JP2908489B2 - Zoom lens and focusing method - Google Patents

Description

The present invention relates to a zoom lens and a focusing method. INDUSTRIAL APPLICABILITY The present invention is applicable to a photographic lens for a 35 mm lens shutter camera and a photographic lens for a video camera.

[Prior Art] Various types of zoom lenses have been conventionally known.
Recently disclosed is a negative, positive and negative three-group configuration disclosed in JP-A-63-25613. This zoom lens performs magnification without a singular point from the wide-angle end to the telephoto end.

The applicant has previously proposed a compact zoom lens having a three-group configuration of negative, positive and negative (Japanese Patent Application No. 63-215967).

[Problem to be Solved by the Invention] The zoom lens disclosed in the above publication has a problem that it is difficult to increase the zoom ratio because the first lens unit is fixed. Further, in the above-mentioned compact zoom lens, the back focus is minimized at the wide-angle end, and the distance between the respective groups is minimized at the telephoto end. Is difficult.

SUMMARY OF THE INVENTION It is an object of the present invention to realize a zoom lens capable of realizing a further shorter focal length while maintaining a longer focal length at the telephoto end in a normal zooming region.

The present invention also includes a half angle of view: a wide angle of about 35 degrees, and approximately 3.3 degrees.
It is an object of the present invention to provide a zoom lens capable of realizing a zoom ratio of up to twice and realizing a compact overall length and diameter from wide angle to telephoto.

It is still another object of the present invention to provide a focusing method having good focusing performance in the entire zoom range of the zoom lens.

[Means for Solving the Problems] Hereinafter, the present invention will be described.

 The invention according to claim 1 is a zoom lens.

As shown in FIG. 1, the zoom lens sequentially moves from the object side (left side in FIG. 1) to the image side (right side in FIG.
A, the second group B, and the third group C are sequentially arranged.

Of the three groups, the first group A has a negative focal length, the second group B has a positive focal length, and the third group C has a negative focal length.

This zoom lens has two magnification ranges for zooming,
That is, it has first and second variable power regions.

In the first zooming area, zooming from the wide-angle side to the telephoto side is performed by moving at least the first lens unit A to the image side.
The second variable power area is continuously connected to the telephoto side of the first variable power area. In the second variable power area, all the groups move toward the object side while changing the distance between each other, so that the wide-angle side changes to the telephoto side. Is changed to. In FIG. 1, the solid line represents the displacement of each group due to zooming. As is clear from the figure, the arrangement of each group is common at the boundary between the first and second variable magnification areas.

In FIG. 1, the horizontal axis (the optical axis direction) is the X direction, and the position of the first unit A on the optical axis is the X direction. In the vertical direction in FIG. 1, f changes the combined focal length of the entire system accompanying zooming. In FIG. 1, the curve representing the displacement of the first group is bent in an inverted shape at the boundary between the first variable power region and the second variable power region. The sign of df / dX is discontinuously inverted at the boundary between the first and second variable power regions. Needless to say, upon zooming from the telephoto side to the wide-angle end, each group reversely follows the displacement shown in FIG. Also, the zoom ratio is more than three times.

Assuming that the focal length of the i-th lens unit (i = 1, 2, 3) is f _i and the focal length of the entire system at the wide-angle end is fw, they are as follows: (I) 0.3 <| f ₃ | / | f ₁ | <1 (II) 1.2 <| f ₁ | / f _W <2.3 (III) 0.9 | f ₃ | / f _W <1.4

A second aspect of the present invention is a focusing method for the zoom lens according to the first aspect, and is characterized by the following points.

That is, focusing is performed by moving the second unit B on the optical axis in the first variable power region, and focusing is performed by moving the first unit A on the optical axis in the second variable power region. In Figure 1
X ₁ and X ₂ indicate the focusing movement amounts of the first unit A and the second unit B, respectively.

[Operation] The above conditions (I) to (III) are the same as the conditions of the small zoom lens disclosed in Japanese Patent Application No. 63-215967.

That is, the condition (I) defines the relationship between the refractive powers of the first and third units. If the upper limit is exceeded, compactness is lost, and if the lower limit is exceeded, the F / No at telephoto increases.

When the value exceeds the upper limit of the condition (II), the F / No at the time of telephoto increases, and the amount of movement of the third lens unit at the time of zooming increases. If the lower limit is exceeded, the second at wide angle,
The distance between the third lens units increases, and the diameter of the third lens units increases, or the amount of peripheral light decreases.

When the value exceeds the upper limit of the condition (III), the telephoto property is deteriorated, and the compactness of the system is lost. Becomes difficult.

The zoom lens according to the present invention, like the small zoom lens disclosed in Japanese Patent Application No. 63-215967, preferably satisfies the following conditions together with the above conditions (I) to (III) in order to perform efficient zooming. .

That is, when the distance between the principal points of the first and second lenses at the time of wide angle is d _12w and the distance between the principal points of the second and third groups is d _23w , these are d _12w / f ₂ <1 (IV) d _23w It is desirable to satisfy the condition of /fw<0.85 (V).

Condition (IV) is to keep the Petzval sum of the whole system properly,
This is an effective condition to keep the system at wide angle small. Condition (V) is to prevent an increase in the ray height of the third lens unit at the time of wide angle, and
This is an effective condition for preventing an increase in the diameter of the group or a shortage of the peripheral light amount.

Now, in a three-group lens system, if the focal length of the first group is f ₁ and the magnifications of the second and third groups are m ₂ and m ₃ , respectively, the focal length f of the entire system is f = f ₁ · m ₂ · m ₃ (1)

The magnifications m ₂ and m ₃ are obtained by setting the refractive power of the _first lens unit to φ ₁ (<0),
The refractive power of the second group is φ ₂ (> 0), the combined refractive power of the first and second groups is φ _1,2 (> 0), the distance between the principal points of the first and second groups is d ₁₂ ,
The distance from the rear principal point of the third group to the image plane of the entire system is bf,
The group is represented by the following equation, where the refractive power is φ ₃ (<0).

m ₂ = φ ₁ / (φ ₁ + φ ₂ -φ ₁ · φ ₂ · d ₁₂ (2) m ₃ = 1 -φ ₃ · bf (3) The above-mentioned compact zoom proposed in Japanese Patent Application No. 63-215967. With a lens, when zooming from the wide-angle side to the telephoto side, d
Decreasing ₁₂ increases | m ₂ | and increasing bf | m ₃
| Is increased. Also performed image surface correction by the second interval of the third group to reduce the d _23. Therefore, it is the bf, is in d _12, d ₂₃ is smaller lens arrangement to the telephoto end it becomes difficult at the wide-angle end when an attempt is made to obtain a large zoom ratio of more than 3 times.

Considering that this small zoom lens further reduces the focal length from the wide-angle end, in this small zoom lens, the zoom ratios of m ₂ and m ₃ are appropriately distributed, and d ₁₂ at the telephoto end.
When the focal length is further reduced from the wide-angle end, the third lens unit comes closer to the image plane than the position at the wide-angle end. For this reason, if it is attempted to further reduce the focal length from the wide-angle end, the lens configuration becomes extremely difficult.

In order to solve this difficulty, the amount of movement of the third lens unit to the image plane side when the focal length is further reduced from the wide-angle end must be reduced.

Assuming that the focal length of the entire system at the wide-angle end is fw, the magnification of the second lens unit is m _2w , and the magnification of the third lens unit is m _3w , the following expression is established from Expression (1).

fw = f ₁ · m _2w · m _3w (4) It is assumed that the focal length is further reduced from this state and the magnification is changed until the focal length of the entire system becomes f′w. The second at this time
Assuming that the magnifications of the group and the third group are m ′ ₂ and m ′ ₃ respectively, f′w,
m ′ ₂ and m ′ ₃ satisfy the following equations.

f′w = f ₁ · m ′ _2w · m ′ _3w (5) Magnification m _2w and focal length f ′ of the second lens unit when the focal length is fw
The ratio between the same magnification m ′ _{2w at} the time of w, that is, the magnification ratio a _2, is the distance between the principal points of the second and third lens groups at the focal length fw of d _12w and the focal length at the focal length f′w. (2) If the principal point interval of the third lens unit is d' _12w , the following expression is obtained from equation (2).

a ₂ = (m _2w / m ' _2w ) = (φ ₁ + φ ₂ -φ ₁・ φ ₂・ d ′ _12w )
/ (Φ ₁ + φ ₂ -φ ₁ · φ ₂ · d _12w ) (6) Magnification m _3w and focal length f ′ of the third lens unit when the focal length is fw
The ratio with respect to the same magnification m ′ _{3w at} the time of w, that is, the zoom ratio a _3, is such that the distance from the rear principal point of the third group to the image plane of the entire system at the focal length fw is bfw, and the focal length f Assuming that the distance from the rear principal point of the third group at the time of 'w to the image plane of the entire system is bf'w, the following is obtained from the equation (3).

a ₃ = (m _3w / m ′ _3w ) = (1−φ ₃ · bfw) / (1−φ ₃ · b
f′w) (7) If Δd _12w = d ′ _12w −d _12w , a ₂ becomes a ₂ = 1− (φ ₁ φ ₂ Δd _12w / φ _12w ) (8) However, in (8), _φ12w is the combined focal length of the first and second lens groups at the focal length fw.

according [Delta] d _12w increases because _{φ 1 · φ 2 · / φ} 12w is negative, the zoom ratio a ₂ is increased monotonously in accordance with equation (8).

Also, if Δbf′w−bfw, a ₃ becomes a ₃ = {(1 / φ ₃ ) −bfw} / {(1 / φ ₃ ) −bfw−Δbfw} (9). When Δbfw approaches 0 from negative, a ₃ monotonously decreases and approaches 1.

The zoom ratio a of the entire system is given by a = (fw / f′w) = a ₂ · a ₃ (10)

Accordingly, when further shortening the focal length from the wide-angle end, it reduces the amount of movement of the image plane side of the third group, reducing the magnification ratio a ₃ of the third group, the zoom ratio a of the entire system is Become smaller. First to keep the magnification ratio a of the entire system to be constant, the increase of distance between the principal points of the second group is increased, is good to increase the zoom ratio a ₂ of the second group. At this time, image plane position correction is performed using the second and third intervals. It is also possible to make the distance between the second and third lens groups constant. In this case, the entire first to third lens groups are moved to perform image plane position correction.

To increase the effect of reducing the amount of movement of the third lens unit toward the image side when further reducing the focal length from the wide-angle end, first and second
It is good to make the increment of the principal point interval of the group as large as possible.

According to the first aspect of the invention, the amount of increase in the interval between the principal points of the first and second units is increased and the amount of movement of the third unit toward the image is reduced until the first unit moves to the object side. . This makes the first
The lens configuration in the variable power range is possible.

 Next, the invention of claim 2 will be described.

As well known, a focusing method for a zoom lens includes a front focusing method and an inner focusing method. However, one of these methods is generally adopted consistently in the entire zooming region.

However, according to the second aspect of the present invention, as described above, the focusing system is made different between the first and second variable power regions, and
An inner focusing method is used in the variable power region, and a front focusing method is used in the second variable power region.

The first variable magnification area is further on the wide angle side from the wide-angle end of the second variable magnification area and can include a wide angle of about half a field angle of 35 degrees. When the front focusing method is adopted, the aperture efficiency decreases when focusing on a short distance, and the negative distortion increases. However, since the second variable magnification area does not include the remarkable wide angle as described above, good focusing can be performed in the second area by employing the front focusing method.

In other words, by adopting the front focusing method in the second variable magnification area, the amount of extension of the first group, that is, the focusing group, is substantially the same for the same photographing distance in the second variable magnification area, so that the focusing mechanism can be easily simplified. Yes, you can take advantage of the front focusing method. In the zoom lens according to the first aspect, since the first lens unit has a negative focal length, the lens diameter does not increase so much even if focusing is performed at a short distance by front focusing in the second variable power range.

Focusing in the first variable power region will be described.

In a three-unit lens system, the focal length of the first group is generally
If f ₁ , the magnification of the second group, and the magnification of the third group are m ₂ and m ₃ respectively,
The focal length F of the entire system is given by the above equation (1).

Further, the magnification m ₃ of the third group is such that the focal length of the third group is f ₃ ,
Assuming that the distance from the rear principal point of the group to the image plane of the entire system is bf, m ₃ = 1− (bf) / f ₃ (11) can be expressed by the above-described equation (3).

In the zoom lens of the present invention, since f ₃ <0 and bf <0, m ₃ > 1 according to equation (11).

In the first zooming range, the relationship between the focal length F of the entire system and the focal length f ₁ of the first group A, F <| a (12) | f _1. Therefore, the magnification m ₂ of the second lens unit B in the first variable magnification area
Becomes as follows.

| M ₂ | = | F / (f ₁ · m ₃ ) | <1 (13) Therefore, when focusing is performed by moving the second unit B on the optical axis in the first variable magnification area, the equation (13) is established. Therefore, the second lens unit B moves to the object side.

In the second variable power range, the distance between the first lens unit A and the second lens unit B is reduced on the telephoto side as shown in FIG. 1, and in the inner focusing method, the moving distance of the second lens unit B is secured on the telephoto side. Is difficult, but there is no such problem in the first variable magnification area.

When the second lens unit B has an aperture, if the second lens unit B moves to the object side and performs focusing, first, the amount of negative distortion generated by the first lens unit A decreases, and the third lens unit C moves. Has the advantage that the amount of positive distortion generated at the point (1) increases, so that the negative distortion at the time of focusing on infinity decreases when focusing on a short distance. Second, the diameter of the entrance pupil moves to the object side. There is an advantage that the aperture efficiency is increased without increasing.

[Examples] Hereinafter, three specific examples will be given.

In each of the embodiments, when zooming from the telephoto end to the wide-angle end of the first zooming region, the distance between the first and second units is increased, and the third unit is slightly moved closer to the image plane side of the entire system. I have. Further, each of these examples satisfies the above conditions (I) to (V).

In each example, r _i is the i-th surface and the curvature radius counted from the object side, d _i is the axial distance between the i-th surface and the i + 1-th surface counted from the object side, N _j and v _j represent the refractive index and Abbe number of the j-th lens.

K ₁ , K ₂ , and K ₃ indicate parameters under the conditions (I), (II), and (III). Further, f represents a combined focal length of the entire system, and ω represents a half angle of view (unit: degree).

The aspherical surface (indicated by an asterisk in the embodiment) represents the distance from the optical axis as Y, the displacement in the optical axis direction as X, and the aspherical coefficients as A ₄ , A ₆ , A ₈ ,
Well-known formula as A ₁₀ Thus, for the adopted aspherical surface, each aspherical surface coefficient is given.

In FIGS. 2, 6, and 10 showing the lens configuration of each embodiment, reference numeral A indicates a first unit, reference numeral B indicates a second unit, and reference numeral C indicates a third unit.

Example 1 f = 30.9~102,1: 2.98~7.66, ω = 37.3~11.9 i r i d i jN j ν j 1 -62.707 1.6 11.6968 55.5 2 25.15 1.0 3 26.914 3.4 21.6727 32.2 4 190.818 Variable 5 21.753 3.0 31.6968 55.5 6 −45.165 1.2 41.84666 23.8 7 −103.608 1.7 8 16.48 2.2 51.58913 61.3 9 70.199 0.9 10 −58.132 3.5 61.834 37.3 11 14.91 1.6 12 37.767 2.5 71.58913 61.3 13 −24.651 Variable 14 −35.226 4.0 81.5927 35.5 15 −15.474 0.8 16 −18.709 1.6 91.7725 49.6 17 −28.541 3.8 18 ^* −14.119 1.4 101.7725 49.6 19 −265.794 4.0 111.7725 28.3 20 −41.557 Variable Aspherical (eighteenth ^{_{surface) A 4 = 8.34986 · 10 -6}} , A 6 = 1.38658 · 10 -7 A 8 = -1.50623 · 10 -9, A 10 = 1.015821 · 10 -11 condition value K ₁ = 0.647, K ₂ = 1.667, K ₃ = 1.078 Incidentally, in example _{1, f 1 = -60.000, f} 2 = 26.00
0, f ₃ = −38.806, and the focal length: fw of the entire system at the wide-angle end of the second area is 36.001 as described above. The values of the parameters of the conditions (I), (II) and (III) are as follows.

| F ₃ | / | f ₁ | = 0.647 | f ₁ | /fw=1.939 | f ₃ | /fw=1.254 FIG. 2 shows a lens configuration diagram of this embodiment at the wide-angle end of the second variable power region. Show.

In the first embodiment, the value of the variable amount when focusing on an object distance of 1 m by the method of claim 2 is as follows.

FIG. 3 shows an object distance of 1 m at the wide-angle end of the first variable magnification area.
2 shows the lens configuration and the on-axis and most peripheral light beams in the focused state.

4 and 5 show aberration diagrams for the first embodiment. FIG. 4A is an aberration diagram at the wide-angle end of the first variable power region at infinity focusing, and FIG. 4B is an aberration diagram at the wide-angle end of the second region at infinity focusing. Figures (C) and (D)
7A and 7B are aberration diagrams at the time of focusing on infinity at the middle and telephoto ends of the second variable power region, respectively. FIG. 5 (A),
(B), (C), and (D) show FIGS. 4 (A), (B),
It is an aberration figure in the state where focusing was carried out to the object distance of 1 m in the state of (C) and (D).

ΔS and ΔM in the figure of astigmatism indicate a sagittal image plane and a meridional image plane, respectively. In the figures of the spherical aberration, the sine condition, and the distortion, d and g represent the d-line and the g-line, respectively. Y 'indicates the image height.

Example 2 f = 30.9~102,1: 2.98~7.63, ω = 37.2~11.9 i r i d i jN j ν j 1 -62.707 1.6 11.6968 55.5 2 23.134 1.2 3 24.394 3.4 21.64769 33.8 4 183.184 Variable 5 24.14 3.0 31.713 53.9 6 −50.0 1.2 41.84666 23.8 7 −132.466 1.7 8 17.138 2.2 51.58913 61.3 9 66.643 0.9 10 −62.259 4.5 61.834 37.3 11 15.206 1.1 12 25.6 3.0 71.58913 61.3 13 −25.67 Variable 14 −55.581 4.0 81.62004 36.3 15 −17.752 1.5 16 −18.748 1.6 91.713 53.9 17 −87.503 4.6 18 −15.624 1.4 101.7725 49.6 19 −62.392 3.0 111.72825 28.3 20 −30.602 Variable Condition values K ₁ = 0.639, K ₂ = 1.611, K ₃ = 1.03 By the way, in the second embodiment, f ₁ = −58.000, f ₂ = 25.70
0, f ₃ = −37.065, and the focal length: fw of the entire system at the wide-angle end of the second area is 36.001 as described above. The values of the parameters of the conditions (I), (II) and (III) are as follows.

| F ₃ | / | f ₁ | = 0.639 | f ₁ | /fw=1.874 | f ₃ | /fw=1.198 FIG. 6 shows a lens configuration diagram of this embodiment at the wide-angle end of the second variable power range. Show.

In the second embodiment, the value of the variable amount when focusing on an object distance of 1 m by the method of claim 2 is as follows.

FIG. 7 shows an object distance of 1 m at the wide-angle end of the first variable magnification area.
2 shows the lens configuration and the on-axis and most peripheral light beams in the focused state.

8 and 9 show aberration diagrams relating to the second embodiment. FIG. 8A is an aberration diagram at the wide-angle end of the first variable power region at infinity focusing, and FIG. 8B is an aberration diagram at the wide-angle end of the second region at infinity focusing. Figures (C) and (D)
7A and 7B are aberration diagrams at the time of focusing on infinity at the middle and telephoto ends of the second variable power region, respectively. FIG. 9 (A),
(B), (C) and (D) show FIGS. 8 (A), (B),
It is an aberration figure in the state where focusing was carried out to the object distance of 1 m in the state of (C) and (D).

Example 3 f = 30.9~102,1: 2.97~7.73, ω = 37~11.9 i r i d i jN j ν j 1 -63.0 1.6 11.6968 55.5 2 21.715 1.3 3 23.58 3.4 21.64769 33.8 4 192.9 Variable 5 23.584 3.0 31.6968 55.5 6 −46.0 1.2 41.84666 23.8 7 −107.601 1.7 8 17.151 2.2 51.58913 61.3 91.0 70.9 10 −58.315 4.2 61.834 37.3 11 15.287 0.9 12 30.694 3.2 71.58913 61.3 13 −24.511 Variable 14 −52.789 4.0 81.5927 35.5 15 ^* −17.689 1.5 16 −19.616 1.6 91.713 53.9 17 −78.448 4.6 18 −16.143 1.4 101.7725 49.6 19 −80.432 3.0 111.72825 28.3 20 −33.488 Variable Aspheric surface (15th surface) A ₆ = −3.56724 · 10 ⁻⁹ , A ₈ = −2.80937 · 10 ⁻¹⁰ Conditional value K ₁ = 0.662, K ₂ = 1.556, K ₃ = 1.029 Second variable power in this embodiment FIG. 10 shows a lens configuration diagram at the wide-angle end of the region.

Incidentally, in Example 2, f ₁ = −56.000, f ₂ = 25.40
0, f ₃ = −37.057, and the focal length: fw of the entire system at the wide-angle end of the second area is 36.0 as described above. Condition (I),
The values of the parameters (II) and (III) are as follows.

| F ₃ | / | f ₁ | = 0.662 | f ₁ | /fw=1.810 | f ₃ | /fw=1.198 In the third embodiment, when focusing is performed to an object distance of 1 m by the method of claim 2, The value of the variable amount is as follows.

FIG. 11 shows an object distance of 1 m at the wide-angle end of the first variable magnification area.
2 shows the lens configuration and the on-axis and most peripheral light beams in the focused state.

12 and 13 show aberration diagrams relating to the third embodiment. FIG. 12A is an aberration diagram at the wide-angle end of the first variable power region at infinity focusing, and FIG. 12B is an aberration diagram at the wide-angle end of the second region at infinity focusing. Figures (C) and (D)
7A and 7B are aberration diagrams at the time of focusing on infinity at the middle and telephoto ends of the second variable power region, respectively. FIG. 13 (A),
(B), (C), and (D) show FIGS. 12 (A), (B),
It is an aberration figure in the state where focusing was carried out to the object distance of 1 m in the state of (C) and (D).

As a comparative example, the front focusing method was also used for the first variable power region in the above-described first to third embodiments, and the object distance was changed.
The group intervals d ₄ and d ₁₃ when focusing to 1 m are shown.

In this comparative example, an example corresponding to the first embodiment is described.
FIG. 14 shows a lens configuration in a state where the first lens unit is moved in the object direction at the wide-angle end of the zooming area at an object distance of 1 m toward the object and focusing and on-axis and outermost peripheral light beams correspond to FIG. FIG. 16 shows a similar diagram in the case of this example, and FIG. 18 shows a similar diagram in the example corresponding to the third embodiment. The aberration diagrams corresponding to FIGS. 14, 16, and 18 are shown in FIGS.
FIG. 19 and FIG.

As is clear from comparison of each embodiment and the corresponding comparative example with an optical path diagram and an aberration diagram,
When the inner focusing method is used in the first variable magnification area as in the invention of the first aspect, distortion is small and the aperture efficiency is large compared to the case where front focusing is used in this area.

[Effects of the Invention] As described above, according to the present invention, a novel zoom lens and a focusing method can be provided. Since the zoom lens has the first variable power region in addition to the second variable power region, which is a normal variable power region, as described above, the wide angle range can be further expanded, and the variable power ratio can be increased. Moreover, compactness is possible. In addition, since the focusing method in each magnification area is the inner focusing method in the first magnification area and the front focusing method in the second magnification area, good focusing can be realized in each area.

[Brief description of the drawings]

FIG. 1 is a view for explaining the displacement and focusing of each group due to zooming in the zoom lens of the present invention, and FIG.
FIG. 3, FIG. 3 is a diagram showing a lens configuration relating to Example 1, FIG. 4, FIG. 5 is an aberration diagram relating to Example 1, FIG.
FIG. 8 is a diagram showing a lens configuration according to Example 2, FIG.
FIG. 9 is an aberration diagram for Example 2, and FIGS.
FIGS. 12, 13 are aberration diagrams for Example 3, FIGS. 14 and 15 are diagrams for explaining a comparative example corresponding to Example 1, and FIGS. Fig. 16, No.
FIG. 17 is a diagram for explaining a comparative example with respect to Example 2,
FIG. 18 and FIG. 19 are diagrams for explaining a comparative example with respect to the third embodiment. A: First group, B: Second group, C: Third group

Claims (2)

(57) [Claims] 1. A lens system according to claim 1, further comprising a first lens unit, a third lens unit, a negative lens unit, a third lens unit, and a negative lens unit. A first variable magnification area in which at least the first lens unit moves to the image side to perform magnification from the wide-angle side to the telephoto side, and further from a telephoto end of the first variable magnification area. A second variable magnification area in which all the units move toward the object side while changing the distance from each other to perform magnification from the wide angle side to the telephoto side, and the position of the first group on the optical axis Where X is the combined focal length of the entire system accompanying the scaling, and the sign of the differential: df / dX is discontinuous at the boundary between the first and second scaling regions. to invert has three times greater zoom ratio, the focal length f _i of the i group (i = 1 to 3), the focal point of the entire system at the wide angle end of the second variable magnification region When the release and f _W, these are conditions: (I) 0.3 <| f 3 | / | f 1 | <1 (II) 1.2 <| f 1 | / f W <2.3 (III) 0.9 | f 3 | A zoom lens that satisfies / f _W <1.4. 2. A focusing method for a zoom lens according to claim 1, wherein focusing is performed by moving the second lens unit on the optical axis in the first variable power region, and the first lens is moved in the second variable power region. A focusing method, wherein a group is moved on an optical axis to perform focusing.