JPH08146297A - Zoom lens of large aperture diameter - Google Patents

Abstract

PURPOSE: To realize a zoom lens of a large diameter which is low in cost, is compact, has excellent performance and as bright as <=6 in F/NO. at a telephoto end. CONSTITUTION: The first and second groups I, II arranged successively from an object side toward an image side have a positive focal length and the third group III has a negative focal length. The second and third groups II, III move in the entire part to the object side while narrowing the spacing therebetween according to zooming from the wide angle end where the first and second groups I, II approximate to a proximate telephoto end. Both of the first group I and the third group III have at least one element of positive and negative lenses. The second group II is arranged, successively from the object side, with a meniscus negative lens, positive lens and biconvex positive lens of which the strong concave faces are directed to the object side. The focal length of the second group II, defined as f2 , and the combined focal length of the entire system at the wide angle end, defined as fW, satisfy the condition 0.6<f2 /fW<0.85 and >=2 faces of the second group and >=1 faces of the third group are aspherical faces.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-diameter zoom lens. The zoom lens of the present invention can be used as a zoom lens for a lens shutter camera.

[0002]

2. Description of the Related Art Recently, a lens shutter camera having a zoom lens is becoming mainstream, and the zoom ratio of the mounted zoom lens is increasing to about 3 times.

However, most zoom lenses having a zoom ratio of about 3 times have a dark F / No of 8 to 10 at the telephoto position, and the shutter speed at the time of shooting at the telephoto position becomes slow. It is easy to cause.

The focal length on the telephoto side: 100 mm
The front and back are suitable for "portrait", but if the F / No is large, the "blur effect" before and after is small, and it is not possible to shoot with the subject emphasized.

In Japanese Patent Laid-Open No. 5-150161, F /
No. = 3.5 to 5.5 / f = 36 to 102 zoom lenses are disclosed, but conditions for controlling coma aberration, which is particularly likely to occur when the aperture is increased, are not disclosed. A large amount of coma remains, which is not enough for practical use.

[0006] Further, in many conventionally known zoom lenses having a zoom ratio of about 3 times, the number of lens components exceeds 10, and it is difficult to reduce the size and cost.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has a variable power ratio of approximately 3 and a small number of about 7 lenses, which is compact.
The objective is to provide a large-diameter zoom lens with a bright F / No of 6 or less at the telephoto end.

[0008]

As shown in FIG. 1, the "large-diameter zoom lens" of the present invention is arranged such that, as shown in FIG. ~ A third group is arranged.

The first group I and the second group II have a "positive focal length", and the third group have a "negative focal length". Therefore, the total power distribution is “positive / positive / negative”.

As shown in the upper diagram of FIG. 1, "the first lens group I and the second lens group II are close to each other" at the wide-angle end, and the second lens group II and the third lens group III are spaced from each other. There is.

Further, as shown in the lower diagram of FIG. 1, at the telephoto end, "the second group II and the third group III are close to each other, and the first group I
And the second group II is open.

During zooming from the wide-angle end to the telephoto end, as shown in FIG. 1, all of the first to third groups move toward the object side, but the first group I and the third group III In comparison,
Since the moving speed of the second group II is slow, follow the zooming
The distance between the second group II and the third group III "is gradually narrowing"
Will be.

The first lens group I has at least one positive lens and at least one negative lens. The second group II includes, sequentially from the object side to the image side, a meniscus negative lens having a strong concave surface facing the object side, a positive lens, and a biconvex positive lens.
The third group III has at least one positive lens and at least one negative lens.

When the focal length of the second lens group II is f ₂ and the combined focal length of the entire system at the wide-angle end is f _W , these conditions are: (1) 0.6 <f ₂ / f _W <0.85 To be satisfied.

Two or more surfaces of the second lens group II and one or more surfaces of the third lens group III are aspherical surfaces.

The second group II has two or more aspherical surfaces as described above, but both surfaces of the most image-side "biconvex positive lens" in the second group can be aspherical. Item 2).

In the first group I, which has the smallest number of sheets,
It is composed of one positive lens and one negative lens, and at this time, the first group can be composed by "bonding the positive lens and the negative lens" (claim 3).

In the third lens group III, one or more aspherical surfaces are adopted, but "at least one surface of the positive lens disposed on the object side" of the third lens group III can be an aspherical surface ( Claim 4).

In the zoom lens according to any one of claims 1 to 4, it is possible to "move the second lens group II to the object side for focusing" (claim 5). In this case, in front of the second lens group II, that is, between the first lens group I and the second lens group II, a diaphragm S is provided, which is integrated with the second lens group II during zooming. Can be moved (claim 6).

When the diaphragm S is provided in this way, "at the time of focusing", that is, when the second group is moved to the object side to perform the focusing operation, "the diaphragm S is fixed and the second group II is set to the object". The distance between the diaphragm S and the second group II can be narrowed by moving it to the side ”(claim 7).

[0021]

As described above, the large-diameter zoom lens of the present invention has a "positive / positive / negative" three-group structure, and the second group mainly has an "imaging effect".

In a zoom lens having a high zoom ratio, it is important to correct the basic aberration in units of a group that moves with zooming. From this viewpoint, the present invention also
Each of the first group and the third group has at least one positive lens and at least one negative lens.

Since the first lens unit has a positive focal length,
Although the light flux is narrowed down by the first lens group, in the telephoto mode, since the distance between the first lens group and the second lens group is widened, the light beam width becomes narrow and is incident on the second lens group.

By making the first surface of the second lens group a "strong concave surface" of the negative meniscus lens, it is possible to correct distortion aberration on the wide angle side and the coma aberration of the focusing property of the lower ray. Also, by configuring the "positive lens component" of the second group with two positive lenses (a positive lens on the object side and a biconvex positive lens on the image side), the sharing of the spherical aberration coefficient is reduced, and these two lenses In cooperation with a negative lens (meniscus negative lens) placed on the object side of the positive lens of, the sum of the spherical aberration coefficients of the second group is set small.

By doing so, “variation of spherical aberration during zooming, variation of spherical aberration during focusing”
Can be kept small.

The condition (1) defines an appropriate range of the focal length of the second lens group in the lens structure as described above. When the parameter of condition (1): f ₂ / f _W becomes small, the positive power of the second lens group becomes relatively strong, which is advantageous for downsizing the zoom lens, but when the lower limit of (1) is exceeded, "The spherical aberration on the wide angle side is insufficiently corrected". In order to eliminate such insufficient correction on the wide angle side, it is necessary to increase the number of lenses, which is a problem in any case.

When the value exceeds the upper limit of the condition (1), the focusing power of the axial light becomes insufficient and the back focus becomes large, so that "it is difficult to achieve miniaturization in the entire zoom range".

By satisfying the condition (1), the zoom lens can be "realized with a small number of constituent elements and with sufficient practical performance".

Further, the parameter of condition (1): f ₂ /
When f _W is in the range of “0.7 <f ₂ / f _W <0.8”, it is possible to realize extremely good performance, as will be apparent from the examples described later.

In order to suppress the sum of the intra-group aberrations to a small value, the second lens group has a meniscus negative lens having a strong concave surface directed toward the object side in order from the object side to the image side, a positive lens, as described above.
Although a biconvex positive lens is arranged, it is necessary to use an aspherical surface to achieve a large aperture and high performance.

In the second lens group, two or more aspherical surfaces are employed, but at least one aspherical surface has a shape in which the positive refractive power becomes weaker toward the periphery, and the second lens group is likely to be underexposed. Reduces spherical aberration.

Image-side surface of the second group (image-side surface of biconvex lens)
Then, since the refraction angle of the ambient light on the axis is large, it is particularly effective to use an aspherical surface for this surface (claim 2). Further, since the object-side surface of the second group (the strong concave surface of the negative meniscus lens) largely refracts the lower ray, it is also effective to employ an aspherical surface for this surface.

On the wide-angle side, the second group and the third group are spaced apart from each other, and the off-axis rays pass through the third group while being separated from each other. Therefore, if an aspherical surface is adopted for the third group, "individual angle of view" It is possible to control the curvature of field in accordance with the above, and to correct the distortion that tends to be positively increased on the wide angle side. "

In this case, the "at least one surface of the positive lens on the object side of the third lens group" has a small effective diameter, and if this surface is an aspherical surface (claim 4), the lens diameter is small, so that the workability is improved. Is good.

As described above, since the sum of the aberration coefficients in the second lens group can be reduced by adopting the lens structure of the second lens group and the aspherical surface, "focusing" can be performed by the second lens group. ),
Aberration variation due to focusing can be minimized.

Further, it is preferable that the "stop" is arranged in the vicinity of the second group which is an image forming group, but if it is provided in the second group, the mechanism for providing the lenses on both sides of the stop becomes complicated. However, when it is placed on the image side of the second lens unit, the diameter of the front lens becomes large in order to maintain the peripheral light amount.

Furthermore, if the aperture (shutter) is fixed during focusing, the second group, which is the focusing group, can be moved by the focus motor carried by the shutter unit, so the focus ring can be eliminated. .

[0038]

[Examples] Five specific examples will be given below.

As shown in FIG. 1, i-th counting from the object side
The radius of curvature of the th surface (including the diaphragm surface) is R _i (the radius of curvature on the optical axis for an aspherical surface), the distance on the optical axis between the i-th surface and the (i + 1) th surface is D _i , from the object side. The refractive index and Abbe number for the d-line of the material of the j-th lens counted are represented by N _j and ν _j , respectively. Further, f is the focal length of the entire system, Bf is the back focus, F / No is the brightness, and ω is the half angle of view.

The aspherical surface is aligned with the optical axis to take the Z axis,
When the Y axis is set orthogonally to the optical axis, the paraxial radius of curvature on the optical axis: R, the conic constant: K, the high-order aspherical coefficient: A,
Using B, C and D, Z = (1 / R) ² Y ² / [1 + √ {1- (1 + K) (Y /
R) ² }] + A · Y ⁴ + B · Y ⁶ + C · Y ⁸ + D · Y ¹⁰ is a curved surface obtained by rotating around the optical axis, and a paraxial radius of curvature R on the optical axis. And the conic constant: K,
The shape is specified by giving aspherical surface coefficients: A, B, C, and D.
In the display of the aspherical surface, "E and the number following it" represent "power". For example, if "E-9",
This represents "10 to ⁹ ", and this number is multiplied by the value immediately before it.

Example 1 f = 39.134 to 111.517, F / No = 4.2 to 5.9 iR _i D _i j N _j ν _j 1 20.111 4.5 1 1.846666 23. 78 2 14.535 2.866 2 1.51454 54.54 3 35.283 Variable 4 ∞ (aperture) 3.964 5 -9.3325 0.7 3 1.58590 48.86 6 -90.02 0. 1 7 179.208 2.817 4 1.48749 70.44 8 -15.498 0.1 9 56.528 3.181 5 1.48749 40.44 10 -13.541 Variable 11 -14.629 2. 708 6 1.84666 23.78 12 -12.977 0.3 13 -16.991 1.6 7 1.69680 55.46 14 -789.366.

Aspherical fifth surface: K = −0.16225, A = 1.66929E-5, B = 9.98605E-7, C = −1.35942E-8, D = 1.42480E-10 Tenth Surface: K = −0.37403, A = 6.74599E-5, B = 5.63113E-7, C = −7.887490E-9, D = 4.41710E-11 11th surface: K = −1. 65911, A = 6.29761E-6, B = 3.70790E-7, C = -4.448256E-10, D = -2.87261E-12 12th surface: K = -1.42928, A = -2 14110E-5, B = 1.63875E-7, C = 1.49530E-9, D = −7.292855E-12.

Variable amount Wide-angle end Medium focal length Telephoto end f 39.134 66.062 111.517 D ₃ 1.646 13.547 22.826 D ₁₀ 19.11.9.805 3.172 Bf 7.0 29. 0 63.642 F / No 4.2 5.0 5.9 ω 28.22 17.67 10.81.

Value of parameter of conditional expression f ₂ / f _W = 0.76 FIGS. 1A, 1B and 1C show lens arrangements at the wide-angle end, the intermediate focal length and the telephoto end according to the first embodiment. Further, FIG. 6 is an aberration diagram at the wide-angle end regarding Example 1.
FIG. 7 shows an aberration diagram at the intermediate focal length, and FIG. 8 shows an aberration diagram at the telephoto end. SA in the spherical aberration diagram
(D) and SA (g) are spherical aberrations with respect to d-line and g-line, SC is a sine condition, and DS in the diagram of astigmatism.
(D) and DS (g) represent sagittal image planes for the d and g lines, and DM (d) and DM (g) represent meridional image planes for the d and g lines.

Example 2 f = 39.141 to 111.549, F / No = 4.2 to 5.9 i R _i D _i j N _j ν _j 1 20.006 5.55 1 1.84666 23. 78 2 14.126 3.12 2 1.51680 64.20 3 33.962 Variable 4 ∞ (diaphragm) 3.71 5 -10.388 1.51 3 1.83500 42.98 6 -28.373 0. 1 7 240.042 3.30 4 1.48749 70.44 8 -14.170 0.1 9 148.087 2.81 5 1.58313 59.46 10 -18.143 Variable 11 -12.162 2. 68 6 1.80518 25.46 12 -11.757 0.74 13 -16.030 1.6 7 1.62041 60.34 14 -742.849.

Aspherical ninth surface: K = 0.0, A = 2.52343E-5, B = 4.869888E-7, C = −5.449378E-9, D = 1.57781E-10 Tenth surface : K = -0.35713, A = 5.9652E-5, B = 4.97286E-7, C = -3.72441E-9, D = 1.43817E-10 Eleventh surface: K = -1.67133. , A = 2.57263E-5, B = 6.20233E-7, C = −2.65833E-9, D = 3.83586E-12 12th surface: K = −1.49651, A = −9.04572E -6, B = 4.03925E-7, C = -3.72942E-10, D = -2.20656E-12.

Variable amount Wide-angle end Medium focal length Telephoto end f 39.141 66.079 111.549 D ₃ 2.0 12.932 33.962 D ₁₀ 17.745 9.9.1 3.0 Bf 7.049 28. 313 63.056 F / No 4.2 5.0 5.9 ω 28.63 17.68 10.8.

Parameter value of conditional expression f ₂ / f _W = 0.71 FIG. 2 shows the lens arrangement at the wide-angle end according to the second embodiment. FIG. 9 is an aberration diagram at the wide-angle end regarding Embodiment 2.
FIG. 10 shows an aberration diagram at the intermediate focal length, and FIG. 11 shows an aberration diagram at the telephoto end.

Example 3 f = 39.149 to 111.59, F / No = 4.0 to 6.0 i R _i D _i j N _j ν _j 1 20.664 4.50 1 1.84666 23. 80 2 13.321 0.10 3 13.087 3.029 2 1.65825 42.22 4 28.604 Variable 5 ∞ (Aperture) 3.908 6 -8.990 0.7 3 1.70780 44.60 7 -27.452 0.10 8 -128.474 2.522 4 1.48749 70.44 9 -14.898 0.10 10 65.099 3.093 5 1.48749 70.44 11 -12.725 Variable 12-29.356 2.605 6 1.846666 23.80 13 -19.552 0.50 14 -19.483 1.60 7 1.66460 56.86 15 51.849.

Aspherical sixth surface: K = −0.12323, A = 4.64043E-5, B = −4.82754E-7, C = 4.03037E-8, D = −4.54852E-10th 11th surface: K = −0.55683, A = 8.46036E-5, B = −1.16346E-7, C = 3.80708E-9, D = −4.64408E-11 13th surface: K = − 0.31593, A = 2.03291E-5, B = -4.87759E-7, C = 5.17506E-9, D = -1.59706E-11 Fifteenth surface: K = -85.776145, A = 2.29936E-5, B = -1.08112E-7, C = -2.18335E-10, D = 1.19264E-12.

Variable amount Wide-angle end Medium focal length Telephoto end f 39.149 66.094 111.59 D ₄ 1.753 11.852 18.3 D ₁₁ 17.077 8.192 1.976 Bf 8.505 30. 005 65.79 F / No 4.0 4.9 6.0 ω 28.34 17.64 10.83.

Parameter value of conditional expression f ₂ / f _W = 0.75 FIG. 3 shows the lens arrangement at the wide-angle end according to the third embodiment. FIG. 12 is an aberration diagram at the wide-angle end regarding Embodiment 3.
FIG. 13 shows an aberration diagram at the intermediate focal length, and FIG. 14 shows an aberration diagram at the telephoto end.

Example 4 f = 39.154 to 111.618, F / No = 3.7 to 6.0 i R _i D _i j N _j ν _j 1 19.349 4.50 1 1.846666 23. 80 2 14.525 2.804 2 1.51222 67.52 3 31.499 Variable 4 ∞ (aperture) 3.977 5 -9.285 0.7 3 1.69206 46.03 6 -33.672 0. 17-1301.072 2.667 4 1.48749 70.44 8-15.893 0.1 9 62.963 3.216 5 1.48749 70.44 10-12.863 Variable 11 -30.193 2 .464 6 1.84666 23.80 12 -20.561 0.5 13 -20.546 1.6 7 1.64761 57.66 14 47.970.

Aspherical fifth surface: K = −0.13525, A = 4.11071E-5, B = −3.92581E-7, C = 3.51089E-8, D = −3.62064E-10th 10th surface: K = −0.57203, A = 8.34273E-5, B = −7.18623E-8, C = 3.71880E-9, D = −4.666675E-11 12th surface: K = − 0.09495, A = 2.04990E-5, B = -3.79364E-7, C = 3.85309E-9, D = -1.04658E-11 14th surface: K = -63.664590, A = 2.36156E-5, B = -1.17437E-7, C = -2.56071E-11, D = 5.82982E-13.

Variable amount Wide-angle end Intermediate focal length Telephoto end f 39.154 66.105 111.618 D ₃ 1.774 12.395 19422 D ₁₀ 17.328 8.286 1.926 Bf 8.508 30. 027 65.554 F / No 3.7 4.67 6.0 ω 28.18 17.63 10.82.

Value of Parameter of Conditional Expression f ₂ / f _W = 0.76 FIG. 4 shows the lens arrangement at the wide-angle end according to the fourth embodiment. FIG. 15 is an aberration diagram at the wide-angle end regarding Example 4.
FIG. 16 shows an aberration diagram at the intermediate focal length, and FIG. 17 shows an aberration diagram at the telephoto end.

Example 5 f = 39.143 to 111.469, F / No = 4.2 to 5.9 iR _i D _i j N _j ν _j 1 20.860 4.455 1 1.846666 23. 78 2 13.368 0.287 3 13.446 3.099 2 1.59686 40.76 4 32.947 Variable 5 ∞ (Aperture) 3.801 6-9.670 0.7 3 1.60096 47.95 7-48.8882 0.18 337.796 3.112 41.48749 70.44 9-12.383 0.110 1296.886 2.582 5 1.48749 70.44 11-15.621 Variable 12-34.297 3.437 6 1.81838 29.59 13 -21.541 1.43 14 -16.356 1.6 7 1.69680 55.46 15 269.140.

Aspherical sixth surface: K = −0.23538, A = 3.50393E-5, B = 5.81822E-7, C = −1.89349E-8, D = −6.74816E-11th 7th surface: K = -81.7790881, A = 3.62887E-5, B = 9.777226E-8, C = -3.62239E-9, D = 5.51733E-11 Tenth surface: K = 32915. 75194, A = 3.356648E-5, B = -9.63915E-7, C = -1.896736E-9, D = 3.31658E-11 11th surface: K = 0.19656, A = 4.60605E -5, B = 1.10709E-7, C = -1.89108E-8, D = 1.21098E-10 13th surface: K = -0.12169, A = -2.04834E-5, B =- .48543E-8, C = 2.63102E-10, D = -1.59087E-12.

Variable amount Wide-angle end Intermediate focal length Telephoto end f 39.143 66.064 111.469 D ₄ 1.673 13.487 27.713 D ₁₁ 17.701 8.032 1.4 Bf 7.58 29. 728 60.1 F / No 4.2 5.0 5.9 ω 28.19 18.0 11.0.

Parameter value of conditional expression f ₂ / f _W = 0.79 FIG. 5 shows the lens arrangement at the wide-angle end according to the fifth embodiment. FIG. 18 is an aberration diagram at the wide-angle end regarding Example 5.
FIG. 19 shows an aberration diagram at the intermediate focal length, and FIG. 20 shows an aberration diagram at the telephoto end.

As is clear from the aberration diagrams of FIGS. 6 to 20, the performance is good in each of the examples.

[0062]

As described above, according to the present invention, a novel large-diameter zoom lens can be provided.
~ 7).

Since the zoom lens of the present invention is constructed as described above, it can be realized compactly and at low cost because it has a construction of at least 7 and a small number of constructions, and has good performance, and even at the telephoto end. F / No: Bright as 6 or less.

If both surfaces of the positive lens closest to the image side of the second lens group are aspherical surfaces as in the second aspect of the present invention, two lenses other than this positive lens can be manufactured by a polishing method, so that performance is improved. It is possible to achieve both maintenance and cost reduction. In the second group having a negative / positive / positive configuration, it is most effective to make both surfaces of the biconvex lens closest to the image side aspherical.

When the first group is a cemented lens of positive and negative lenses as in the third aspect of the invention, the influence of decentering in the first group is minimized, so that the assemblability is improved and the internal reflection is improved. Can effectively prevent performance deterioration. Further, the tolerance of the joint surface is widened and the workability is improved.

If an aspherical surface is adopted as the surface of the positive lens element on the object side of the third lens unit as in the fourth aspect of the present invention, the diameter of the lens becomes smaller, so the cost for forming an aspherical surface can be reduced. You can

As in the fifth aspect of the present invention, by performing focusing by moving the second lens unit, it is possible to minimize fluctuations in aberrations associated with focusing. Since the second group has the smallest effective diameter,
The focusing mechanism can be made compact.

As in the invention described in claim 6, the aperture is set to the second position.
When the diaphragm is placed in front of the second group, there is no complication of the mechanism due to disposing the lens before and after the diaphragm which occurs when the diaphragm is arranged in the second group. There is no problem of increasing the front lens diameter.

By fixing the diaphragm during focusing as in the seventh aspect of the invention, the focus ring can be omitted as described above, and the camera with the zoom lens can be further downsized.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the lens configuration of a large-diameter zoom lens according to the present invention and the movement of each group during zooming.

FIG. 2 is a diagram illustrating a lens arrangement at a wide-angle end of a zoom lens according to a second exemplary embodiment.

FIG. 3 is a diagram illustrating a lens arrangement at a wide-angle end of a zoom lens according to a third exemplary embodiment.

FIG. 4 is a diagram illustrating a lens arrangement at a wide-angle end of a zoom lens according to a fourth exemplary embodiment.

FIG. 5 is a diagram showing a lens arrangement at a wide-angle end of a zoom lens according to a fifth exemplary embodiment.

FIG. 6 is an aberration diagram for the wide-angle end in Embodiment 1.

FIG. 7 is an aberration diagram for Example 1 regarding the intermediate focal length.

FIG. 8 is an aberration diagram for Example 1 regarding the telephoto end.

FIG. 9 is an aberration diagram for Example 2 at the wide-angle end.

FIG. 10 is an aberration diagram for Example 2 regarding the intermediate focal length.

FIG. 11 is an aberration diagram for Example 2 at the telephoto end.

FIG. 12 is an aberration diagram for a wide-angle end in Example 3.

FIG. 13 is an aberration diagram for Example 3 regarding the intermediate focal length.

FIG. 14 is an aberration diagram for Example 3 at the telephoto end.

FIG. 15 is an aberration diagram for a wide-angle end in Example 4.

FIG. 16 is an aberration diagram related to the intermediate focal length of Example 4.

FIG. 17 is an aberration diagram for the telephoto end according to example 4.

FIG. 18 is an aberration diagram for a wide-angle end in Example 5.

FIG. 19 is an aberration diagram regarding an intermediate focal length in Example 5.

FIG. 20 is an aberration diagram for Example 5 at the telephoto end.

[Explanation of symbols]

 I 1st group II 2nd group III 3rd group

Claims (7)

[Claims] 1. A first to a third lens group are sequentially arranged from the object side to the image side, and the first and second lens groups have a positive focal length and a third lens group.
The group has a negative focal length, and from the wide-angle end where the first group and the second group are close to each other, the second group and the third group are arranged.
With zooming toward the telephoto end where the group is close to each other, the second group and the third group move toward the object side while narrowing the interval, and the first group includes at least one positive lens and at least one negative lens. The second group comprises a meniscus negative lens having a strong concave surface facing the object side, a positive lens, and a biconvex positive lens arranged in order from the object side to the image side. When at least one positive lens and at least one negative lens are provided, and the focal length of the second group is f ₂ and the combined focal length of the entire system at the wide-angle end is f _W , these are the conditions: (1) 0 .6 <f ₂ / f _W <0.85, and at the same time, at least two surfaces of the second lens group and at least one surface of the third lens group are aspherical surfaces. . 2. The zoom lens according to claim 1, wherein both surfaces of the biconvex positive lens in the second lens group are aspherical surfaces. 3. The zoom lens according to claim 1, wherein the first lens unit is composed of a positive lens and a negative lens cemented to each other. 4. The zoom lens according to claim 1, 2 or 3, wherein at least one surface of the object-side positive lens of the third group is an aspherical surface. 5. The zoom lens according to claim 1, 2 or 3 or 4, wherein the second lens unit is moved toward the object side for focusing. 6. The zoom lens according to claim 5, wherein a diaphragm is provided in front of the second lens group, and the diaphragm moves integrally with the second lens group during zooming. lens. 7. The zoom lens according to claim 6, wherein the diaphragm is fixed during focusing, and the second lens unit moves toward the object side to narrow the distance between the diaphragm and the second lens unit. A caliber zoom lens.