JPH10274739A - Zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure a focal distance at which the sufficiently wide photographing angle of view is secured and necessary and sufficient back focus and to obtain high optical performance all over the variable power range by appropriately setting the lens constitution of respective lens groups. SOLUTION: This zoom lens is constituted of the lens groups, that is, the 1st group L1 having negative refractive power and the 2nd group L2 having positive refractive power; and the 1st group L1 is moved to an image surface side while keeping a projected locus and the 2nd group L2 is moved to an object side while a distance between both lens groups is changed. The 1st group L1 corrects the fluctuation of an image surface caused by variable magnification and performs focusing. The 2nd group L2 has variable power action. In such a case, the 2nd group L2 is constituted of three lenses having specified lens shape, that is, a positive 21st lens which turns its convex surface to the object side, a negative 22nd lens which turns its concave surface to the image surface side and a positive 23rd lens which turns its convex surface to the object side. Then, the 21st lens and the 22nd lens are joined. Thus, the fluctuation of specifications given by the production error of a holding mechanism is restrained to the minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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, an SV camera (electronic still camera), and the like. And these two
By appropriately setting the lens configuration of the two lens groups, the number of lenses is reduced and the overall lens system is miniaturized. The shooting angle of view is 66 ° to 25 °, the zoom ratio is about 3, and the F-number is about 1.8. It relates to a zoom lens.

[0002]

2. Description of the Related Art In recent years, video cameras and electronic still cameras capable of digital recording even for consumer use have emerged, and recording with higher image quality than conventional so-called analog recording type video cameras and electronic still cameras has become available. It has become possible.

Accordingly, there is a demand for a photographic lens used in these cameras having higher optical performance. Further, in the electronic still camera, the recording medium is changed from a conventional floppy disk to an IC card or an IC memory, and the space of a recording unit to be tightened to the whole camera is remarkably reduced. .

In particular, there is a tendency for a CCD for imaging to be smaller, and a zoom lens having a shorter focal length at the wide-angle end than before is demanded.

[0005] As a zoom lens having a relatively high optical performance, a small size, a zoom ratio of about 2 to 3 times, and an F-number of about 2.5 to 3.0, a two-group zoom type is better than a conventional zoom lens. Are known. The two-unit zoom lens includes, in order from the object side, a first unit having a negative refractive power and a second unit having a positive refractive power. Zooming is performed by changing the air gap between these two lens units. .

Since the two-unit zoom lens can be constituted by a relatively small number of lenses, it is often used as a zoom type aiming at miniaturization.

As a two-zoom lens, for example, in the Japanese Patent Application Laid-Open No. Sho 62-266511, the present applicant has attempted to reduce the size by disposing a fixed stop during zooming between two lens groups. .

Also, Japanese Patent Application Laid-Open Nos. 1-183616, 1-183618 and 1-183419
JP, JP-A-1-210914, JP-A-4-4914
In Japanese Patent Application Laid-Open Nos. 6308, 4-46310 and 4-56814, the first lens unit includes two lenses, a negative lens and a positive lens, and the second lens unit includes a positive lens. , Negative lens, and positive lens
It proposes a zoom lens composed of two lenses.

[0009]

In the two-unit zoom lens proposed in each of the above publications, the second unit has a triplet configuration or a configuration in which a positive lens is added to the triplet configuration, and furthermore, a negative lens The positive lenses on both sides of the lens did not have a contact structure at the marginal portion. Also, if a marginal contact structure is taken, the outer diameter of those lenses tends to increase.
In addition, if it is attempted to maintain the accuracy of the distance or the like only by the lens barrel structure without taking a marginal contact, there is a tendency that a manufacturing error of the lens barrel is added and the aberration is deteriorated.

Japanese Patent Application Laid-Open No. 1-183618 proposes disposing a stop behind the second lens unit.
In this case, the position of the entrance pupil at the wide-angle end becomes deep, and it becomes difficult to widen the angle and reduce the size.

Also, the focal length at the wide-angle end has to be much shorter than the length of the back focus for inserting a low-pass filter or an infrared cut filter and for obtaining a sufficient working distance. For this reason, it is important to obtain a good balance between the focal length and the back focus.

The present invention generally relates to a zoom lens composed of two negative-lead type lens groups which are preceded by a lens group having a negative refractive power, by appropriately setting the lens configuration of each lens group. It is possible to secure a sufficient focal length and a necessary and sufficient back focus for a CCD with an effective diagonal screen size of 4 "size, which is about 4.5 mm to 4.6 mm for a CCD. It is an object of the present invention to provide a zoom lens that can be made and has high optical performance over the entire zoom range.

[0013]

The zoom lens according to the present invention comprises, in order from the object side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power. In a zoom lens that changes magnification by changing an air interval, the first group includes a negative eleventh lens and a positive twelfth lens,
The second group includes a positive twenty-first lens, a negative twenty-second lens, and a positive twenty-third lens, and the twenty-second lens is the second lens.
It is characterized by being joined to the first lens or the twenty-third lens.

[0014]

1 to 6 are sectional views of a lens at a wide angle end according to Numerical Examples 1 to 6 of the present invention. 7 to 1
2 is an aberration diagram of each of Numerical Examples 1 to 6 of the present invention. In the aberration diagrams, (A) shows the wide-angle end, and (B) shows the telephoto end. In the figure, L1 denotes a first group having a negative refractive power, L2 denotes a second group having a positive refractive power, and arrows show the movement trajectories of the respective lens groups during zooming from the wide-angle end to the telephoto end. In the first example, the first unit L1 has a locus convex toward the image plane while the distance between the two lens units is changed, and the second unit L2 moves toward the object side. The first lens group corrects the image plane fluctuation due to zooming and performs focusing. The second group has a zooming action.

G is a glass block such as a low-pass filter or an infrared cut filter. SP is the aperture,
Provided between the first lens unit L1 and the second lens unit L2,
It is moved together with the group L2. IP is an image plane.

In general, there are four types of zoom lenses which are composed of a first lens unit having a negative refractive power and a second lens unit having a positive refractive power of the second lens unit, and perform zooming by moving both lens units. With good optical performance over a range, and downsizing of the lens,
In order to achieve low cost, it is necessary to appropriately set the lens configuration of the second group, which has a zooming action and has a relatively large moving amount from the wide-angle end to the telephoto end, and minimizes aberration fluctuations during zooming. There is.

Therefore, in this embodiment, as described above, the second lens unit is a positive twenty-first lens having a convex surface facing the object side, a negative twenty-second lens having a concave surface facing the image surface side, and having a convex surface facing the object side. It is composed of three lenses having a predetermined lens shape of the positive twenty-third lens.

In the first to third numerical embodiments, the twenty-first lens and the twenty-second lens are cemented. Numerical examples 4 to
In No. 6, the 22nd lens and the 23rd lens are joined.
In this way, by forming the second group, the second group has a small, lightweight, easy-to-manufacture configuration having good optical performance over the entire zoom range despite the small number of lenses of three. , Low-cost zoom lens.

The first lens unit comprises a negative meniscus eleventh lens having a convex surface facing the object side and a positive meniscus twelfth lens having a convex surface facing the object side.

Also, in the present embodiment, the negative lens, whose spacing and eccentricity most affect the aberration in the second lens unit, and the positive lens are cemented to each other to prevent mutual eccentricity and prevent deterioration of optical performance. Zoom lens. Alternatively, the positive lens in the second group and the negative lens are joined, and the negative lens and the positive lens on the image plane side are brought into a marginal contact state, thereby forming a lens barrel or a mechanical space holding means (for example, a separation ring or the like). ) Facilitates minimizing variations in optical performance and specifications, such as aberrations, caused by manufacturing errors of the holding mechanism that occur when passing through.

In Numerical Embodiments 1 to 3, the positive first lens closest to the object side and the negative second lens in the second lens unit are cemented, and the positive third lens closest to the image plane and the negative second lens are connected. The lens is held by marginal contacts. In the case of this type, the negative second lens has a marginal error that is minimized by a marginal contact with the positive twenty-third lens on the image plane side, the distance error of which is the most sensitive to aberration, and at the same time, the maximum diameter Since the lens is also cemented to the positive 21st lens closest to the object side, it can be held in a very stable state.

In Numerical Examples 4 to 6, the negative second lens in the second lens unit and the positive 23rd lens closest to the image plane are held in a cemented state. In the case of this type, the above-mentioned negative second lens is cemented with the positive twenty-third lens on the image plane side where the spacing error is most sensitive to aberration, so that no “interval error” occurs, and the lens is extremely polarized. It has a strong holding structure for the core.

In the present invention, a zoom lens in which the size of the entire lens system is reduced is achieved by setting each element as described above. However, it is preferable that at least one of the following conditions be satisfied. According to this, while trying to reduce the size,
Higher optical performance can be obtained.

(1-1) Assuming that the radius of curvature of the i-th lens surface counted from the object side is Ri, the following condition should be satisfied: -0.2 <R8 / R7 <0.3 (1) It is.

This conditional expression (1) specifies the lens shape of the negative second lens in the second lens unit, which is suitable for achieving the object of the present invention. The refractive power of the lens surface on the image surface side of the 22nd lens is made stronger than the refractive power of the lens surface on the object side, and the negative 22nd lens and the positive 23rd lens on the image surface side are marginally contacted or joined. To provide a suitable shape for Therefore, the negative second
The two lenses have a negative meniscus shape with a strong refraction surface facing the image surface, or a negative lens shape with a substantially flat lens surface on the object side.

If the lower limit of conditional expression (1) is exceeded, the refracting power of the lens surface on the image plane side is too weak, and it is impossible to join the next positive 23rd lens or to make a marginal contact. Chromatic aberration is in a state of insufficient correction. On the other hand, if the value exceeds the upper limit, the refractive power of the lens surface on the image plane side becomes too strong, and even if a marginal contact structure is employed, aberration fluctuations are likely to occur.

If the conditional expression (1) satisfies the following condition: -0.1 <R8 / R7 <0.2 ‥‥ (1a), the object of the present invention can be achieved more effectively. It will be easier.

(1-2) Refractive index of the material of the positive twenty-third lens,
When the Abbe numbers are Np and νp, it is preferable to satisfy the following conditional expressions: Np> 1.70, νp> 35 (2) According to this, chromatic aberration can be favorably corrected.

In particular, if conditional expression (2) is satisfied, chromatic aberration can be satisfactorily corrected even when the twenty-second lens is near the lower limit of conditional expression (1).

(1-3) The radius of curvature of the i-th lens surface counted from the object side is Ri, the distance between the 22nd and 23rd lenses is DL, and the focal length of the entire system at the wide-angle end is fw.
1 ≦ R9 / R8 <2 ‥‥ (3) 0 ≦ DL / fw <0.15 ‥‥ (4)

The conditional expressions (3) and (4) define a marginal condition or a joining condition of the negative twenty-second lens and the following positive twenty-third lens. In addition, when both types exceed the upper limit, in order to make marginal contact, the refractive power of the positive twenty-third lens becomes too strong, and it becomes difficult to correct the coma flare of the upper ray of the off-axis luminous flux in a wide angle range. Absent.

The conditional expressions (3) and (4) satisfy the following condition: 1.0 ≦ R9 / R8 <1.5 ‥‥ (3a) 0.0 ≦ DL / fw <0.1 ‥‥ (4a) Is satisfied, it is easy to effectively achieve the object of the present invention.

(1-4) When the focal length of the i-th lens unit is fi and the focal length of the entire system at the wide-angle end is fw, 2 <| f1 | / fw <3 (5) 1.8 <f2 /Fw<2.5‥‥(6) 0.9 <| f1 | / f2 <1.4 ‥‥ (7)

The conditional expressions (5) to (7) are equivalent to the first group and the second group.
This is related to the power (refractive power) of the group. In such a zoom lens, a back focus for securing a sufficient space for a low-pass filter such as a crystal or an infrared cut filter disposed immediately before the CCD and an infrared cut filter is provided. This is a condition or a condition for achieving a zoom ratio of about three times.

Conditional expression (5) is the refractive power of the first lens unit, conditional expression (6) is the refractive power of the second lens unit, and conditional expression (7) is the ratio of the refractive powers of the first and second lens units. Has been identified.

Exceeding the lower limit of condition (5) is the first condition.
This means that the diverging power of the group becomes too strong, and the back focus becomes unnecessarily long, which hinders a reduction in the overall length of the lens. If the focal length of the first lens group becomes longer than the upper limit, the refractive power of the second lens group must be weakened in order to secure an optimum back focus. To achieve the zoom lens of the present invention, It is necessary to increase the distance between the first and second lens units and the amount of movement, which also hinders the reduction of the overall length of the lens.

If the lower limit of conditional expression (6) is exceeded, it will be difficult to secure a sufficient back focus. If the upper limit is exceeded and the focal length of the second lens unit becomes too long, the back focus will be unnecessarily long, and Inhibits shortening of the overall length.

Conditional expression (7) is a focal length that can secure a wide angle of view even if the size of the CCD is reduced, while maintaining a good balance between the refractive powers of the first and second groups. It is for.

By satisfying conditional expressions (5) to (7) at the same time, the object of the present invention is effectively achieved.

A zoom lens having a wide angle of view with respect to the above-mentioned 1/4 "size CCD image pickup screen has a focal length at the wide angle end shorter than the effective diagonal screen size of the CCD. In the case of a single-lens reflex camera and a zoom lens with a focal length of about 35 mm at the wide-angle end, the wide-angle of the camera for a 1/4 "CCD (about 4.5 mm) is used. Even in the case of a zoom lens having a focal length of about 3.6 mm at the end, the thickness of the edge at the outer peripheral portion, which is a ring portion of the lens unit, is about 1 mm to 2 mm. Therefore, the relationship between the thickness of the lens and the focal length also differs greatly from the conventional one.

It is another object of the present invention to satisfy at least one of the following conditional expressions in order to secure a lens shape which is easy to manufacture even if the CCD size is reduced.

(1-5) The positive 21st object on the object side in the second group
Assuming that the lens thickness of the lens is D21, the following conditional expression is satisfied: 0.6 <D21 / fw <0.8 (8)

If the thickness of the lens is reduced beyond the lower limit value of the conditional expression, the production of the lens becomes very costly, and the ring portion becomes thin, so that the lens cannot be held stably. Further, increasing the thickness beyond the upper limit is not particularly necessary for correcting the aberration, and it is not preferable because the effect of shortening the entire length of the lens is hindered.

(1-6) The positive second lens element from the image plane in the second lens group
Assuming that the lens thickness of the three lenses is D23, the following conditional expression is satisfied: 0.4 <D23 / fw <0.9 (9)

In the case of this conditional expression as well, as in conditional expression (8) above, if the lens is thinned beyond the lower limit value, the production of the lens becomes very costly, and the coupling portion becomes thin and the lens becomes stable. Inadequate retention. Also, it is not particularly necessary to increase the thickness beyond the upper limit for aberration correction,
This is not preferable because the effect of shortening the entire length of the lens is impaired.

(1-7) When the refractive index and Abbe number of the material of the negative eleventh lens in the first group are N1 and ν1, respectively, N1> 1.7 and ν1> 46 ‥‥ (10) are satisfied. It is to be.

Conditional expression (10) is mainly for correcting aberration. If the lower limit of conditional expression (10) is exceeded, the Petzval sum of the entire system will increase in the positive direction, and the field curvature will be extremely generated, which is not preferable, and the chromatic aberration of magnification will be overcorrected, which is also not preferable.

(1-8) When the back focus at the wide-angle end of the whole system is Bf, the following conditional expression is satisfied: 1.7 <Bf / fw <2.2 (11)

According to this, a zoom lens which sufficiently achieves the object of the present invention can be obtained.

(1-9) An aspheric lens is provided for each of the first and second groups. This enables further aberration correction.

With the recent reduction in the size of the CCD, CC
It has been practiced to arrange a lens array or the like on the D element to increase the light energy collection rate and compensate for the lack of sensitivity. In the case of these CCD elements, it is known that the sensitivity rapidly decreases when the light beam incident on the CCD element exceeds a certain angle. If this is replaced with the photographing lens side and the exit pupil position of the photographing lens becomes too short, it appears that the peripheral light amount is insufficient on the TV screen even though the peripheral light amount is actually set to be sufficient. , So-called shading occurs.

Therefore, in a zoom lens for a video camera or an electronic still camera as in the embodiment of the present invention, in order to avoid this shading, the diaphragm moves on the optical axis when zooming, and the exit pupil position is set to an appropriate position. It is desirable to keep. However, since it is generally necessary to arrange a mechanism for electrically opening and closing the aperture, fixing the aperture during zooming does not hinder the object of the present invention.

In the present invention, the focus is moved by the first lens group. However, the panning focus is not hindered by the whole extension system or the pan focus when aiming for a wider angle.

Next, numerical examples of 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, Di is the i-th lens thickness and air gap in order from the object side, and Ni and νi are the i-th lens in order from the object side. Are the refractive index and Abbe number of the glass.

In the numerical examples, the last two lens surfaces are glass blocks such as a face plate and a filter.

The aspherical shape has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis, a positive traveling direction of light, R represents a paraxial radius of curvature, and B, C, D, and E each represent an aspherical coefficient. And when

[0057]

(Equation 1) It is represented by the following expression. Table 1 shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples.

[0058]

[Outside 1]

[0059]

[Outside 2]

[0060]

[Outside 3]

[0061]

[Outside 4]

[0062]

[Outside 5]

[0063]

[Outside 6]

[0064]

[Table 1]

[0065]

As described above, according to the present invention, in a zoom lens composed of two negative-lead type lens groups which are preceded by a lens group having a negative refractive power, the lens configuration of each lens group is appropriately set. In general, 1 /
Effective diagonal screen size called 4 "size is φ4.5m
It has a focal length that can secure a sufficiently large shooting angle of view for a CCD of about m to 4.6 mm, and a sufficient and sufficient back focus, and has high optical performance over the entire zoom range. Zoom lens can be achieved.

In particular, in a two-group type zoom lens,
As mentioned above, the lens unit has a predetermined configuration, maintaining an appropriate back focus, accurately maintaining sensitive parts to aberrations, and shortening the overall length. A zoom lens having an F number of about 2.8 and a magnification of about 2.8 can be achieved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens at a wide angle end according to Numerical Embodiment 1 of the present invention.

FIG. 2 is a sectional view of a lens at a wide-angle end according to a second numerical embodiment of the present invention.

FIG. 3 is a sectional view of a lens at a wide-angle end according to a third numerical embodiment of the present invention;

FIG. 4 is a sectional view of a lens at a wide-angle end according to a fourth embodiment of the present invention.

FIG. 5 is a sectional view of a lens at a wide angle end according to Numerical Example 5 of the present invention.

FIG. 6 is a sectional view of a lens at a wide-angle end according to a sixth numerical embodiment of the present invention.

FIG. 7 is an aberration diagram at a wide angle end and a telephoto end according to Numerical Embodiment 1 of the present invention.

FIG. 8 is an aberration diagram at a wide angle end and a telephoto end according to Numerical Embodiment 2 of the present invention.

FIG. 9 is an aberration diagram at a wide angle end and a telephoto end according to Numerical Example 3 of the present invention.

FIG. 10 is an aberration diagram at a wide angle end and a telephoto end according to Numerical Embodiment 4 of the present invention.

FIG. 11 is an aberration diagram at a wide angle end and a telephoto end according to Numerical Example 5 of the present invention.

FIG. 12 is an aberration diagram at a wide angle end and a telephoto end according to Numerical Example 6 of the present invention.

[Explanation of symbols]

 L1 First lens unit L2 Second lens unit d d-line g g-line S sagittal image plane M meridional image plane C sine condition

Claims (5)

[Claims] 1. A zoom lens comprising two lens groups, a first group having a negative refractive power and a second group having a positive refractive power, in order from the object side, and performing zooming by changing the air gap between both lens groups. In the first group, the first group includes a negative eleventh lens and a positive twelfth lens, and the second group includes a positive twenty-first lens, a negative twenty-second lens, and a positive twenty-third lens. A zoom lens, wherein the lens is cemented with the 21st lens or the 23rd lens. 2. The lens according to claim 1, wherein a condition of −0.2 <R8 / R7 <0.3 is satisfied, where Ri is a radius of curvature of the i-th lens surface counted from the object side. Zoom lens. 3. When the radius of curvature of the i-th lens surface counted from the object side is Ri, the distance between the 22nd lens and the 23rd lens is DL, and the focal length of the entire system at the wide-angle end is fw. 2. The zoom lens according to claim 1, wherein the following condition is satisfied: .ltoreq.R9 / R8 <30.ltoreq.DL / fw <0.15. 4. When the focal length of the i-th lens unit is fi and the focal length of the entire system at the wide-angle end is fw, 2 <| f1 | / fw <3 1.8 <f2 / fw <2.50. 4. The zoom lens according to claim 2, wherein a condition of 9 <| f1 | / f2 <1.4 is satisfied. 5. The lens thickness of the 21st lens is D21,
4. The zoom lens according to claim 2, wherein a condition of 0.6 <D21 / fw is satisfied when the focal length of the entire system at the wide-angle end is fw.