JP2925301B2 - 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.

[Prior Art] In a zoom lens having a focal length at a wide-angle end shorter than a diagonal on both sides 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, and a second lens unit having a positive refractive power. A four-unit zoom lens including three lens units and a fourth lens unit having a positive 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 compact high-power zoom lens of the present invention comprises, in order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power.
The first lens group and the fourth lens group move toward the object side during zooming from the short focal length side to the long focal length side, and the second lens group includes a fourth lens group having a positive refractive power. Move the air gap between the first lens group and the third lens group so that the air gap between the third lens group and the second lens group is monotonically decreased, and move the fourth lens group toward the image plane in order from the object side. An image of the first positive lens, which is composed of a cemented lens in which a first positive lens with a strong convex surface facing toward the object, a meniscus positive lens with a concave surface facing the object side and a negative lens with a strong concave surface facing the object side are bonded. When the refractive power of the surface-side lens surface is φ _A and the refractive power of the cemented surface of the cemented lens is φ _C (φ _C <0), the condition 0.5 <| φ _C | / φ _A <1.5 is satisfied. It is characterized by.

Embodiment An embodiment of the present invention will be described below 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. Then, 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.

Conventionally, in a zoom lens according to the present invention, a method of increasing the refractive power of each lens group 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 the 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 minimize aberrations occurring in each lens group and to eliminate aberrations particularly between the lens groups. It becomes. That is, in the four-group zoom lens according to the present invention, taking the cancellation relation between the lens groups of the spherical aberration as an example, the fluctuation of the remaining spherical aberration from the first lens group to the third lens group is not sufficient between the lens groups. However, in the present invention, the fourth lens group is moved toward the object side, and the fourth lens group is configured as described above to change the residual spherical aberration. Has been removed.

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 when zooming from the short focal length to the long focal length. Further, the absolute value of the coma aberration is also negative (introverted coma), and is similarly displaced in the negative direction. The astigmatism also has a negative absolute value, and is displaced in the positive direction. Therefore, as the condition of the aberration generated in the fourth lens group, the absolute value of the spherical aberration is positive, and the spherical aberration is displaced in the positive direction during zooming from the short focal length side to the long focal length side, and the absolute value of the coma aberration is also positive. Similarly, the absolute value of the astigmatism which is displaced in the positive direction is also positive, and conversely, the astigmatism needs to be displaced in the negative direction. However, due to the action of the positive refractive power inherent in the fourth lens group, in particular, spherical aberration
The absolute value becomes negative, and the lens system tends to promote a negative amount as a whole.

Therefore, in the present invention, the fourth lens group is arranged in order from the object side, the first positive lens having a strong convex surface (A surface) facing the image surface side, and the meniscus positive lens having the concave surface (B) facing the object side and strong toward the object side. It is composed of a cemented lens having a negative refractive power (the cemented surface is defined as a C surface) as a whole, and a negative lens having a negative refractive power generated on the C surface. 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.

In the present invention, the ratio of the refractive power of the surface A to the refractive power of the surface C satisfies the following condition. That is, when the refractive power of the A-plane phi _A, C surface power was _{φ C 0.5 <| φ C |} / φ A <1.5 (φ C <0) ... (1) Conditions may satisfy the.

When the refracting power of the surface A is increased beyond the lower limit of the expression (1), the amount of negative spherical aberration generated on the surface A increases, so that the addition of the surface C and the surface B cannot be canceled, and the entire lens system becomes impossible. Of the spherical aberration including negative fluctuations in the negative direction.
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 (1), the occurrence of positive spherical aberration increases as a whole of the fourth lens unit, and the correction on the short focal length side becomes slightly more 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 (1), the occurrence of positive coma increases, and in particular, strong introvert coma occurs on the long focal length side, which is not good.

More preferably, the following conditions should be satisfied.

_{0.1 <N B -N A <0.35} (2) where, N _A is the refractive index of the positive lens constituting the cemented lens in the fourth lens group, the N _B constituting the cemented lens in the fourth lens group This is the refractive index of the negative lens.

This condition mainly corrects coma aberration more favorably, and the coma aberration becomes worse even when the value exceeds the upper limit value or exceeds the lower limit value.

Although the object of the present invention is achieved by the above configuration, in order to reduce the size of the entire lens system, it is preferable to set the focal length of the first lens unit as follows. That focal length of the long focus side f _T, when the f ₁ and the focal length of the first lens group, may satisfy _{0.25 <f 1 / f T <} 0.39 ... (3) The condition.

If the positive refractive power of the first lens unit is increased beyond the lower limit of the conditional expression (3), it is advantageous in terms of the total length of the lens, the diameter of the front lens, 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 refractive power of the first lens unit is weakened beyond the upper limit of the conditional expression (3), zooming fluctuation of spherical aberration and focus fluctuation will be advantageous, but it is not good because the entire lens system becomes large.

More preferably, when the movement amounts of the first lens unit and the second lens unit during zooming from the wide-angle end to the telephoto end are A ₁ and A ₂ , respectively, it is preferable to set as follows.

0.11 <A ₂ / (A ₂ −A ₁ ) <0.26 (4) Conditional expression (4) satisfies the condition that the total lens length on both the short focal length side and the long focal length side is compact and the lens diameter of the first lens group is small. That is, conditional expression (4)
Exceeds the lower limit of the second lens group, the amount of movement of the first lens unit is large,
When the amount of movement of the 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. If the amount of movement of the first lens group is small and the amount of movement of the second lens group is large beyond the upper limit of conditional expression (4), a strong telephoto type is obtained on the long focal length side, which is advantageous for shortening the overall length of the lens on the long focal length side. However, not only does the overall length of the lens on the short focal length side increase, but also the distance between the first lens unit and the stop disposed in the rear lens unit (third lens unit) increases, so that the lens diameter of the first lens unit increases. 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 meniscus negative lens having a concave surface facing the image surface and a biconvex lens, and a meniscus positive 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 biconvex 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 refractive power as a whole, which is formed by laminating a biconvex lens and a biconcave men's lens having a strong curvature of the lens surface on the object side. , A cemented lens having a negative refractive power as a whole, consisting of bonding a meniscus positive lens with a strong convex surface facing the image surface and a meniscus negative 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 spacing of the first lens group is always monotonically increased. The reason why the second lens group is moved in a curved manner is to improve performance at an intermediate focal length.
In particular, in order to keep the spherical aberration favorable, the curve is moved 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 to the object side in a curved manner, and in the second embodiment, the third lens group is moved to the object side once in a curved manner, then moved to the image plane side, and further moved to the object side. The field curvature is corrected. In this embodiment, focusing is performed by moving the first lens group to the object side. However, the third lens group may be moved to the image plane side, or the fourth lens group may be moved to the object side. May be.

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 amount in the optical axis direction at the height h from the optical axis, r is The radii of curvature A, B, C, and D of the reference spherical surface are aspherical coefficients, 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. FIGS. 2 and 4 show various aberration diagrams in the object processing 実 施 state of Numerical Examples 1 and 2. FIG. 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 (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (2)

(57) [Claims] A first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. During zooming from the focal point to the long focal point, 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. The third lens group moves so that the air gap with the second lens group monotonously decreases, and the fourth lens group moves from the object side in order from the object side to the first positive lens having a strong convex surface facing the image surface side, and the concave surface toward the object side. A positive meniscus lens and a negative lens with a strong concave surface facing the object side are composed of a cemented lens, and the refractive power of the image-side lens surface of the first positive lens is φ _A− , when the refractive power of the cemented surface phi and _{C (φ C <0),} 0.5 <| φ C | / φ a <1.5 Compact high zoom zoom lens satisfies the that condition. 2. The refractive index of a positive lens constituting the cemented lens is N _A , and the refractive index of a negative lens constituting the cemented lens is
When the _{N B, 0.1 <N B -N} A < compact high magnification zoom lens of the claims claim 1 wherein, characterized by satisfying the 0.35 condition:.