JP3365837B2 - Focusing method of 3-group 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 focusing system for a three-group zoom lens suitable for a lens shutter camera and the like. In particular, it has a small number of constituent lenses, is small in size, and is low in cost. The present invention relates to a focusing method for a three-group zoom lens having excellent performance.

[0002]

2. Description of the Related Art Conventionally, a positive / negative two-group zoom type is known as a type suitable for a compact and low-cost zoom lens. As a focusing method for this zoom lens, front lens feeding by the first group is generally known.

The positive / negative two-group zoom type has a field curvature variation associated with zooming. For example, when the aberrations on the wide-angle side and the telephoto side are properly corrected, the image plane tends to be under at the intermediate focal length. However, when the front lens is extended, the curvature of field varies greatly with focusing, and the image surface further falls to the under side. Therefore, it is difficult to correct the field curvature in the entire focal length region when focusing on a very close object.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 2-10307, the front group having a positive refracting power is divided into two parts and they are separately fed to the object side, whereby focusing with little fluctuation of the field curvature is possible. Is known to become.

It is well known that the cost can be significantly reduced by using a plastic lens instead of a glass lens, but the plastic lens has a large change in the refractive index and the lens shape due to temperature and humidity, and thus the image forming performance is greatly affected. There is a problem that has a large impact. As a countermeasure, there is known a method in which the refractive power of a plastic lens is made extremely small, as shown in JP-A-5-113537.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
No. 0307 has a positive refractive power front lens group with a positive / negative lens 2
It is composed of a first group composed of one or more sheets and a second group having a positive refractive power. If the glass lens is replaced with a plastic lens for cost reduction, the plastic lens has a strong refractive power, and there is a problem that the influence of temperature and humidity becomes large.

Further, in the above-mentioned Japanese Patent Laid-Open No. 5-113537, since the field curvature at the intermediate focal length is originally on the under side on the infinity side, it becomes larger on the under side in close-up photography. There is.

The present invention has been made in view of the above problems of the prior art, and an object thereof is that the number of constituent lenses is small, the size is small and the cost is low, and the performance is good from a long distance to a very close distance. It is to provide a focusing method for a zoom lens.

[0009]

A focusing system for a three-group zoom lens according to the present invention, which achieves the above object, comprises a first lens group, a second lens group having a positive refractive power, and a second lens group having a negative refractive power in order from the object side. In a zoom lens composed of three lens groups and performing zooming by changing the distance between the lens groups, the first lens group includes at least one aspherical surface, and all the lenses included in the first lens group are as follows. The first lens group and the second lens group move along the optical axis when focusing is achieved. | F _T / f ₁ | <0.1 (3) where f _T is the focal length of the entire system on the telephoto side, and f ₁ is the first
It is the focal length of each lens in the lens group.

In this case, when focusing, the first lens group and the second lens group
It is also possible to integrally move the lens group on the optical axis.

Further, another focusing system for a three-group zoom lens according to the present invention is, in order from the object side, the first lens group,
A zoom lens comprising a second lens group having a positive refracting power and a third lens group having a negative refracting power to change the magnification by changing the distance between the lens groups, wherein the first lens group includes at least one aspherical surface, and , All the lenses included in the first lens group are lenses that satisfy the following conditional expression (3), the second lens group includes at least one aspherical surface, and the second lens group includes the second lens when focusing.
It is characterized in that the lens group moves on the optical axis. | F _T / f ₁ | <0.1 (3) where f _T is the focal length of the entire system on the telephoto side, and f ₁ is the first
It is the focal length of each lens in the lens group.

[0012]

The reason why the above configuration is adopted in the present invention and the action thereof will be described below. The first group described above effectively generates aberration by using an aspherical surface, and particularly cancels spherical aberration, coma aberration, field curvature, etc., which occur in the second group,
It works to reduce the aberration of the entire lens system. In addition, the use of a powerless lens can reduce the occurrence of chromatic aberration in the first group.

The second and third groups are lens groups having paraxial action (magnification / focus adjustment action) similar to those of the conventional positive / negative two-group zoom type. Spherical aberration,
The coma aberration, the curvature of field, etc. are similar to those of the conventional positive / negative two-group zoom type, and it is difficult to sufficiently correct them with only the second and third groups, and the second and third groups have aspherical surfaces. Is not sufficient to correct these aberrations. Therefore, the aberration of the entire lens system is reduced by the correction effect of the aspherical surface of the first group described above.

By moving the second group and the third group having a paraxial action toward the object side while shortening the distance between them, the magnification is changed from the wide-angle end to the telephoto end. If the group and the second group are moved integrally, aberration variation due to zooming will occur as in the conventional two-group zoom type. Therefore, by changing the distance between the first group and the second group in conjunction with zooming, it is possible to change the aberration balance and improve aberration fluctuations associated with zooming.

Further, during focusing, the aberration balance between the combined group of the first group, the second group and the third group is lost, and the image surface falls to the under side. Therefore, in order to change the occurrence of the field curvature, the interval between the first group and the second group is changed in association with focusing as in the case of zooming. That is, the field curvature is improved by moving the first group and the second group separately on the optical axis.

When the field curvature is generated on the under side in the second lens unit, the aspherical surface of the first lens unit is corrected in the over direction.
The curvature of field fluctuates further to the under side due to the extension of the first group and the second group, but if the distance between the first group and the second group is widened, the field curvature returns to the over direction and the entire image surface The curvature can be reduced.

Further, since the zoom lens is a three-group zoom type, and the field curvature variation due to zooming at infinity is small, even if the first and second groups are integrally focused, the conventional positive and Less performance degradation than the negative 2 group zoom type. In the case of focusing in this way, although the shortest distance cannot be shortened so much, there is an advantage that the focusing frame mechanism can be simplified.

Further, when an aspherical surface is used for the second lens unit,
Aberration occurrence in the second group is very small. In particular, the curvature of field can be generated from the under side to the over side, and the curvature of field of the first group is also on the under side, which is the opposite of the above. In this case, if the distance between the first group and the second group becomes small, the field curvature will fluctuate to the over side. Therefore, after the first group and the second group are extended, the first group for the field curvature correction is made. As a result, the first lens group is lowered, and as a result, even if focusing is performed only by the second lens group, the fluctuation of the field curvature becomes small. In this focusing method, since there is only one focus moving group,
The focusing frame is preferable because it can be mechanically most simplified.

Further, the relationship between the focusing movement amounts ΔG ₁ and ΔG ₂ of the first group and the second group preferably satisfies the following conditional expression. −1 <ΔG ₁ / ΔG ₂ <2 (1) If the condition (1) is not satisfied, the field curvature is overcorrected, which is not preferable. Further, when the lower limit of -1 is exceeded, the entrance surface at the time of focusing at infinity is separated from the stop, the marginal ray height becomes high, and the lens diameter becomes large. On the other hand, if the upper limit of 2 is exceeded, the entrance surface will be separated from the diaphragm at the closest focusing, and the lens diameter will be large.

Further, in order to shorten the total length of the lens on the telephoto side, it is necessary to strengthen the negative refractive power of the third lens group, enhance the zooming effect, and reduce the zoom movement amount. But in this case,
In order to maintain good image surface characteristics, it is desirable that the negative lens in the third lens group satisfy the following conditions. 1.65 <n _N <1.90 (2) where n _N is the average value of the refractive index of the negative lens of the third group.

When the lower limit of 1.65 to condition (2) is exceeded, the Petzval sum becomes negatively large, and the upper limit of 1.9 is reached.
When it exceeds 0, it becomes positively large, and in both cases, astigmatism becomes large, which is not preferable.

Further, it is desirable that the first lens group satisfies the following conditional expression. | F _T / f ₁ | <0.1 (3) where f _T is the focal length of the entire system on the telephoto side, and f ₁ is the first
It is the focal length of each lens in the group. If the conditional expression (3) is not satisfied, chromatic aberration will be increased in the first lens group, and the overall chromatic aberration will be deteriorated.

Further, when the first lens group is made up of a plastic lens which is cheaper than a glass lens, if the conditional expression (3) is satisfied, even if the refractive index or the lens shape changes due to temperature and humidity, the lens group There is also an advantage that the influence on the imaging performance can be minimized since the refractive power hardly changes. Further, the chromatic aberration of the first group can be made sufficiently small with the one-lens configuration without achromatizing with two or more positive and negative lenses as in the prior art.

[0024]

EXAMPLES Examples 1 to 3 of the focusing system for a zoom lens according to the present invention will be described below. The lens data of each example will be described later, but the wide-angle end (a), the intermediate focal length (b), and the telephoto end (c) when focusing on infinity in Examples 1 to 3 are described.
1 to 3 are sectional views of the lens of FIG.

Regarding the lens arrangement, in Example 1, from the object side, the first group G1 consisting of a plastic lens having a very weak refractive power, the second group G2 consisting of a cemented positive lens of a negative lens and a positive lens, and a positive lens. And a negative lens which has a negative refracting power and is composed of a total of 5 lenses of the third group G3, and the aspherical surface is the first.
One surface is used for each of the group G1 and the third group G3. Since the entrance surface of the first group G1 where the central light flux and the peripheral light flux are separated is an aspherical surface, the field curvature and the coma aberration fluctuation in the peripheral portion are well corrected. In particular, the field curvature is generated under in the second group G2, but the aspherical surface of the first group G1 is formed in a shape in which the positive refractive power becomes weaker as the distance from the optical axis is increased, and the field curvature is generated on the over side. The first group G1 and the second group G
The field curvature at 2 is reduced.

At the time of zooming, the distance between the second lens group G2 and the third lens group G3 is shortened, and at the same time, the distance between the first lens group G1 and the second lens group G2 is changed to improve the field curvature at the intermediate focal length. ing. For focusing, the first group G1 and the second group G2 may be extended together (FIG. 5). However, when the shortest distance is shortened, both the first group G1 and the second group G2 are widened to increase the distance between the first group G1 and the second group G2. Get out (Fig. 6). At this time, the first group G1 and the second group G2
If the moving amount ratio of is constant, the payout control system can be simplified while the imaging performance remains good.

It should be noted that an aspherical surface may be used for the glass lens provided in the second lens group G2 of this embodiment, particularly the most object side surface of the second lens group G2. In this case, the aberration performance can be improved by using an aspherical surface for the glass lens of the second group G2, so that the focusing of the present embodiment can be performed only by extending the second group G2. Therefore, it is possible to obtain the effect that the configuration can be simplified and the cost can be reduced.

In the second embodiment, from the object side, a first group G1 composed of a plastic lens having a very weak refractive power, a second group G2 composed of a powerless plastic lens and a cemented positive lens composed of a negative lens and a positive lens, It is composed of a total of 6 pieces of the 3rd group G3 which has a negative refractive power which consists of a positive lens and a negative lens,
One aspherical surface is used for each group. The plastic lens in the second group G2 is an aspherical lens and is powerless in order to reduce the performance change due to temperature and humidity. Further, due to the aberration correction effect of this aspherical surface, the aberration generation in the second lens group G2 is much smaller than that in the first embodiment, and in particular the field curvature slightly occurs on the over side. Therefore, in the first group G1, the aspherical surface has a shape in which the positive refracting power becomes stronger as the distance from the optical axis increases, and the field curvature is generated on the under side. However, since the amount of aberration generated is small in both groups, the performance is not degraded even if the positional accuracy between the first group and the second group is low.

The zooming is the same as that of the first embodiment, but the focusing provides good performance only by extending the second lens group G2 (FIG. 8). Further, if the first group is retracted at the same time, the performance is further improved (FIG. 9).

The third embodiment comprises, from the object side, a first group G1 consisting of a plastic lens having a very weak refractive power, a second group G2 consisting of a positive lens cemented with a positive lens and a negative lens, a positive lens and a negative lens. It is composed of a total of 5 pieces of the third lens group G3 having a negative refracting power, and the aspheric surface has two surfaces in the first lens group G1 and the third lens group G3
One side is used every three. The first group G1 is a double-sided aspherical lens, but there are differences in the marginal ray heights on both sides, and the generated aberrations are different. Therefore, the degree of freedom in correction balance in the central portion (spherical aberration) and the peripheral portion (field curvature, etc.) is increased, and more favorable aberration correction is possible. Also, both aspherical surfaces have a shape in which the positive refracting power becomes stronger as the distance from the optical axis increases,
The lens has an over curvature of field. Zooming and focusing are the same as in the first embodiment (FIG. 1).
1, FIG. 12)).

The lens data of each embodiment are shown below. The symbols are the above, f is the focal length of the entire system, F _NO is the F number, 2ω is the angle of view, f _B is the back focus, r ₁ and
r ₂ ... curvature radius of each lens surface, d _1, d ₂ ... the spacing between the lens surfaces, n _d1, n _d2 ... d-line refractive index of each lens, ν _d1, ν _d2 ... Each lens Is the Abbe number. In addition,
The aspherical shape is x in the optical axis direction and y in the direction orthogonal to the optical axis.
Then, it is expressed by the following equation. x = (y ² / r) / [1+ {1-P (y /
r) ² } ^1/2 ] + A ₄ y ⁴ + A ₆ y ⁶ + A ₈ y ⁸ + A ₁₀ y ¹⁰ However, r is a paraxial radius of curvature, P is a conic coefficient, A ₄ , A ₆ ,
A ₈ and A ₁₀ are aspherical coefficients.

Example 1 f = 39.2 to 58.6 to 87.1 F _NO = 4.66 to 6.21 to 8.28 2ω = 56.4 ° to 40.0 ° to 27.7 ° f _B = 9.51 to 28.13 to 55.49 r ₁ = 25.4210 (aspherical surface) d ₁ = _{3.000 n d1 = 1.49241 ν d1 =} 57.66 r 2 = 24.5490 d 2 = ( _{variable) r 3 = 157.8680 d 3 =} 1.500 n d2 = 1.83400 ν d2 = 37.16 r 4 = 13.6650 d 4 = 9.980 n d3 = 1.69680 ν d3 = 55.52 r ₅ = -17.5900 d ₅ = 1.000 r ₆ = ∞ (aperture) d ₆ = (variable) r ₇ = -29.8590 d ₇ = 3.000 n _d4 = 1.57501 ν _d4 = 41.49 r ₈ = -16.2750 d ₈ = 3.660 r ₉ = -13.0430 (aspherical surface) d ₉ = 1.800 n _d5 = 1.72916 ν _d5 = 54.68 r ₁₀ = -640.3870 Aspheric coefficient Coefficient 1st surface P = 0.2454 A ₄ = -0.49968 × 10 ^-4 A ₆ = -0.28262 × 10 ^-6 A ₈ = -0.12628 × 10 ^-8 A ₁₀ = -0.47657 × 10 ^-11 9th surface P = 0.2409 A ₄ = -0.19743 x 10 ^-4 A ₆ = 0.55883 x 10 ^-8 A ₈ = -0.12032 x 10 ^-8 A ₁₀ = 0.59178 x 10 ^-11
.

Example 2 f = 39.1 to 59.1 to 87.0 F _NO = 4.64 to 6.15 to 8.28 2ω = 56.8 ° to 39.8 ° to 27.8 ° f _B = 9.28 to 29.66 to 57.96 r ₁ = 129.4710 (aspherical surface) d ₁ = _{2.000 n d1 = 1.49241 ν d1 =} 57.66 r 2 = 137.4900 d 2 = ( variable) r ₃ = 37.7490 _{(aspherical) d 3 = 2.000 n d2 =} 1.49241 ν d2 = 57.66 r 4 = 36.6790 d 4 = 1.420 r 5 = -119.5760 d ₅ = 9.020 n _d3 = 1.67270 ν _d3 = 32.10 r ₆ = 25.8550 d ₆ = 3.830 nd ₄ = 1.62041 ν _d4 = 60.27 r ₇ = -15.3280 d ₇ = 1.000 r ₈ = ∞ (aperture) d ₈ = ( Variable) r ₉ = -30.1720 d ₉ = 4.100 n _d5 = 1.62004 ν _d5 = 36.25 r ₁₀ = -15.3450 d ₁₀ = 3.290 r ₁₁ = -11.3270 (aspherical) d ₁₁ = 1.800 n _d6 = 1.69680 ν _d6 = 55.52 r ₁₂ = -142.9020 Aspherical coefficient 1st surface P = 1.0000 A ₄ = 0.47463 × 10 ^-6 A ₆ = 0.72872 × 10 ^-7 A ₈ = -0.36425 × 10 ^-9 A ₁₀ = 0 3rd surface P = 7.1963 A ₄ = -0.99201 × 10 ^-4 A ₆ = -0.10089 × 10 ^-5 A ₈ = 0.53956 × 10 ^-8 A ₁₀ = -0.64608 × 10 ^-10 11th surface P = -1.7719 A ₄ = -0.20403 × 10 ^-3 A ₆ = 0.81619 × 10 ^-6 A ₈ = -0.43485 × 10 ^-8 A ₁₀ = 0.95073 × 10 ^-11
.

Example 3 f = 38.9 to 70.2 to 101.9 F _NO = 4.61 to 7.52 to 9.99 2ω = 57.3 ° to 34.3 ° to 24.0 ° f _B = 1.60 to 42.09 to 74.02 r ₁ = -34.6220 (aspherical surface) d _{1 = 5.000 n d1 = 1.49241 ν d1} = 57.66 r 2 = -37.2550 ( aspherical) d ₂ = _{(variable) r 3 = -59.6620 d 3 =} 8.010 n d2 = 1.51633 ν d2 = 64.15 r 4 = -9.2970 d 4 = 1.130 n _d3 = 1.84666 ν _d3 = 23.78 r ₅ = -11.5560 d ₅ = 1.000 r ₆ = ∞ (aperture) d ₆ = (variable) r ₇ = -57.1080 d ₇ = 3.760 nd ₄ = 1.61293 ν _d4 = 37.00 r ₈ = -19.1090 d ₈ = 4.160 r ₉ = -12.0390 (aspherical surface) d ₉ = 1.800 n _d5 = 1.75500 ν _d5 = 52.33 r ₁₀ = -304.1050 Aspheric surface Coefficient 1st surface P = 1.0000 A ₄ = 0.65037 × 10 ^-5 A ₆ = 0.67721 × 10 ^-6 A ₈ = -0.11970 × 10 ^-8 A ₁₀ = 0 2nd surface P = 1.0000 A ₄ = 0.13326 × 10 ^-3 A ₆ = 0.13112 × 10 ^-5 A ₈ = 0.12946 × 10 ^-7 A ₁₀ = 0 9th surface P = 0.7782 A ₄ = 0.25048 × 10 ^-4 A ₆ = 0.11094 × 10 ^-6 A ₈ = 0.12981 × 10 ^-8 A ₁₀ = -0.695 10 × 10 ^-11
.

Next, the pay-out amounts ΔG ₁ and ΔG _{2 of the first} and second groups G1 and G2 during focusing in the first to third embodiments.
Is shown in the table below. Note) The sign − represents the movement toward the object side, the sign + represents the movement toward the image plane side, and the value in parentheses represents the ratio of the movement amounts of the first group G1 and the second group G2 (conditional expression (1)). .

Next, the values of the conditional expressions (2) and (3) of each embodiment are shown in the following table.

Aberrations representing spherical aberration, astigmatism, distortion, and lateral chromatic aberration at the wide-angle end (a), the intermediate focal length (b), and the telephoto end (c) when focusing on infinity in Examples 1 to 3 above. The drawings are shown in FIGS. 4, 7, and 10, respectively. In addition, Example 1
A similar aberration diagram at the time of 1 m focusing by the first group G1 and the second group G2 integrally extended is shown in FIG. 5, and when the first group G1 and the second group G2 of the first example are independently extended and 0.6 m are focused. A similar aberration diagram is shown in FIG. Further, FIG. 8 shows a similar aberration diagram at the time of focusing 0.6 m by the second group G2 extension in Example 2, and at the time of 0.6 m focusing by the first group G1 extension and second group G2 extension in Example 2. A similar aberration diagram of is shown in FIG. Further, FIG. 11 is a similar aberration diagram at the time of 1 m focusing by the first group G1 and the second group G2 integrally extended in Example 3, and FIG. 11 shows the same aberration diagrams when the first group G1 and the second group G2 are independently extended. A similar aberration diagram at the time of focusing at 6 m is shown in FIG.

[0038]

As is clear from the above description, according to the present invention, there is provided a focusing system for a zoom lens, which has a small number of constituent lenses, is small in size, and is low in cost, but has good performance from a long distance to a very close distance. can get.

[Brief description of drawings]

FIG. 1 is a wide-angle end (a) at the time of focusing at infinity of a three-group zoom lens of Example 1 to which a focusing system of the present invention is applied,
It is sectional drawing of an intermediate focal length (b) and a telephoto end (c).

2 is a sectional view similar to FIG. 1 of Example 2. FIG.

3 is a sectional view similar to FIG. 1 of Example 3. FIG.

FIG. 4 is an aberration diagram showing spherical aberration, astigmatism, distortion, and chromatic aberration of magnification at the wide-angle end (a), the intermediate focal length (b), and the telephoto end (c) when focusing on infinity in Example 1. is there.

FIG. 5 is an aberration diagram similar to FIG. 4 at the time of 1 m focusing by the first group and second group integrally feeding in Example 1.

FIG. 6 is an aberration diagram similar to that of FIG. 4 at the time of focusing at 0.6 m by the first group and the second group independent extension in Example 1;

FIG. 7 is an aberration diagram for Example 2 at the time of focusing at infinity, which is similar to FIG.

FIG. 8 is an aberration diagram similar to FIG. 4 at the time of focusing at 0.6 m by the second lens unit extension in Example 2;

FIG. 9 is an aberration diagram similar to that of FIG. 4 when the first lens unit is retracted and the second lens unit is extended to focus at 0.6 m in Example 2;

FIG. 10 is an aberration diagram similar to FIG. 4 at the time of focusing on infinity in Example 3.

FIG. 11 is an aberration diagram similar to that of FIG. 4 at the time of 1 m focusing by the first group and second group integrally moving out in Example 3;

FIG. 12 is an aberration diagram similar to FIG. 4 at the time of focusing at 0.6 m by the first group and the second group independent extension in Example 3;

[Explanation of symbols]

G1 ... First lens group G2: Second lens group G3 ... Third lens group

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-10307 (JP, A) JP-A-2-48622 (JP, A) JP-A-3-127010 (JP, A) JP-A-3- 150518 (JP, A) JP 4-78814 (JP, A) JP 4-153613 (JP, A) JP 5-134180 (JP, A) JP 4-95912 (JP, A) JP-A-6-294932 (JP, A) International publication 92/21048 (WO, A1) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02- 21/04 G02B 25/00-25/04

Claims (9)

(57) [Claims] From 1. A object side, a first lens group, a positive refractive power second lens group, a third lens unit having a negative refractive power,
In a zoom lens that performs zooming by changing the distance between lens groups, the first lens group includes at least one aspherical surface, and all the lenses included in the first lens group have the following conditional expressions:
A focusing system for a three-group zoom lens, comprising a lens satisfying the condition (3), wherein the first lens group and the second lens group move on the optical axis at the time of focusing. | F _T / f ₁ | <0.1 (3) where f _T is the focal length of the entire system on the telephoto side, and f ₁ is the first
It is the focal length of each lens in the lens group. 2. The focusing system for a three-group zoom lens according to claim 1, wherein the first lens group and the second lens group move integrally on the optical axis during focusing. From wherein object side, a first lens group, a positive refractive power second lens group, a third lens unit having a negative refractive power,
In a zoom lens that performs zooming by changing the distance between lens groups, the first lens group includes at least one aspherical surface, and all the lenses included in the first lens group have the following conditional expressions:
A focusing system for a three-group zoom lens, comprising a lens satisfying the condition (3) , wherein the second lens group includes at least one aspherical surface, and the second lens group moves on the optical axis during focusing. | F _T / f ₁ | <0.1 (3) where f _T is the focal length of the entire system on the telephoto side, and f ₁ is the first
It is the focal length of each lens in the lens group. 4. The focusing system for a three-group zoom lens according to claim 1, wherein the first lens group and the second lens group are separately moved along the optical axis in conjunction with focusing. 5. The focusing system for a three-group zoom lens according to claim 1, wherein the focusing system is arranged such that the first lens group and the second lens group are extended while being extended during focusing. 6. The moving amounts ΔG ₁ and ΔG _{2 of} the first lens group and the second lens group during focusing satisfy the following conditional expressions: Focusing method for 3 group zoom lens. -1 <ΔG ₁ / ΔG ₂ <2 (1) 7. The focusing system for a three-group zoom lens according to claim 1, wherein the negative lens in the third lens group satisfies the following conditional expression. 1.65 <n _N <1.90 (2) where n _N is the average value of the refractive index of the negative lens of the third lens group. 8. any of claims 1 to 7, characterized in that said first lens group is constituted by a plastic lens 1
Focusing method for the three-group zoom lens described in the item . 9. The focusing system for a three-group zoom lens according to claim 1, wherein the first lens group is composed of one lens.