JP3339912B2 - Focusing method of 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 zoom lens, and more particularly to a zoom lens mainly used for a compact camera having a short back focus, which has a zoom ratio of about 2 to 3 times. The present invention relates to a focusing method effective for a small zoom lens.

[0002]

2. Description of the Related Art As a zoom lens used for a compact camera having a short back focus, Japanese Patent Application Laid-Open No.
No. 28911, JP-A-62-264019, JP-A-62-264019
2. Description of the Related Art A large number of two-unit zoom lenses including a positive lens unit and a negative lens unit in order from the object side, as known in JP-A-3-266413 and the like, are used.

In recent years, further wide-angle and high-power zooming of compact cameras have been desired, and the present applicant has also filed Japanese Patent Application No. Hei 4-28769.
No. 2 and Japanese Patent Application No. 5-80536, the zoom ratio is twice or more, while being as large as the conventional positive / negative two-unit zoom lens.
We have proposed a compact zoom lens with the wide-angle end angle of view expanded to about 70 degrees.

In each of the above-described zoom lenses, a focusing method from an object point at infinity to a close photographing distance is as follows.
Conventionally, focus adjustment has generally been performed by moving the positive lens group of the front group.

[0005]

FIG. 2 shows a paraxial arrangement of a focusing system of a conventional two-unit zoom lens comprising a positive front unit G _F and a negative rear unit G _R. Thus, the front group G
If the _F a focus lens group, the fluctuation amount of the image plane position relative to the focusing lens group stopping accuracy, i.e., the lateral magnification of the image surface sensitivity of the focus lens group epsilon _F is the rear group G _R beta _R
, Ie, ε _F = β _R ² . However, the object point is considered as infinity.

The lateral magnification of the rear unit at the telephoto end increases as the zoom ratio of the zoom lens increases. As a result, the image surface sensitivity of the focus lens group at the telephoto end also increases, so the stopping accuracy of the focus lens group must be increased in accordance with the high zoom ratio, and the movement control of the focus lens group is mechanically controlled. It will be difficult.

[0007] The present invention has been made in view of such problems of the prior art, and has as its object the application of Japanese Patent Application Nos. Hei 4-287692 and Hei 5-80536 proposed by the present applicant.
It is an object of the present invention to provide a focusing system in which a small zoom lens such as the one described above has an image surface sensitivity of a focus lens group which is large enough to be mechanically controllable at the telephoto end.

[0008]

In order to achieve the above object, a focusing system of a zoom lens according to the present invention comprises, in order from the object side, a negative first lens group, a positive second lens group, and a negative third lens group.
The first lens group and the second lens group move integrally, and the distance between the second lens group and the third lens group is monotonously reduced. In a zoom lens that performs zooming from the wide-angle end to the telephoto end by setting only the distance between the second lens group and the third lens group, focusing from an object point at infinity to a close distance is performed by moving the second lens group toward the object side. And the first lens group is composed of one negative lens satisfying the following condition. −2.5 <(r ₁ + r ₂ ) / (r ₁ −r ₂ ) <− 0.3 where r ₁ : object-side radius of curvature of the negative lens in the first lens group G 1 r ₂ : first lens group This is the radius of curvature of the image side of the negative lens at G1.

According to another focusing method of the zoom lens of the present invention, a negative first lens unit,
The second lens group includes a positive second lens group and a negative third lens group, and the second lens group includes, in order from the object side, a positive 2A
A lens group and a positive second B lens group, the distance between the second B lens group and the third lens group is monotonously reduced, and
The distance between the variable lens groups at the time of zooming is the distance between the first lens group and the second A lens group, and the distance between the second A lens group and the second
Telephoto from the wide-angle end by setting only the distance between the B lens group, the distance between the 2B lens group and the third lens group, and integrating the first lens group and the second B lens group during zooming. In a zoom lens that performs zooming to an end, the second A lens group moves closer to the first lens group from the wide-angle end to the intermediate focal length, and moves closer to the second B lens group from the intermediate focal length to the telephoto end. Focusing from an object point at infinity to a close distance is performed by moving the second lens group to the object side, and the first lens group includes one negative lens satisfying the above conditions. It is characterized by having.

[0010]

FIG. 1 shows a paraxial arrangement of a zoom lens adopting the focusing method according to the present invention.
Is a focus lens group. Here, assuming that the lateral magnifications of the second lens group G2 and the third lens group G3 at the object point at infinity are respectively β ₂ and β ₃ , the image plane at the object point at infinity of the second lens group G2 that is the focus lens group is set. sensitivity epsilon _{2 ^{is, ε 2 = β 3 2 -}} (β 2 β 3) 2 ... become.

[0011] Since the third lens group in FIG. 1 G3 corresponds to the rear group G _R in FIG. 2, each of the lateral magnification beta _3,
β _R has the same value. That is, since β _R = β ₃ , ε ₂ = β _R ² − (β ₂ β ₃ ) ² = ε _F − (β ₂ β ₃ ) ² .

Therefore, by adopting the focusing method of the present invention, the image surface sensitivity ε ₂ of the focus lens group is made smaller by (β ₂ β ₃ ) ² than the image surface sensitivity ε _F of the conventional focus lens group. Can be.

The shorter the focal length f ₁ of the first lens group G ₁ , the larger β ₂ can be, so that ε ₂
Can be reduced. However, if ε ₂ is too small, the amount of movement of the second lens group G2 during focusing becomes too large.
The distance between the first lens group G1 and the second lens group G2 must be increased, and the lens system becomes large. Therefore,
The focal length f ₁ of the first lens group G1 is preferably satisfies the following conditions.

1.2 <| f ₁ | / F _W <1.8, where F _{W is} the focal length of the entire system at the wide-angle end. In addition, ε ₂ becomes too small, and the lens system becomes large. If the upper limit of 1.8 is exceeded, β ₂ cannot be increased, and ε ₂ increases, making it difficult to control the movement of the focus lens group at the telephoto end.

Furthermore, it is desirable to satisfy the following conditions in order to achieve the object of the present invention.

0.6 <f ₂ / | f ₃ | <1.2 2.5 <β _3T <4.0 where f _{2 is} the focal length of the second lens group G2 at the telephoto end f _{3 is the} second Focal length β _3T of the third lens group G3: lateral magnification of the third lens group G3 at the telephoto end The above conditions relate to the focal length of the second lens group G2.
If the focal length of the second lens group exceeds the 0.6 lower limit G2 is short, group first lens to beta ₂ increases G
The distance between the first lens group G2 and the second lens group G2 must be reduced, and it becomes difficult to secure a moving space by focusing of the second lens group G2. If the focal length of the second lens group G2 becomes longer than the upper limit of 1.2, the distance between the second lens group G2 and the third lens group G3 becomes unnecessarily long, so that the total length of the lens system becomes long. Would.

In addition, the above-mentioned condition relates to the lateral magnification of the third lens group G3. If the lower limit of 2.5 is exceeded, it becomes difficult to secure the back focus at the wide-angle end. Can not be. When the value exceeds the upper limit of 4.0, ε ₂ becomes large, and it becomes difficult to control the movement of the focus lens unit at the telephoto end.

In order to satisfactorily correct the aberration of the entire lens system even with a small number of lenses, it is preferable that the first lens group G1 comprises one negative lens and satisfies the following conditions.

[0019] _{-2.5 <(r 1 + r 2} ) / (r 1 -r 2) <- 0.3 ... However, r _1: curvature at the object side of the negative lens in the first lens group G1 radius r _2: the The image-surface-side radius of curvature of the negative lens in the first lens group G1 is a conditional expression for correcting the astigmatism and distortion generated in the third lens group G3 in a balanced manner by the first lens group G1. Exceeds -0.3, the astigmatism and distortion generated in the first lens group G1 become too small, and if the lower limit of -2.5 is exceeded, they become too large.

The second lens group G2 includes, in order from the object side, a positive second A lens group and a positive second B lens group, and the second A lens group includes at least two positive lenses and at least one positive lens. The second B lens group includes at least one biconvex positive lens and at least one negative meniscus lens, so that spherical aberration and axial chromatic aberration can be satisfactorily corrected.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 1 to 4 of a zoom lens adopting a focusing system according to the present invention will be described below with reference to the drawings. Embodiments 1 and 2 are zoom lenses in which the first lens group G1 and the second lens group G2 move integrally when zooming from the wide-angle end to the telephoto end.

FIG. 3 is a cross-sectional view of the zoom lens of Embodiment 1 at the wide-angle end (FIG. 3A) and the telephoto end (FIG. 3B). The lens arrangement in this embodiment is the same as that of the first lens group G1. Is
The second lens group G2 is composed of a double-convex lens, a double-concave lens, a double-convex lens, and a cemented lens of a double-convex lens and a negative meniscus lens having a concave surface facing the object side. Consisting of
The third lens group G3 includes three positive meniscus lenses having a concave surface facing the object side and two negative meniscus lenses having a concave surface facing the object side.

The lens data of the first embodiment will be described later. The spherical surface at the wide-angle end (FIG. 9A), the intermediate focal length (FIG. 9B), and the telephoto end (FIG. 9C) with respect to the object point at infinity. FIG. 6 is an aberration diagram showing aberration, astigmatism, distortion, and chromatic aberration of magnification.

FIG. 4 is a sectional view of the zoom lens of Embodiment 2 at the wide-angle end (a) and the telephoto end (b). In this embodiment, the sixth surface (from the object side) in the second lens unit G2 is used. An aspherical surface is used for the image surface side of the third biconvex lens,
The lens arrangement is such that the first lens group G1 is composed of one negative meniscus lens having a concave surface facing the object side, and the second lens group G2 is composed of a positive meniscus lens having a convex surface facing the object side, a biconcave lens, and a biconcave lens. The third lens group G3 includes a convex lens, a biconvex lens, and a cemented lens of a negative meniscus lens having a concave surface facing the object side. The third lens group G3 includes a positive meniscus lens having a concave surface facing the object side and a concave surface facing the object side. Turned to 2
It consists of three negative meniscus lenses.

The lens data of Example 2 will be described later. The spherical surface at the wide-angle end (FIG. 9A), the intermediate focal length (FIG. 9B), and the telephoto end (FIG. 9C) with respect to the object point at infinity. FIG. 7 is an aberration diagram showing aberration, astigmatism, distortion, and lateral chromatic aberration.

The following third and fourth embodiments use the second A lens group G2.
A moves from the wide-angle end to the intermediate focal length so as to approach the first lens group G1, and from the intermediate focal length to the telephoto end so as to approach the second B lens group G2B. This is a zoom lens with a larger size. Further, the first lens group G1 and the second B lens group G2B are moving integrally.

FIG. 5 is a sectional view of the zoom lens of Embodiment 3 at the wide-angle end (FIG. 7A), the intermediate focal length (FIG. 8B), and the telephoto end (FIG. 8C). The fourth embodiment is the same as the third embodiment and is not shown. In the lens arrangements of Examples 3 and 4, the first lens group G1 includes one biconcave lens, and the second A lens group G2A includes a biconvex lens, a negative meniscus lens having a convex surface facing the image surface side, and a biconcave lens. The second B lens group G2B is composed of a cemented lens of a biconvex lens and a negative meniscus lens having a convex surface facing the image surface side. The second lens group G2B has a stop after that. Also has three lenses, a positive meniscus lens, a negative meniscus lens, and a negative meniscus lens, with the convex surface facing the image plane side.
The aspherical surface is used as the one surface closest to the image plane of the second A lens group G2A in both the third and fourth embodiments.

The lens data of the third and fourth embodiments will be described later. The wide-angle end (FIG. 10A), the intermediate focal length (FIG. 10B), and the telephoto end (FIG. FIGS. 8 and 9 show spherical aberration, astigmatism, distortion, and lateral chromatic aberration in c)), respectively.

When comparing the image plane sensitivity ε _F (conventional focusing method) and ε ₂ (focusing method of the present invention) at the object point at infinity at the telephoto end in each of the above embodiments, the following is obtained. is there.

Next, lens data of each embodiment will be shown.
In the following, the symbols are the above, f is the focal length of the entire system, F _NO is the F number, ω is the half angle of view, f _B is the back focus, r ₁ , r _2, ... 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 ... is the Abbe number of each lens. The aspherical shape is represented by the following equation, where x is the optical axis direction and y is the direction orthogonal to the optical axis. ^{x = cy 2 / {1+ (} 1-c 2 y 2) 1/2} + A 4 y 4 +
However ^{_{A 6 y 6 + A 8 y}} 8, a c = 1 / r, r is the radius of curvature on the optical axis,
A ₄ , A ₆ , and A ₈ are aspherical coefficients.

[0031] Example 1 f = 28.51 ~ 40.00 ~ 60.00 F NO = 3.58 ~ 5.02 ~ 7.53 ω = 37.12 ~ 28.84 ~ 20.09 ° f B = 5.31 ~ 16.23 ~ 35.24 r 1 = -25.388 d 1 = 1.32 n d1 = 1.65844 _{ν d1 = 50.86 r 2 = -156.695} d 2 = 1.80 r 3 = 19.750 d 3 = 2.94 n d2 = 1.60342 ν d2 = 38.01 r 4 = -146.970 d 4 = 1.72 r 5 = -15.786 d 5 = 1.22 n d3 = _{1.80100 ν d3 = 34.97 r 6 =} 35.907 d 6 = 0.11 r 7 = 41.019 d 7 = 4.42 n d4 = 1.58913 ν d4 = 61.18 r 8 = -13.393 d 8 = 0.10 r 9 = 18.228 d 9 = 3.80 n d5 = 1.57099 ν _d5 = 50.80 r ₁₀ = -11.757 d ₁₀ = 1.20 nd ₆ = 1.78590 ν _d6 = 44.18 r ₁₁ = -34.270 d ₁₁ = 0.80 r ₁₂ = ∞ (aperture) d ₁₂ = (variable) r ₁₃ = -49.372 d _{13 = 3.30 n d7 = 1.63930 ν d7} = 44.88 r 14 = -12.430 d 14 = 0.10 r 15 = -15.452 d 15 = 1.20 n d8 = 1.65844 ν d8 = 50.86 r 16 = -152.470 d 16 = 4.94 r 17 = -9.446 _{d 17 = 1.80 n d9 = 1.69680} ν d9 = 55.52 r 18 = -31.784 | F ₁ | / F _W = 1.62 f ₂ / | f ₃ | = 0.89 β _{3 T} = 2.88 (r ₁ + r ₂ ) / (r ₁ -r ₂ ) = -1.39
.

[0032] Example 2 f = 34.99 ~ 54.96 ~ 79.92 F NO = 3.60 ~ 5.65 ~ 8.22 ω = 31.69 ~ 21.45 ~ 15.12 ° f B = 6.08 ~ 24.65 ~ 47.86 r 1 = -26.612 d 1 = 1.30 n d1 = 1.65830 _{ν d1 = 57.33 r 2 = -84.007} d 2 = 1.80 r 3 = 17.236 d 3 = 2.34 n d2 = 1.66892 ν d2 = 44.98 r 4 = 47.927 d 4 = 1.20 r 5 = -43.062 d 5 = 1.20 n d3 = 1.80100 ν _d3 = 34.97 r ₆ = 38.181 d ₆ = 0.70 r ₇ = 51.336 d ₇ = 2.16 n _d4 = 1.57250 ν _d4 = 57.76 r ₈ = -39.936 (aspherical surface) d ₈ = 2.40 r ₉ = 31.441 d ₉ = 4.38 n _d5 = 1.61765 ν _d5 = 55.05 r ₁₀ = -8.940 d ₁₀ = 1.20 nd ₆ = 1.79952 ν _d6 = 42.24 r ₁₁ = -18.730 d ₁₁ = 0.80 r ₁₂ = ∞ (aperture) d ₁₂ = (variable) r ₁₃ =- _{20.146 d 13 = 2.04 n d7 =} 1.66998 ν d7 = 39.27 r 14 = -13.667 d 14 = 2.82 r 15 = -13.740 d 15 = 1.20 n d8 = 1.63854 ν d8 = 55.38 r 16 = -52.693 d 16 = 3.61 r 17 _{= -12.882 d 17 = 1.80 n d9} = 1.69680 ν d9 = 55.52 r 18 = -26.219 Aspheric surface 8th surface A ₄ = 1.8719 × 10 ^-5 A ₆ = 1.4994 × 10 ^-7 A ₈ = -6.9203 × 10 ^-9 | F ₁ | / F _W = 1.7 f ₂ / | f ₃ | = 0.91 β _{3 T} = 3.22 (r ₁ + r ₂ ) / (r ₁ -r ₂ ) = -1.93
.

[0033] Example 3 f = 28.84 ~ 48.0 ~ 78.0 F NO = 4.6 ~ 6.0 ~ 8.0 ω = 37.765~ 24.632~ 15.646 ° f B = 4.21 ~ 21.48 ~ 51.05 r 1 = -29.935 d 1 = 1.300 n d1 = 1.65844 _{ν d1 = 50.86 r 2 = 276.982} d 2 = ( _{variable) r 3 = 29.323 d 3 =} 2.447 n d2 = 1.60342 ν d2 = 38.01 r 4 = -875.557 d 4 = 1.912 r 5 = -17.829 d 5 = 2.088 n d3 = 1.83400 ν _d3 = 37.16 r ₆ = -61.216 d ₆ = 0.179 r ₇ = 125.064 d ₇ = 3.688 nd ₄ = 1.51633 ν _d4 = 64.15 r ₈ = -20.368 (aspheric) d ₈ = (variable) r ₉ = 27.111 _{d 9 = 5.774 n d5 = 1.53996} ν d5 = 59.57 r 10 = -10.124 d 10 = 1.200 n d6 = 1.80610 ν d6 = 40.95 r 11 = -18.618 d 11 = 0.800 r 12 = ∞ ( stop) d ₁₂ = (variable _{) r 13 = -39.357 d 13 =} 3.517 n d7 = 1.59551 ν d7 = 39.21 r 14 = -14.726 d 14 = 1.092 r 15 = -16.637 d 15 = 1.193 n d8 = 1.74400 ν d8 = 44.73 r 16 = -43.461 d ₁₆ = 4.546 r ₁₇ = -12.395 d ₁₇ = 1.800 n _d9 = 1.69680 ν _d9 = 55.52 r ₁₈ = -62.066 Aspherical coefficients eighth surface ^{_{A 4 = 0.10209 × 10 -4 A}} 6 = -0.59663 × 10 -8 A 8 = -0.21208 × 10 -8 | F ₁ | / F _W = 1.42 f ₂ / | f ₃ | = 0.89 β _{3 T} = 3.43 (r ₁ + r ₂ ) / (r ₁ -r ₂ ) = -1.80
.

[0034] Example 4 f = 28.84 ~ 49.98 ~ 82.5 F NO = 4.6 ~ 6.0 ~ 8.0 ω = 37.792~ 23.737~ 14.811 ° f B = 4.13 ~ 23.26 ~ 54.91 r 1 = -32.658 d 1 = 1.300 n d1 = 1.65844 _{ν d1 = 50.86 r 2 = 119.635} d 2 = ( _{variable) r 3 = 28.959 d 3 =} 2.802 n d2 = 1.60342 ν d2 = 38.01 r 4 = -193.846 d 4 = 1.958 r 5 = -17.020 d 5 = 2.115 n d3 = 1.83400 ν _d3 = 37.16 r ₆ = -63.355 d ₆ = 0.220 r ₇ = 159.253 d ₇ = 3.750 nd ₄ = 1.51633 ν _d4 = 64.15 r ₈ = -19.431 (aspheric) d ₈ = (variable) r ₉ = 26.696 _{d 9 = 5.890 n d5 = 1.53996} ν d5 = 59.57 r 10 = -10.193 d 10 = 1.200 n d6 = 1.80610 ν d6 = 40.95 r 11 = -18.823 d 11 = 0.800 r 12 = ∞ ( stop) d ₁₂ = (variable _{) r 13 = -41.680 d 13 =} 3.583 n d7 = 1.59551 ν d7 = 39.21 r 14 = -14.813 d 14 = 1.127 r 15 = -16.380 d 15 = 1.200 n d8 = 1.74400 ν d8 = 44.73 r 16 = -44.980 d ₁₆ = 4.517 r ₁₇ = -12.606 d ₁₇ = 1.800 n _d9 = 1.69680 ν _d9 = 55.52 r ₁₈ = -65.414 Aspheric surface coefficient 8th surface A ₄ = 0.75716 × 10 ^-5 A ₆ = 0.17822 × 10 ^-7 A ₈ = -0.22029 × 10 ^-8 | F ₁ | / F _W = 1.35 f ₂ / | f ₃ | = 0.90 β _{3 T} = 3.63 (r ₁ + r ₂ ) / (r ₁ -r ₂ ) = -0.57
.

[0035]

As is clear from the above description, according to the focusing method of the present invention, the backing system as disclosed in Japanese Patent Application Nos. 4-287792 and 5-80536 previously proposed by the present applicant has been disclosed. In a small zoom lens with a zoom ratio of about 2 to 3 times used in a compact camera or the like having a short focus, the image surface sensitivity of the focus lens group can be made smaller than that of the conventional focus lens group. Even at the telephoto end, the movement of the focus lens group can be made large enough to be mechanically controllable.

[Brief description of the drawings]

FIG. 1 is a paraxial arrangement diagram of a zoom lens employing a focusing system of the present invention.

FIG. 2 is a paraxial arrangement diagram of a focusing system of a conventional two-unit zoom lens.

FIG. 3 is a sectional view of a zoom lens according to a first embodiment of the present invention at a wide-angle end (a) and at a telephoto end (b).

FIG. 4 is a sectional view of a zoom lens according to a second embodiment at a wide-angle end (a) and at a telephoto end (b).

FIG. 5 is a sectional view of a zoom lens according to a third exemplary embodiment at a wide-angle end (a), an intermediate focal length (b), and a telephoto end (c).

FIG. 6 shows 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) with respect to an object point at infinity of the zoom lens according to the first embodiment. FIG.

FIG. 7 is an aberration diagram similar to FIG. 6 of the zoom lens according to the second embodiment.

FIG. 8 is an aberration diagram similar to FIG. 6 of the zoom lens according to the third embodiment.

FIG. 9 is an aberration diagram similar to FIG. 6 of the zoom lens according to the fourth embodiment.

[Explanation of symbols]

 G1: first lens group G2: second lens group G3: third lens group G2A: second A lens group G2B: second B lens group

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (5)

(57) [Claims] 1. A first lens group, a second positive lens group, and a third negative lens group are arranged in order from the object side .
Lens group and the second lens group monotonically decreases the distance between the third lens group and the second lens group vital move integrally, during zooming
A variable lens group spacing is set between the second lens group and the third lens.
In a zoom lens that performs zooming from the wide-angle end to the telephoto end by setting only the distance to the zoom lens group, focusing from an object point at infinity to a close distance is performed by moving the second lens group to the object side , and said first lens group one negative les satisfying the following conditions
Zoom lens lens
Focusing method. −2.5 <(r ₁ + r ₂ ) / (r ₁ −r ₂ ) <− 0.3 where r ₁ is a negative lens in the first lens group G1.
Body-side radius of curvature r ₂ : image-side curvature of the negative lens in the first lens group G1
Radius . 2. A negative first lens unit and a positive first lens unit in order from the object side.
A second lens group and a negative third lens group,
The second lens group includes, in order from the object side, a positive second A
The second B lens unit includes a lens unit and a positive second B lens unit.
The distance between the lens group and the third lens group is monotonously reduced, and
The first lens group and the second lens group
The distance between the 2A lens group, the second A lens group and the second
The distance from the B lens group, the 2B lens group and the third lens
Only the distance between the first lens group and the
Wide angle by integrating the second B lens group during zooming
In the zoom lens that performs zooming from the end to the telephoto end, the second A lens unit is located at the wide-angle end to the intermediate focal length.
When approaching the first lens group, moving from the intermediate focal length to the telephoto end
The second lens unit is moved so as to approach the second B lens unit , and focusing from an object point at infinity to a close distance is performed by a second lens unit.
The lens group is moved to the object side, and the first lens group is a single negative lens satisfying the following condition.
Of a zoom lens characterized by
Focusing method. −2.5 <(r ₁ + r ₂ ) / (r ₁ −r ₂ ) <− 0.3 where r ₁ is a negative lens in the first lens group G1.
Body-side radius of curvature r ₂ : image-side curvature of the negative lens in the first lens group G1
Radius . 3. A focusing method according to claim 1 or 2, wherein the zoom lens is characterized in that the focal length f ₁ of the first lens unit satisfy the following condition. 1.2 <| f ₁ | / F _W <1.8 where F _W is the focal length of the entire system at the wide-angle end. 4. The zoom lens focusing method according to claim 3 , wherein the following condition is satisfied. 0.6 <f ₂ / | f ₃ | <1.2 2.5 <β _3T <4.0 where f _{2 is} the focal length of the second lens group G2 at the telephoto end. F _{3 is} the third lens group. Focal length _{β3T of} G3: lateral magnification of the third lens group G3 at the telephoto end. 5. The second lens group includes, in order from the object side,
The second A lens group includes at least two positive lenses and at least one negative lens, and the second B lens group includes at least one positive lens. convex positive lens and at least one focusing system according to claim 1, wherein the zoom lens characterized by being composed of a negative meniscus lens.