JP2832092B2 - Rear focus zoom lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear focus type zoom lens, and more particularly to a rear focus type zoom lens which is used for a photographic camera or a video camera and has a high zoom ratio of about 8 to 10 or a large aperture ratio of about F2. The present invention relates to a rear-focus type zoom lens that has a small size while being held.

[0002]

2. Description of the Related Art Conventionally, various types of zoom lenses such as a photographic camera and a video camera adopt a so-called rear focus type in which a lens group other than the first lens group on the object side is moved to perform focusing. Have been.

In general, a rear focus type zoom lens has a smaller effective diameter of the first lens group than a zoom lens which performs focusing by moving the first lens group, so that the entire lens system can be easily miniaturized. Since close-up photography, particularly very close-up photography, is facilitated and the relatively small and lightweight lens group is moved, the driving force of the lens group is small, so that quick focusing can be performed.

As such a rear focus type zoom lens, for example, in JP-A-63-247316, a positive first lens group, a negative second lens group, a positive third lens group, and a positive Having a fourth lens group,
A zoom lens is disclosed in which a lens group is moved to perform zooming, and a fourth lens group is moved to focus an image plane variation accompanying zooming.

In Japanese Patent Laid-Open Publication No. 58-160913, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and
The zoom lens has four lens groups, a third lens group having a positive refractive power and a fourth lens group having a positive refractive power. The first and second groups are moved to perform zooming, and the image plane varies with zooming. Is performed by moving the fourth group. Then, focusing is performed by moving one or more of these lens groups.

Further, Japanese Patent Application Laid-Open Nos. 58-129404 and 61-258217 disclose a positive first lens unit, a negative second lens unit, a positive third lens unit, and a positive third lens unit. A zoom lens that includes a fourth lens group and a negative fifth lens group and performs focusing by moving the fifth lens group or a plurality of lens groups including the fifth lens group is disclosed. Japanese Patent Application Laid-Open No. 60-6914 discloses a zoom lens having the same refractive power arrangement as described above, and the position of the focus lens group on the optical trajectory for a specific finite distance becomes constant regardless of zooming. Such a zoom lens is disclosed.

[0007]

Generally, when a rear focus method is used in a zoom lens, the overall lens system is reduced in size and quick focusing is possible as described above, and further advantages such as close-up photographing are facilitated. Can be

On the other hand, however, aberration fluctuations during focusing increase, and it becomes very difficult to obtain high optical performance while miniaturizing the entire lens system over the entire object distance from an object at infinity to an object at a short distance. The problem arises.

In particular, a zoom lens having a large aperture ratio and a high zoom ratio has a problem that it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance.

The present invention employs a rear focus system to achieve a large aperture ratio and a high zoom ratio while preventing a further increase in the size of the entire lens system. It is an object of the present invention to provide a rear-focus type zoom lens having a simple configuration having excellent optical performance over a double range and over an entire object distance from an object at infinity to an object at a short distance.

[0011]

[Means for Solving the Problems] The feature of the present invention is that a first lens having a positive refractive power is arranged in order from the object side.
A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a positive refractive power, a fifth lens group having a negative refractive power, A rear-focusing zoom lens that performs zooming by moving the second and fourth lens groups and performs focusing by moving the fourth lens group.
The focal lengths of the lens group and the fifth lens group are f ₃ and f ₅ , respectively, and the imaging magnification of the fifth lens group when the object distance is infinity is β.
_When it is set to ₅ , 1.25 <| f ₅ / f ₃ | <2.10 (1) 1.09 <β ₅ <1.45 (2)

[0012]

FIG. 1 is a schematic view of an embodiment showing a paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention.

In the figure, I is a first lens unit having a positive refractive power,
I is a second lens group having a negative refractive power, III is a third lens group having a positive refractive power, IV is a fourth lens group having a positive refractive power, and V is a fifth lens group having a negative refractive power. SP denotes an aperture stop, which is arranged in front of the third lens group III.

At the time of zooming from the wide-angle end to the telephoto end, the second lens group is moved to the image plane side as indicated by an arrow, and the image plane fluctuation caused by zooming is corrected by moving the fourth lens group. .

Also, a rear focus system is employed in which the fourth lens group is moved on the optical axis to perform focusing. A solid line curve 4a and a dotted line curve 4b of the fourth lens shown in the same figure show the image plane fluctuation caused by zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance, respectively. A moving miracle for correction is shown.

The first lens group, the third lens group and the fifth lens group
The lens group is fixed during zooming and focusing.

In the present embodiment, the fourth lens group is moved to correct the image plane fluctuation caused by zooming, and the fourth lens group is moved to perform focusing. In particular, as shown by the curves 4a and 4b in the figure, the zoom lens is moved so as to have a convex trajectory toward the object side when zooming from the wide-angle end to the telephoto end. Thereby, the space between the third lens unit and the fourth lens unit is effectively used, and the overall length of the lens is effectively reduced.

In this embodiment, for example, when focusing from an object at infinity to an object at a short distance at the telephoto end,
This is performed by turning the fourth lens group forward as indicated by the straight line 4c in FIG.

In this embodiment, the effective lens diameter of the first lens group is increased by adopting the rear focus method as described above, compared with the conventional four-group zoom lens in which the first group lens is extended and focused. Is effectively prevented.

By arranging the aperture stop immediately before the third lens group, aberration fluctuation due to the movable lens group can be reduced, and the distance between the lens groups in front of the aperture stop can be reduced to reduce the diameter of the front lens. Achieved easily.

By specifying the optical constants of each lens unit as described above, a zoom lens having a high zoom ratio having good optical performance over the entire zoom range and over the entire object distance is obtained.

Next, the technical meaning of each of the above conditional expressions will be described.

Conditional expression (1) is a condition relating to the ratio of the focal length of the third lens group to the fifth lens group. Mainly, it is to shorten the lens length of the third and subsequent lens groups and maintain good optical performance. belongs to. Beyond the lower limit of condition (1), the fifth
If the refractive power of the lens group becomes too strong, the negative Petzval sum increases and it becomes difficult to correct the field curvature. On the other hand, if the refractive power of the fifth lens group becomes too weak beyond the upper limit, it becomes difficult to sufficiently reduce the overall length of the lens.

Conditional expression (2) relates to the magnification of the fifth lens unit and is intended to obtain a predetermined optical performance while shortening the overall length of the lens. If the magnification of the fifth lens group is reduced below the lower limit, it becomes difficult to reduce the overall length of the lens. On the other hand, when the magnification is increased beyond the upper limit, it is advantageous for shortening the overall length of the lens, but it becomes difficult to confirm a predetermined back focus or the distance between the exit pupil and the image plane becomes short. That is, the telecentricity deteriorates considerably, and it becomes difficult to apply this zoom lens to a video camera.

The zoom lens of the present invention is achieved by satisfying the above conditions, but it is desirable to further satisfy the following conditions in order to obtain better optical performance.

That is, the focal length of the second lens group is f ₂ ,
The focal lengths of the entire system at the telephoto end and the wide-angle end are f _T and f _W , respectively.

[0027]

[Outside 2]

Further, the focal lengths of the third lens unit and the fourth lens unit are f ₃ and f _{4 at the} wide-angle ends, respectively, and the third lens unit and the fourth lens unit are focused on an object at infinity. When the air spacing of the four lens units is D, 0.6 <f ₃ / f ₄ <1.0 (4) By satisfying 0.65 <D / f _W <0.95 (5) is there.

Conditional expression (3) is for effectively obtaining a predetermined zoom ratio while reducing aberration fluctuations caused by zooming with respect to the refracting power of the second lens unit. If the refractive power of the second lens group is too high beyond the lower limit, the size of the entire lens system can be easily reduced. However, the Petzval sum increases in the negative direction, the curvature of field increases, and aberrations associated with zooming increase. Fluctuations increase. If the refracting power of the second lens group becomes too weak beyond the upper limit, the fluctuation of aberration due to zooming is reduced, but the amount of movement of the second lens group for obtaining a predetermined zoom ratio increases,
It is not good because the entire length of the lens becomes longer.

Conditional expression (4) relates to the refractive power of the third lens unit and the fourth lens unit, and is for shortening the lens length of the third and subsequent lens units while maintaining a predetermined optical performance. .

If the refractive power of the third lens unit is too strong beyond the lower limit, it is advantageous for shortening the overall length of the lens, but it is difficult to satisfactorily correct spherical aberration and coma, and it is difficult to secure a back focus. It is not good because it becomes difficult. On the other hand, if the refractive power of the third lens group becomes too weak beyond the upper limit, the reduction of the overall length of the lens becomes insufficient.

Conditional expression (5) relates to the air gap between the third lens unit and the fourth lens unit, and obtains good optical performance while shortening the total length after the stop. If the distance between the third lens unit and the fourth lens unit becomes shorter than the lower limit, it is advantageous for shortening the overall length of the lens, but it becomes difficult to correct spherical aberration from the wide-angle end to the middle of zooming. Conversely, if the value exceeds the upper limit, the reduction of the overall length after the stop becomes insufficient.

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 spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass.

Table 1 shows the relationship with each conditional expression in each numerical example. In addition, R2 in Numerical Examples 1 to 3
0 and R21 are glass materials such as a face plate.

The aspherical surface 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 ₀ is a paraxial radius of curvature, and B, C, D, and E are aspherical coefficients, respectively. And when

[0036]

[Outside 3] It is represented by the following equation.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

According to the present invention, as described above, the lens arrangement for setting the refractive power of the five lens units and the moving conditions of the second and fourth units in zooming and moving the fourth unit during focusing. By adopting the optical system, it is possible to achieve high aberration performance while achieving good aberration correction over the entire zoom range with a zoom ratio of about 8 to 10 while reducing the size of the entire lens system, and with little aberration fluctuation during focusing. Thus, a rear focus zoom lens having an F number of 2.0 and a large aperture ratio can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a paraxial arrangement of a zoom lens and a movement locus of each lens according to the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 1 according to the present invention.

FIG. 3 is a diagram illustrating various aberrations of the zoom lens illustrated at Numerical Example 1 at the wide-angle end.

FIG. 4 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 1.

FIG. 5 is a diagram illustrating various aberrations of the zoom lens illustrated at Numerical Example 1 at a telephoto end.

FIG. 6 is a diagram illustrating various aberrations at the wide-angle end of the zoom lens described in Numerical Example 2;

FIG. 7 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 2;

FIG. 8 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 2;

FIG. 9 is a diagram illustrating various aberrations at the wide-angle end of the zoom lens described in Numerical Example 3;

FIG. 10 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 3;

FIG. 11 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 3;

[Explanation of symbols]

 I First lens group II Second lens group III Third lens group IV Fourth lens group ΔM Meridional image plane ΔS Sagittal image plane d d line g g line

Claims (3)

(57) [Claims] 1. A first lens having a positive refractive power in order from the object side.
A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a positive refractive power, a fifth lens group having a negative refractive power, A rear-focusing zoom lens that performs zooming by moving the second and fourth lens groups and performs focusing by moving the fourth lens group.
The focal lengths of the lens group and the fifth lens group are f ₃ and f ₅ , respectively, and the imaging magnification of the fifth lens group when the object distance is infinity is β.
When a _{5, 1.25 <| f 5 /} f 3 | <2.10 1.09 <β 5 < rear focus type zoom lens that satisfies the 1.45 condition:. 2. When the focal length of the second lens unit is f ₂ , and the focal lengths of the entire system at the wide-angle end and the telephoto end are f _W and f _T , respectively. 2. The rear focus type zoom lens according to claim 1, wherein the following conditional expression is satisfied. 3. The third lens unit and the fourth lens unit have focal lengths of f ₃ and f ₄ , respectively, and the object distance is an infinite distance, and the air gap between the third lens unit and the fourth lens unit at the wide-angle end. when the D _{W a, 0.6 <f 3 / f 4} <1.0 0.65 <D W / f W < and satisfies 0.95 the condition claim 2 rear according Focus type zoom lens.