. From a cemented lens having a negative refracting power as a whole consisting of bonding a positive meniscus lens with a strong convex surface facing the image surface and a negative meniscus lens with a strong concave surface facing the object side In zooming from the short focus to the long focus, the first lens unit and the fourth lens unit are integrally moved to the object side, and the second lens unit is curvedly moved to one end and the object side. To move to
The air gap from the first lens group is always monotonically increased. The reason why the second lens group is moved in a curved manner is to improve the performance at the intermediate focal length, and in particular, in order to maintain a favorable spherical aberration, the second lens group is moved in a curved manner to bulge toward the object side.

The third lens group is also moved in a curved manner, but this movement is for correcting the field curvature. That is, in the first embodiment, the third lens group is moved in a curve toward the object side in the first embodiment.
In this example, the image is curvedly moved to the object side, then moved to the image side, and further moved to the object side to correct the field curvature.

In the present embodiment, focusing is performed by moving the first lens group to the object side.
The lens group may be moved to the image plane side, and the fourth lens group may be moved to the object side.

Next, examples related to the present invention will be described. In the numerical examples, _Ri is the radius of curvature of the i-th lens surface in order from the object side,
D _i is the i-th lens thickness and air gap in order from the object side,
N _i and v _i are the refractive index and Abbe number of the glass of the i-th lens in order from the object side. Note that the aspheric shape in this embodiment is given by the following equation.

X = r ｛1− (1−h ² / r ² ) ^0.5 ｝ + Ah ² + Bh ⁴ + Ch ⁶ + Dh ⁸ where X is the displacement in the optical axis direction at a height h from the optical axis, and r is the reference spherical surface Are the aspherical coefficients A, B, C, and D, respectively. [Effects of the Invention] As described above, according to the present invention, it is possible to provide a zoom lens having high zoom ratio and compact but excellent optical performance.

[Brief description of the drawings]

1 and 3 are lens cross-sectional views of Numerical Examples 1 and 2 according to the present invention, respectively. 2 and 4 show various aberration diagrams of the numerical examples 1 and 2 in an object distance ∞ state. In the drawing, (A),
(B) shows aberrations at the wide-angle end and the telephoto end, respectively. S denotes a sagittal image plane, M denotes a meridional image plane, d denotes a d-line, g denotes a g-line, and SC denotes a sine condition.

Continuation of the front page (56) References JP-A-57-161804 (JP, A) JP-A-2-66509 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 9 / 00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (1)

(57) [Claims] A first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. During zooming from the end to the telephoto end, the first lens group and the fourth lens group move toward the object side, and the second lens group monotonically increases the air gap between the first lens group and the first lens group, and the third lens group The group moves so that the air gap between the second lens group and the second lens group decreases monotonically, and the amount of movement of the first lens group and the second lens group during zooming from the wide-angle end to the telephoto end is represented by A ₁ , respectively. A
_2, when the focal length f _T of the entire system at the telephoto end, the focal length of the first lens group and the _{f 1, 0.11 <A 2 /} (A 2 -A 1) <0.26 0.25 <f 1 / f A compact, high-magnification zoom lens that satisfies the following condition: _T <0.39.