JPH0921954A - Rear focusing type zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rear focusing type zoom lens provided with satisfactory performance extending over the whole zooming area and entire object distance as possessing four lens groups and securing a high variable power ratio and of short entire length. SOLUTION: This lens is provided with the four lens groups of a first group L1 of positive refracting power, a second group L2 of negative refracting power, a third group L3 of positive refracting power, and a fourth group L4 of positive refracting power sequentially from an object side. Variable magnification from a wide angle terminal to a telescopic terminal is performed by moving the second group L2 to an image surface side, and the fluctuation of an image surface according to the variable magnification is corrected by moving the fourth group L4 as holding a projecting locus, and also, focusing is performed by moving the fourth group L4. The lens constitution of the third group L3 and the fourth group L4, the focal distance f2 of the second group L2, the F number and the focal distances fNW, fW of the whole system of at the wide angle terminal, and the focal distance fT of the whole system at the telescopic terminal, etc., are set appropriately.

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 photographic camera, a video camera,
And a zoom ratio of 14 to 15 used for broadcast cameras, etc.
The present invention relates to a compact rear focus type zoom lens having a large aperture ratio of about F / 1.4 at the wide angle end and a high zoom ratio and a short overall lens length.

[0002]

2. Description of the Related Art In recent years, as home video cameras and the like have become smaller and lighter, remarkable progress has been made in miniaturization of zoom lenses for image pickup. Emphasis is placed on simplification.

As one means for achieving these objects, there is known a so-called rear focus type zoom lens in which a lens unit other than the first lens unit on the object side is moved for focusing.

Generally, in a rear focus type zoom lens, the effective diameter of the first lens group is smaller than that of a zoom lens in which the first lens group is moved to perform focusing, which facilitates downsizing of the entire lens system and close-up photography. In particular, since extremely close-up photography is facilitated and the relatively small and lightweight lens group is moved, the driving force of the lens group is small, and quick focusing is possible.

As such a rear focus type zoom lens, for example, JP-A-62-215225, JP-A-62-206516, and JP-A-62-2.
4213, JP-A-63-247316, and JP-A-4-43311, the first group having a positive refractive power, the second group having a negative refractive power, and the second group having a positive refractive power are arranged in this order from the object side. It has four lens units, a third lens unit and a fourth lens unit having a positive refractive power, moves the second lens unit to perform zooming, and moves the fourth lens unit to move the image surface and focus upon zooming. It is carried out.

As a method for shortening the total lens length, for example, in a telephoto lens such as a single focus lens, a lens group having a positive refractive power is arranged on the object side and a lens group having a negative refractive power is arranged on the image plane side. The principal point position of the entire system is located on the object side, whereby the so-called telephoto type lens arrangement in which the telephoto ratio is improved is adopted.

On the other hand, as a zoom lens having a short total lens length, the applicant of the present invention has disclosed in Japanese Unexamined Patent Publication No. 4-26811 and Japanese Unexamined Patent Publication No. 4-88309 that a lens group having a positive refractive power on the object side of the third lens group. By disposing a lens having a negative refractive power at the rearmost part, a telephoto type zoom lens has been proposed in which the total lens length is shortened.

Further, JP-A-4-43311, JP-A-4-153615, JP-A-5-19165, JP-A-5-27167 and JP-A-5-609.
In Japanese Patent Laid-Open No. 73-73, a zoom lens having a short overall lens length is proposed, in which the fourth lens group is composed of one positive lens or two positive lenses. In JP-A-5-60974, the fourth
A zoom lens has been proposed in which the lens group includes two lenses, a positive lens and a negative lens.

JP-A-55-62419, JP-A-6-62
No. 2-24213, No. 62-215225, No. 56-114920, and No. 3-20.
No. 0113, JP-A-4-242707, JP-A-4-343313, and JP-A-5-297275 disclose a positive lens and a negative lens in the third group and the fourth group, respectively. Discloses a zoom lens including two lenses.

[0010]

Generally, when a rear focus system is adopted in a zoom lens, the entire lens system is downsized, quick focusing is possible, and further close-up photography is facilitated.

On the other hand, however, the aberration variation at the time of focusing becomes large, and it becomes very difficult to obtain high optical performance over the entire object distance from an object at infinity to a near object.

Particularly, in a zoom lens having a large aperture ratio and a high zoom ratio, it becomes very difficult to obtain high optical performance over the entire zoom range and the entire object distance.

JP-A-4-26811 and JP-A-4-26811
The zoom lens disclosed in Japanese Patent No. 88309 is the fourth zoom lens.
Since the group is composed of three lenses, a negative lens, a positive lens, and a positive lens, further miniaturization is desired.

JP-A-4-43311 and JP-A-4-43311
In the zoom lenses disclosed in JP-A-153615, JP-A-5-19165, JP-A-5-27167 and JP-A-5-60973, the zoom ratio is about 6 to 8 times and more. When trying to obtain a zoom lens having a high zoom ratio, the variation of chromatic aberration due to zooming becomes too large, and it becomes difficult to satisfactorily correct this.

Japanese Patent Laid-Open Nos. 55-62419 and 5
In the zoom lens disclosed in JP-A-6-114920 and JP-A-3-200113, the first lens group or the third lens group is used.
Since the group moves with zooming, the lens barrel structure becomes complicated,
There was a problem that it was difficult to achieve miniaturization.

In the zoom lenses disclosed in JP-A-4-242707, JP-A-4-343313 and JP-A-5-297275, the third lens unit has a lens structure having a large air gap. Further, since the negative lens in the third lens group has a weak refractive power, when it is applied to a zoom lens having a high zoom ratio, a large amount of chromatic aberration occurs in the third lens group, and it is difficult to sufficiently correct this. there were.

In the zoom lens proposed in Japanese Unexamined Patent Publication No. 5-297275, the meniscus-shaped negative lens in the third lens unit has a lens structure with a strong concave surface facing the image plane side, and therefore is not suitable for telephoto conversion. Although effective, it is difficult to correct high-order flare components generated by the positive lens with the negative lens, and it is difficult to increase the aperture and zoom ratio.

The present invention adopts the rear focus method and achieves a large aperture ratio and a high zoom ratio, over the entire zoom range from the wide-angle end to the telephoto end, and from the infinity object to the ultra close object. An object of the present invention is to provide a compact rear focus type zoom lens having a short overall lens length, which has good optical performance over the entire object distance.

[0019]

A rear focus type zoom lens according to 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. And a fourth lens unit of a fourth group having a positive refractive power.
The lens unit is moved to the image plane side to perform zooming from the wide-angle end to the telephoto end, and the image plane variation due to zooming is corrected by moving the fourth lens unit to the object side while having a convex locus. Together with this, the fourth group is moved to perform focusing, and the third group is composed of a cemented lens in which a positive 31st lens and a meniscus negative 32nd lens having a convex surface facing the image surface side are cemented, The fourth lens unit is composed of a negative meniscus 41st lens having a convex surface directed toward the object side and a positive 42nd lens. The focal length of the second lens unit is f
2. The f-number and focal length of the entire system at the wide-angle end are fNW and fW, respectively, and the focal length of the entire system at the telephoto end is fT,

[0020]

(Equation 3) It is characterized in that the condition of 0.45 <| f2 | × fNW / fM <0.7 (1) is satisfied.

[0021]

1 to 4 are sectional views of numerical examples 1 to 4 of a rear focus type zoom lens according to the present invention, which will be described later. FIGS. 5 to 7 are numerical examples 1 and 8 to 10. Numerical Example 2 and FIGS. 11 to 13 are Numerical Example 3 and FIGS.
6 is a diagram of various types of aberration in Numerical Example 4. FIG.

In the aberration diagrams, FIGS. 5, 8, 11, and 14 are wide-angle ends, FIGS. 6, 9, 12, and 15 are intermediate, and FIGS.
3 and 16 indicate the telephoto end.

In the figure, L1 is a first group having a positive refractive power, L2 is a second group having a negative refractive power, L3 is a third group having a positive refractive power, and L4 is a fourth group having a positive refractive power. . SP denotes an aperture stop, which is arranged in front of the third lens unit L3. G is a glass block such as a face plate or a filter. IP is an image plane.

In this embodiment, the second lens unit is moved to the image plane side as indicated by the arrow when the magnification is changed from the wide-angle end to the telephoto end, and the fourth lens unit is convex toward the object side due to the image plane variation due to the magnification change. It is corrected by moving while having the locus of.

Further, a rear focus type in which focusing is performed by moving the fourth lens unit on the optical axis is adopted. A solid line curve 4a and a 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 first and third units are fixed during zooming and focusing.

In the present embodiment, the fourth lens unit is moved to correct the image plane variation due to zooming, and the fourth lens unit is moved to perform focusing. Curve 4 in the figure
As shown in a and 4b, the zoom lens is moved so as to have a convex locus toward the object side during zooming from the wide-angle end to the telephoto end.
Thereby, the space between the third and fourth units is effectively used, and the overall length of the lens is effectively reduced.

In the present embodiment, when focusing from an object at infinity to a near object at the telephoto end, for example, the fourth lens unit is moved forward as indicated by a straight line 4c in FIG.

In the present invention, in order from the object side, the third lens group is a cemented lens in which a positive thirty-first lens element and a meniscus-shaped negative thirty-second lens element having a convex surface directed toward the image side are cemented together. The lens unit is characterized by being composed of a negative meniscus 41st lens element having a convex surface facing the object side and a positive 42nd lens element, and satisfying conditional expression (1).

This is for strengthening the refracting power of the second lens unit in order to achieve a high zoom ratio, and for effectively correcting the residual axial chromatic aberration by securing a moving amount for performing the zoom ratio. Is achieved by arranging a negative meniscus thirty-second lens element having a concave surface on the object side on the image side of the positive thirty-first lens element of the third lens group.

The refractive power (focal length f2) and the F number of the second lens group are adjusted in order to satisfactorily correct aberration fluctuations, especially coma aberration fluctuations associated with zooming when increasing the aperture and zooming. Each element is set so as to satisfy the conditional expression (1).

This conditional expression (1) is defined by the focal length f2 of the second lens unit.
The F number FNW at the wide-angle end has a great relation with the F number. Since the second lens group mainly has a variable magnification function, it moves on the optical axis by zooming. Therefore, fluctuations in aberration that occur must be corrected well. In particular, coma changes greatly. Conditional expression (1) is for satisfactorily correcting this.

If the lower limit of conditional expression (1) is exceeded and F at the wide-angle end is exceeded,
If the number FNW is made brighter or the focal length of the second lens unit is made shorter, high-order coma flare will occur greatly, making correction difficult. On the contrary, if the focal length of the second lens unit is excessively lengthened beyond the upper limit or the F number FNW at the wide-angle end is darkened, the optical performance is improved, but the total lens length is increased.
Miniaturization becomes difficult. Further, it is desirable to suppress the range of conditional expression (1) to a range of 0.45 to 0.65.

Other features of the lens structure of the rear focus type zoom lens of the present invention will be described.

(B) Since the purpose of the present invention is to achieve high zooming as described above, it is desirable to cancel the chromatic aberration caused by zooming in the first and second groups. However, the way in which chromatic aberration of magnification occurs due to zooming differs greatly between that of the first group and that of the second group, and there is a tendency for overcorrection at the wide-angle end. Therefore, the chromatic aberration of the magnification of the fourth lens group is undercorrected to maintain the balance of the chromatic aberration as a whole.

In this case, the axial chromatic aberration can be corrected without greatly disturbing the balance when the zoom ratio is small. Therefore, it is possible to make the third lens group a positive single lens, but when aiming for a high zoom ratio and a large aperture as in the present invention, chromatic aberration on the axis as a whole is insufficiently corrected and high performance is maintained. Will be difficult.

Therefore, in the present invention, the third lens unit is composed of two lenses, a positive lens having an appropriate refractive power and an Abbe number, and a meniscus-shaped negative lens having a strong concave surface facing the object side, and one lens for the third lens unit. By adopting an aspherical surface, chromatic aberration is optimally corrected over the entire zoom range. In addition, spherical aberration with high-order flare components is kept small.

As described above, according to the present invention, the zoom ratio is 14 to 15 and the F number 1.
It has a high zoom ratio of about 4, a large aperture, and maintains high optical performance.

(B) Basically, if the lenses are cemented in the lens structure of each group, the decentering within the group can be effectively suppressed and the product performance can be stabilized. This reduces the degree of freedom by 1 and makes it difficult to achieve sufficient initial performance while satisfying the specifications of large aperture and small zoom.

Therefore, the present invention comprises a cemented lens in which a positive lens and a meniscus negative lens having a concave surface facing the object side are cemented to the third lens group, and the convex surface having the strongest positive refractive power in the third lens group. By providing an aspherical surface having a shape in which the positive refractive power becomes weaker toward the lens periphery, the high-order flare component of spherical aberration is corrected and the decentering within the group is effectively suppressed, and more accurate Achieving a large aperture with a high zoom lens. Further, by constructing the fourth lens unit with a cemented lens, the decentering within the lens unit is effectively suppressed as in the third lens unit, and a more accurate zoom lens is achieved.

Further, by adopting an aspherical surface having a shape in which the positive refracting power becomes weaker toward the lens peripheral portion on the convex surface of the strongest positive refracting power in the fourth group, it is a zoom lens of large aperture and super high magnification. However, it has achieved a highly accurate zoom lens.

(C) The focal lengths of the entire system at the wide-angle end and the telephoto end are fW and fT, respectively, and the combined focal lengths of the first to third groups at the wide-angle end and the telephoto end are fMW and f, respectively.
MT,

[0042]

(Equation 4) It means that the condition of 0 <fM / fAM <1.0 (2) is satisfied.

Conditional expression (2) means the degree of convergence of the ray bundle from the third group. Generally, the most stable aberration correction method is to make the light flux diverged in the variable power portion substantially afocal in the third lens unit. However, if the bundle of rays that emerges from the third group is made to be substantially parallel rays, it becomes difficult to shorten the total lens length. Therefore, in the present invention, by satisfying the conditional expression (2), the bundle of rays emitted from the third lens group is used as a convergent ray to further shorten the total lens length.

The meaning of the conditional expression (2) is as follows: When the lower limit value is exceeded, the light flux becomes a divergent system and the entire lens length extends.
Since the height of the incident light beam to the group also becomes high, the fourth group becomes large, which is not preferable. If the value exceeds the upper limit, the degree of convergence becomes large, which is effective for downsizing, and the aberration variation due to zooming and focusing becomes large, making it difficult to perform good aberration correction in the entire zoom range.

In the present invention, if the upper limit of conditional expression (2) is set to 0.38 <fM / fAM <0.52 (2a), more stable aberration correction and shortening of the total lens length can be achieved. It becomes easy to achieve both.

(D) In the fourth group, when the radius of curvature of the lens surface on the 41st lens side of the positive 42nd lens is R42a and the radius of curvature of the lens surface on the 42nd lens side of the negative 41st lens is R41b. ,

[0047]

(Equation 5) It is better to satisfy the following conditions.

Conditional expression (3) is for suppressing high-order astigmatism and spherical aberration components occurring in the fourth lens unit between the 42nd lens and the 41st lens, and suppressing them. is there. The lower limit value is in a very stable state with an effect equivalent to that of bonding, and when the upper limit value is exceeded, correction of high-order flare components concentrates on the high-order terms of the aspherical surface, which is extremely high considering manufacturing errors. Prone to instability.

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 between the above-mentioned conditional expressions and various numerical values in the numerical examples.

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 light traveling direction, and R as a paraxial radius of curvature.
When K, B, C, D and E are aspherical coefficients,

[0051]

(Equation 6) It is represented by the following equation. "E-0x" means 10 ^-x . The last two faces show glass blocks such as face plates and filters.

Numerical Example 1 f = 1 to 14.82 Fno = 1.45 to 2.59 2ω = 61.9 ° to 4.6 ° R 1 = 10.453 D 1 = 0.31 N 1 = 1.84666 ν 1 = 23.8 R 2 = 5.756 D 2 = 1.22 N 2 = 1.60311 ν 2 = 60.7 R 3 = 13834.736 D 3 = 0.05 R 4 = 5.404 D 4 = 0.71 N 3 = 1.71299 ν 3 = 53.8 R 5 = 18.917 D 5 = Variable R 6 = 7.597 D 6 = 0.15 N 4 = 1.77249 ν 4 = 49.6 R 7 = 1.296 D 7 = 0.67 R 8 = -3.232 D 8 = 0.15 N 5 = 1.69679 ν 5 = 55.5 R 9 = 3.232 D 9 = 0.24 R10 = 3.193 D10 = 0.37 N 6 = 1.84666 ν 6 = 23.8 R11 = 31.817 D11 = Variable R12 = Aperture D12 = 0.29 R13 = 3.860 D13 = 1.17 N 7 = 1.58312 ν 7 = 59.4 R14 = -3.372 D14 = 0.17 N 8 = 1.84666 ν 8 = 23.8 R15 = -5.267 D15 = Variable R16 = 2.871 D16 = 0.20 N 9 = 1.84666 ν 9 = 23.8 R17 = 1.652 D17 = 0.02 R18 = 1.724 D18 = 0.98 N10 = 1.58312 ν10 = 59.4 R19 = -7.817 D19 = 0.73 R20 = ∞ D20 = 0.81 N11 = 1.51633 ν11 = 64.2 R21 = ∞ W M T \ Focal length 1.00 6.98 14.82 Variable spacing \ D 5 0.23 4.54 5.41 D11 5.47 1.17 0.30 D15 2.14 1.09 2.27 Aspherical R13 surface K = 2.554 e-01 B = -6.490 e-03 C = 2.077 e-04 D = -2.006 e-04 E = 3.713 e-05 Spherical surface R19 surface K = 2.078 e + 01 B = 5.390 e-03 C = 2.933 e-03 D = 1.115 e-04 E = -2.659 e-03 <Numerical example 2> f = 1 to 14.01 Fno = 1.45 to 2.45 2ω = 61.9 ° ~ 4.9 ° R 1 = 10.417 D 1 = 0.31 N 1 = 1.84666 ν 1 = 23.8 R 2 = 5.799 D 2 = 1.22 N 2 = 1.60311 ν 2 = 60.7 R 3 = -1442.930 D 3 = 0.05 R 4 = 5.340 D 4 = 0.71 N 3 = 1.71299 ν 3 = 53.8 R 5 = 13.097 D 5 = Variable R 6 = 8.357 D 6 = 0.15 N 4 = 1.77249 ν 4 = 49.6 R 7 = 1.294 D 7 = 0.67 R 8 =- 3.531 D 8 = 0.15 N 5 = 1.69679 ν 5 = 55.5 R 9 = 3.531 D 9 = 0.24 R10 = 3.086 D10 = 0.37 N 6 = 1.84666 ν 6 = 23.8 R11 = 15.330 D11 = Variable R12 = Aperture D12 = 0.29 R13 = 3.932 D13 = 1.17 N 7 = 1.58312 ν 7 = 59.4 R14 = -3.295 D14 = 0.17 N 8 = 1.84666 ν 8 = 23.8 R15 = -5.014 D15 = 0.00 R16 = ∞ D16 = Variable R17 = 2.917 D17 = 0.20 N 9 = 1.84666 ν 9 = 23.8 R18 = 1.693 D18 = 0.02 R19 = 1.773 D19 = 0.98 N10 = 1.58312 ν10 = 59.4 R20 = -7.570 D20 = 0.73 R21 = ∞ D21 = 0.81 N11 = 1.51633 ν11 = 64.2 R22 = ∞ W M T \ Focal length 1.00 5.60 14.01 Variable spacing \ D 5 0.23 4.23 5.36 D11 5.47 1.47 0.34 D16 2.19 1.12 2.15 Non Surface R13 surface K = 1.509 e-01 B = -6.945 e-03 C = 3.566 e-04 D = -1.911 e-04 E = 3.705 e-05 <Numerical example 3> f = 1 to 14.00 Fno = 1.45 to 2.51 2 ω = 61.9 ° ~ 4.9 ° R 1 = 11.322 D 1 = 0.31 N 1 = 1.84666 ν 1 = 23.8 R 2 = 5.988 D 2 = 1.22 N 2 = 1.60311 ν 2 = 60.7 R 3 = -111.865 D 3 = 0.05 R 4 = 5.241 D 4 = 0.71 N 3 = 1.71299 ν 3 = 53.8 R 5 = 12.638 D 5 = Variable R 6 = 9.339 D 6 = 0.15 N 4 = 1.77249 ν 4 = 49.6 R 7 = 1.316 D 7 = 0.63 R 8 = -3.356 D 8 = 0.15 N 5 = 1.69679 ν 5 = 55.5 R 9 = 3.358 D 9 = 0.25 R10 = 3.238 D10 = 0.37 N 6 = 1.84666 ν 6 = 23.8 R11 = 28.550 D11 = Variable R12 = Aperture D12 = 0.29 R13 = 4.295 D13 = 1.30 N 7 = 1.58312 ν 7 = 59.4 R14 = -2.579 D14 = 0.17 N 8 = 1.83400 ν 8 = 37.2 R15 = -4.545 D15 = Variable R16 = 3.193 D16 = 0.20 N 9 = 1.84666 ν 9 = 23.8 R17 = 1.720 D17 = 1.10 N10 = 1.58312 ν10 = 59.4 R18 = -6.609 D18 = 0.73 R19 = ∞ D19 = 0.81 N11 = 1.51633 ν11 = 64.2 R20 = ∞ W M T \ Focal length 1.00 5.67 14.00 Variable spacing \ D 5 0.23 4.24 5.36 D11 5.48 1.47 0.34 D15 2.01 0.94 1.95 Aspherical R13 surface K = 1.523 e-01 B = -5.363 e-03 C = 5.667 e-04 D = -2.733 e-04 E = 5.658 e-05 Aspherical R18 surface K = 1.104 e + 01 B = 6.484 e-03 C = 2.407 e-03 D = 4.765 e-04 E =- 1.402 e-03 <Numerical Example 4> f = 1 to 14.00 Fno = 1.45 to 2.48 2ω = 61.9 ° to 4.9 ° R 1 = 10.862 D 1 = 0.31 N 1 = 1.84666 ν 1 = 23.8 R 2 = 5.926 D 2 = 1.22 N 2 = 1.60311 ν 2 = 60.7 R 3 = -255.457 D 3 = 0.05 R 4 = 5.234 D 4 = 0.71 N 3 = 1.71299 ν 3 = 53.8 R 5 = 12.577 D 5 = Variable R 6 = 10.508 D 6 = 0.15 N 4 = 1.77249 ν 4 = 49.6 R 7 = 1.315 D 7 = 0.63 R 8 = -3.666 D 8 = 0.15 N 5 = 1.69679 ν 5 = 55.5 R 9 = 3.666 D 9 = 0.25 R10 = 3.176 D10 = 0.37 N 6 = 1.84666 ν 6 = 23.8 R11 = 15.677 D11 = Variable R12 = Aperture D12 = 0.29 R13 = 4.350 D13 = 1.30 N 7 = 1.58312 ν 7 = 59.4 R14 = -2.569 D14 = 0.17 N 8 = 1.83400 ν 8 = 37.2 R15 = -4.208 D15 = Variable R16 = 3.095 D16 = 0.20 N 9 = 1.84666 ν 9 = 23.8 R17 = 1.610 D17 = 1.10 N10 = 1.58312 ν10 = 59.4 R18 = -7.079 D18 = 0.73 R19 = ∞ D19 = 0.81 N11 = 1.51633 ν11 = 64.2 R20 = ∞ W M T \ Focal length 1.00 5.63 14.00 Variable spacing \ D 5 0.23 4.24 5.37 D11 5.48 1.47 0.34 D15 2.08 0.98 2.02 Aspherical surface R13 surface K = -2.721 e-01 B = -5.778 e-03 C = 6.569 e-04 D = -3.000 e-04 E = 6.474 e-05

[0053]

[Table 1]

[0054]

According to the present invention, by setting the lens structure as described above, when a rear focus system is adopted and a large aperture ratio and a high zoom ratio are achieved, from the wide-angle end to the telephoto end. It is possible to achieve a rear focus type zoom lens having good optical performance over the entire zoom range and over the entire object distance from an infinitely distant object to a super close object.

[Brief description of drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 4 is a lens cross-sectional view of Numerical Example 4 of the present invention.

FIG. 5 is an aberration diagram at a wide-angle end according to Numerical Embodiment 1 of the present invention.

FIG. 6 is an intermediate aberration diagram of Numerical example 1 of the present invention.

FIG. 7 is an aberration diagram at a telephoto end according to Numerical Example 1 of the present invention.

FIG. 8 is an aberration diagram at the wide-angle end according to Numerical Example 2 of the present invention.

FIG. 9 is an intermediate aberration diagram of Numerical example 2 of the present invention.

FIG. 10 is an aberration diagram at a telephoto end according to Numerical Example 2 of the present invention.

FIG. 11 is an aberration diagram at a wide-angle end according to Numerical Example 3 of the present invention.

FIG. 12 is an intermediate aberration diagram of Numerical example 3 of the present invention.

FIG. 13 is an aberration diagram at a telephoto end according to Numerical Example 3 of the present invention.

FIG. 14 is an aberration diagram at the wide-angle end according to Numerical Example 4 of the present invention.

FIG. 15 is an intermediate aberration diagram of Numerical example 4 of the present invention.

FIG. 16 is an aberration diagram at a telephoto end according to Numerical Example 4 of the present invention.

[Explanation of symbols]

 L1 1st group L2 2nd group L3 3rd group L4 4th group SP Aperture ΔM Meridional image surface ΔS Sagittal image surface d d line g g line

Claims (4)

[Claims] 1. Four lens groups, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power. The second lens unit is moved to the image plane side to perform zooming from the wide-angle end to the telephoto end, and the image plane variation due to zooming causes the fourth lens unit to have a convex locus on the object side. The third group is made up of a positive 31st lens and a meniscus negative 32nd lens having a convex surface directed toward the image plane side. The fourth lens unit is composed of a negative meniscus 41st lens having a convex surface facing the object side and a positive 42nd lens. The second lens unit has a focal length of f2 and a total length at the wide-angle end. The f-number and focal length of the system are fNW,
fW, the focal length of the entire system at the telephoto end is fT, and A rear focus type zoom lens which satisfies the condition 0.45 <| f2 | × fNW / fM <0.7. 2. The third group and / or the fourth group has at least one aspherical surface.
Rear focus type zoom lens. 3. The rear focus type zoom lens according to claim 1, wherein the forty-first lens and the forty-second lens are cemented together. 4. The focal lengths of the entire system at the wide-angle end and the telephoto end are fW and fT, respectively, and the combined focal lengths of the first group to the third group at the wide-angle end and the telephoto end are fMW and fMT, respectively.
And The rear focus type zoom lens according to claim 1, wherein the condition 0 <fM / fAM <1.0 is satisfied.