JP2623835B2 - Rear focus zoom lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a rear focus type zoom lens,
More particularly, the present invention relates to a rear focus type zoom lens suitable for a zoom lens having a large aperture ratio and an F number of about 1.8, which is used in a photographic camera, a video camera, a broadcast camera and the like, and which has a large aperture ratio and a high zoom ratio.

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

Generally, a rear focus type zoom lens has a first lens position compared to a zoom lens which moves the first lens unit to perform focusing.
Since the effective diameter of the lens group is small, it is easy to reduce the size of the entire lens system, and it is also easy to perform close-up photography, especially very close-up photography. Has a small driving force so that quick focusing can be performed.

For example, Japanese Patent Application Laid-Open No. 63-44614 discloses a rear focus type zoom lens having a first lens unit having a positive refractive power in order from the object side, a second lens unit having a negative refractive power for magnification, and variable power. A so-called 4th lens group including a third lens unit having a negative refractive power and a fourth lens unit having a positive refractive power for correcting the accompanying image plane fluctuation.
In the group zoom lens, the third group is moved to perform focusing. However, this zoom lens is the third
The moving space of the group must be secured, and the overall length of the lens tends to increase.

In Japanese Patent Application Laid-Open No. 58-136012, a variable power unit is composed of three or more lens groups, and focusing is performed by moving some of the lens groups.

In JP-A-63-247316, 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 are sequentially arranged from the object side.
Group, and a fourth lens group having a positive refractive power. The fourth lens group is moved, and the second group is moved to perform zooming. The fourth group is moved to perform image plane fluctuation and focusing caused by zooming. ing.

JP-A-58-160913 discloses 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.
Group, and a fourth lens group having a positive refractive power. The fourth lens group is moved. The first and second groups are moved to perform zooming. Have gone. Then, focusing is performed by moving one or more of these lens groups.

(Problems to be Solved by the Invention) In general, when a rear focus method is adopted in a zoom lens, features such as miniaturization of the whole lens system, quick focusing as described above, and easier close-up photographing are obtained. Can be

However, on the other hand, aberration fluctuations during focusing become large, and it becomes extremely difficult to obtain high optical performance while miniaturizing the entire lens system over the entire object distance from an object at infinity to a close object. Will occur.

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.

2. Description of the Related Art Conventionally, not only a zoom lens but also many photographing systems use an aspheric surface in addition to a spherical surface to reduce the number of lenses and satisfactorily correct various aberrations. However, simply using an aspherical surface instead of a spherical surface does not simplify the optical system, and it is difficult to satisfactorily correct various aberrations and obtain high optical performance.

The present invention reduces the number of lenses by adopting the rear focus method and appropriately setting the aspherical lens group and the aspherical shape to achieve a large aperture ratio and a high zoom ratio. Rear focus type with good optical performance over the entire zoom range from the wide-angle end to the telephoto end, and over the entire object distance from the object at infinity to the close object while preventing the size of The purpose of the present invention is to provide a zoom lens.

(Means for Solving the Problems) The rear focus type zoom lens according to the present invention includes, in order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power,
A fourth lens group having a positive refractive power and a fourth lens group having a positive refractive power. The first lens group is moved toward the object side, and the second lens group is moved toward the image plane. From the zoom lens to the telephoto end, and corrects the image plane variation caused by the zooming by moving the fourth lens unit, and focuses by moving the fourth lens unit. It is characterized in that the lens surface is provided with an aspherical surface having a shape such that the positive refractive power decreases from the center of the lens surface toward the peripheral portion.

Further, in the present invention, in order to obtain good optical performance over the entire zoom range, the focal length of the i-th unit is fi, the focal length at the wide-angle end of the entire system is fw, the zoom ratio is z, and the telephoto end of the second unit is 0.9 <| f2 / fw | <1.35 where β2T is the imaging magnification at 2.0 <f4 / fw <3.1 (3) It is characterized by satisfying the following condition.

(Embodiment) 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 group having a positive refractive power, II is a second group having a negative refractive power, III is a third group having a positive refractive power, and IV is a fourth group having a positive refractive power. SP denotes an aperture stop, which is arranged in front of the third lens group III.

When zooming from the wide-angle end to the telephoto end, the first
The second lens unit is moved to the object side toward the object side, and the image plane fluctuation accompanying zooming is corrected by moving the fourth lens unit to the object side.

In addition, a rear focus type in which the fourth unit is moved on the optical axis to perform focusing is adopted. The solid line curve 4a and the dotted line curve 4b of the fourth lens group 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. The movement locus for correction is shown.

 The third unit is fixed during zooming and focusing.

In the present embodiment, the fourth unit is moved to correct the image plane fluctuation caused by zooming, and the fourth unit is moved to perform focusing. In particular, as shown by curves 4a and 4b in the same 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. This makes the third
By effectively utilizing the space between the group and the fourth group, the overall length of the lens is effectively reduced.

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

In the present embodiment, the rear focus method as described above is employed to effectively prevent an increase in the effective lens diameter of the first group, as compared with a conventional four-group zoom lens in which the first group is extended and focused. doing.

By arranging the aperture stop immediately before the third lens unit, aberration variation due to the movable lens unit is reduced, and the distance between the lens units in front of the aperture stop is shortened, so that the diameter of the front lens can be easily reduced. doing.

In this embodiment, at least one lens surface in the third lens unit is provided with an aspherical surface having a shape in which the positive refractive power becomes weaker from the center of the lens surface toward the peripheral portion, so that aberration variation due to zooming, particularly, Spherical aberration, coma, and the like at the middle zoom position from the wide-angle end are favorably corrected. Further, the third group is constituted by one or two lenses by applying an aspheric surface having a predetermined shape as described above, thereby reducing the number of lenses in the entire lens system.

That is, in the numerical examples described later, the third lens unit is composed of a single positive lens having both lens surfaces convex, or a meniscus negative lens having both lens surfaces convex and a convex surface facing the image surface side. Excellent optical performance is obtained while being composed of two lenses.

When the third group is constituted by a single positive lens, it is preferable to select a material having an Abbe number of 58 or more in order to satisfactorily correct longitudinal chromatic aberration.

In the present embodiment, as shown in Numerical Examples described later, the lens is configured by a total of 10 to 11 lenses,
Zoom ratio 6 and F-number 1.8 while miniaturizing the lens system
A rear focus type zoom lens composed of four groups having a good optical performance is achieved.

By specifying the optical constants of each lens unit as in the above-mentioned conditional expressions (1) to (3), good optical performance over the entire zoom range and over the entire object distance while miniaturizing the entire lens system is achieved. And a high zoom ratio zoom lens having

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

Conditional expression (1) relates to the refractive power of the second lens unit, and is intended to effectively obtain a predetermined zoom ratio while reducing the variation in aberration due to zooming, particularly the variation in astigmatism. 2nd over the lower limit
If the refracting power of the group becomes too strong, it is easy to reduce the size of the entire lens system, but the astigmatism varies greatly with zooming. If the refractive power of the second lens unit becomes too weak beyond the upper limit, aberration fluctuation such as astigmatism due to zooming will decrease, but the amount of movement of the second lens unit for obtaining a predetermined zoom ratio will increase. However, since the overall length of the lens becomes longer, it is not good.

Conditional expression (2) relates to the imaging magnification at the telephoto end of the second lens unit with respect to the zoom ratio. If the imaging magnification becomes too small below the lower limit, a second magnification for obtaining a predetermined zoom ratio is obtained.
The amount of movement of the group increases and the overall length of the lens increases. Conversely, if the imaging magnification becomes too large beyond the upper limit, the overall length of the lens will be shortened, but the moving trajectory of the fourth group near the telephoto end of an infinite object will change abruptly, and driving means such as a motor Is not good because the load on

Conditional expression (3) mainly relates to the positive refracting power of the fourth lens unit, and is used for favorably correcting aberration fluctuation during zooming and focusing. If the positive refractive power of the fourth lens unit becomes too strong below the lower limit, spherical aberration will be insufficiently corrected, and aberration fluctuations due to zooming, particularly fluctuations of chromatic aberration of magnification, will increase. It becomes difficult. On the other hand, if the positive refractive power of the fourth lens unit becomes too weak beyond the upper limit, the amount of movement of the fourth lens unit during zooming and focusing becomes too large, which is not good.

In this embodiment, the aspherical surface provided in the third lens unit has a deviation from the reference spherical surface at 70% of the effective diameter as Δ and a radius of curvature of the paraxial reference spherical surface as R, where 1 × 10 ⁻⁴ <| Δ / R | <4 × 10 ⁻³ . Thereby, good optical performance is obtained over the entire zoom range.

Next, numerical examples of the present invention will be described. In numerical examples
Ri is the radius of curvature of the i-th lens surface in order from the object side,
Is the i-th lens thickness and air gap from the object side, Ni and ν
i is the refractive index and Abbe number of the glass of the i-th lens in order from the object side.

The aspheric surface has an X-axis in the optical axis direction and H in the direction perpendicular to the optical axis.
R and paraxial radius of curvature, A, B, C, D,
When E is each aspheric coefficient It is represented by the following equation.

Table 1 shows the relationship with each conditional expression in each numerical example. R20 and R21 in Numerical Examples 1 and 2 and R22 and R23 in Numerical Example 3 are glass materials such as a face plate.

Numerical Example 1 F = 1.0 to 5.56 FNo = 1: 1.8 to 2.4 2ω = 49.1 ゜ to 9.4
゜ R 1 = 8.927 D 1 = 0.139 N 1 = 1.80518 ν 1 = 25.4 R 2 = 3.812 D 2 = 0.494 N 2 = 1.60311 ν 2 = 60.7 R 3 = -19.739 D 3 = 0.021 R 4 = 3.122 D 4 = 0.322 N 3 = 1.61271 ν 3 = 58.8 R 5 = 8.999 D 5 = Variable R 6 = 2.437 D 6 = 0.086 N 4 = 1.74950 ν 4 = 35.3 R 7 = 0.802 D 7 = 0.360 R 8 = -1.098 D 8 = 0.086 N 5 = 1.53172 ν 5 = 48.9 R 9 = 1.250 D 9 = 0.247 N 6 = 1.84666 ν 6 = 23.9 R10 = 113.103 D10 = Variable R11 = Aperture D11 = 0.1 R12 = Aspherical surface D12 = 0.301 N 7 = 1.51633 ν 7 = 64 .
1 R13 = -5.555 D13 = Variable R14 = 80.968 D14 = 0.086 N 8 = 1.84666 ν 8 = 23.9 R15 = 1.913 D15 = 0.064 R16 = 2.649 D16 = 0.268 N 9 = 1.60311 ν 9 = 60.7 R17 = -2.328 D17 = 0.016 R18 = 1.712 D18 = 0.258 N10 = 1.51633 ν10 = 64.1 R19 = ∞ D19 = 0.537 R20 = ∞ D20 = 0.645 N11 = 1.51633 ν11 = 64.
1 R21 = ∞ 12th surface Aspheric surface Ro = 1.721 B = −4.769 × 10 ⁻² C = −2.347 × 10 ⁻² D = 2.179 × 10 ⁻² Numerical Example 2 F = 1.0 to 5.56 FNo = 1: 1.8 to 2.4 2ω = 49.1 ゜ to 9.4
゜ R 1 = 9.287 D 1 = 0.139 N 1 = 1.80518 ν 1 = 25.4 R 2 = 3.766 D 2 = 0.494 N 2 = 1.62299 ν 2 = 58.1 R 3 = -20,115 D 3 = 0.021 R 4 = 3.155 D 4 = 0.322 N 3 = 1.60729 ν 3 = 59.4 R 5 = 8.904 D 5 = variable R 6 = 2.421 D 6 = 0.085 N 4 = 1.74950 ν 4 = 35.3 R 7 = 0.803 D 7 = 0.363 R 8 = -1.099 D 8 = 0.085 N 5 = 1.52310 ν 5 = 50.8 R 9 = 1.250 D 9 = 0.247 N 6 = 1.84666 ν 6 = 23.9 R10 = 36.218 D10 = Variable R11 = Aperture D11 = 0.1 R12 = Aspherical surface D12 = 0.300 N 7 = 1.51633 ν 7 = 64 .
1 R13 = -5.571 D13 = Variable R14 = 84.072 D14 = 0.085 N 8 = 1.84666 ν 8 = 23.9 R15 = 1.910 D15 = 0.064 R16 = 2.633 D16 = 0.268 N 9 = 1.60311 ν 9 = 60.7 R17 = -2.324 D17 = 0.016 R18 = 1.717 D18 = 0.257 N10 = 1.51633 ν10 = 64.1 R19 = ∞ D19 = 0.537 R20 = ∞ D20 = 0.644 N11 = 1.51633 ν11 = 64.
1 R21 = ∞ 12th surface Aspheric surface Ro = 1.721 B = −4.786 × 10 ⁻² C = −2.339 × 10 ⁻² D = 2.184 × 10 ⁻² Numerical Example 3 F = 1.0 to 5.55 FNo = 1: 1.8 to 2.4 2ω = 49.1 ゜ to 9.4
゜ R 1 = 10.107 D 1 = 0.139 N 1 = 1.80518 ν 1 = 25.4 R 2 = 3.716 D 2 = 0.537 N 2 = 1.51633 ν 2 = 64.1 R 3 = -9.672 D 3 = 0.021 R 4 = 2.896 D 4 = 0.354 N 3 = 1.58484 ν 3 = 50.9 R 5 = 8.874 D 5 = Variable R 6 = 7.391 D 6 = 0.086 N 4 = 1.83400 ν 4 = 37.2 R 7 = 0.783 D 7 = 0.241 R 8 = -0.936 D 8 = 0.086 N 5 = 1.51823 ν 5 = 59.0 R 9 = 1.295 D 9 = 0.247 N 6 = 1.84666 ν 6 = 23.9 R10 = -4.058 D10 = Variable R11 = Aperture D11 = 0.16 R12 = 1.588 D12 = 0.483 N 7 = 1.58313 ν 7 = 59.4 R13 = aspheric surface D13 = 0.555 R14 = -1.373 D14 = 0.086 N 8 = 1.80518 ν 8 = 25.4 R15 = -3.883 D15 = variable R16 = 3.986 D16 = 0.086 N 9 = 1.84666 ν 9 = 23.9 R17 = 1.829 D17 = 0.078 R18 = 6.766 D18 = 0.258 N10 = 1.60311 ν10 = 60.7 R19 = -2.113 D19 = 0.016 R20 = 1.784 D20 = 0.268 N11 = 1.51633 ν11 = 64.1 R21 = ∞ D21 = 0.537 R22 = ∞ D22 = 0.645 N12 = 1.51633 ν12 = 64.
1 R23 = ∞ 13th surface Aspheric surface Ro = −1.542 B = 3.512 × 10 ⁻² C = 3.204 × 10 ⁻² D = −7.259 × 10 ⁻² (Effects of the Invention) According to the present invention, as described above, the refracting power of the four lens units and the moving conditions of the first, second, and fourth units in zooming are set, and the fourth unit is focused during focusing. By adopting a lens configuration to move, at least one of the third group
By using an aspheric surface of a predetermined shape for one lens surface,
While reducing the number of lenses to about 10 to 11 as a whole and reducing the size of the entire lens system, achieving excellent aberration correction over the entire zoom range with a zoom ratio of about 6, and aberration fluctuation during focusing It is possible to achieve a rear-focus type zoom lens having an F-number of 1.8 and a large aperture ratio having a high optical performance with little.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment showing a paraxial refractive power arrangement of the present invention, FIGS. 2 and 3 are lens sectional views of Numerical Embodiments 1 to 3 of the present invention, and FIGS. Are numerical examples 1 to 3 of the present invention.
FIG. 4 is a diagram of various aberrations of FIG. In the aberration diagram, (A) is at the wide-angle end,
(B) is an aberration diagram at an intermediate position, and (C) is an aberration diagram at a zoom position at a telephoto end. I, II, III, and IV in FIG. 1, FIG.
The first, second, third, and fourth lens units, d is a d-line, g is a g-line, ΔM is a meridional image plane, ΔS is a sagittal image plane, and SP is an aperture stop.

Claims (1)

(57) [Claims] 1. 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. The first lens unit is moved to the object side, and the second lens unit is moved to the image plane side to perform zooming from the wide-angle end to the telephoto end. An aspheric surface having a shape in which positive refractive power decreases from the center of the lens surface toward the peripheral portion of at least one lens surface in the third group while correcting by moving and performing focusing by moving the fourth group. When the focal length of the i-th unit is fi, the focal length at the wide-angle end of the entire system is fw, the zoom ratio is z, and the imaging magnification at the telephoto end of the second unit is β2T, 0.9 <| f2 / fw | <1.35 A rear focus zoom lens that satisfies the condition 2.0 <f4 / fw <3.1.