Zoom lens

• US 5,383,058 A
• Filed: 07/20/1993
• Issued: 01/17/1995
• Est. Priority Date: 03/11/1992
• Status: Expired due to Fees

• Current Assignee: Nikon Corporation
• Original Assignee: Nikon Corporation
• Inventors: Yonezawa, Yasuo
• Primary Examiner(s): Sugarman, Scott J.
• Application Number:
  US08/093,723
• Time in Patent Office: 546 Days
• Field of Search: 359/677, 359/687, 359/684, 359/659, 359/656, 359/379, 359/380
• US Class Current: 359/687
• CPC Class Codes: G02B 15/144113 arranged +-++

Abstract

A zoom lens of a four-group structure comprises the following sequentially from the object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power. When a variable power takes place from the wide angle end to the telescopic end, at least the second lens group is driven along the optical axis relatively so that the distance between-the first lens group and second lens group is increased and at the same time, the fourth lens group is driven toward the image side.

Claims

1. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

given the lens group spaces on the axis between said first lens group and said second lens group at the wide angle end and telescopic end as d_12W and d_12T respectively and the back focuses at the wide angle end and telescopic end as Bf_W and Bf_T respectively, said first lens group and said second lens group are structured to satisfy the following conditions;

space="preserve" listing-type="equation">0<

(d.sub.12T -d.sub.12W)/(Bf.sub.W -Bf.sub.T)≦

1, Bf.sub.W >

Bf.sub.T.

2. A zoom lens according to claim 1, wherein

given the magnification of said second lens group at the wide angle end as β

_2W, said second lens group is structured to satisfy the following condition;

space="preserve" listing-type="equation">β

.sub.2W >

1 or β

.sub.2W ≦

-1.

3. A zoom lens according to claim 2, wherein

given the magnification of said fourth lens group at the wide angle end as β

_4W, said fourth lens group is structured to satisfy the following condition;

space="preserve" listing-type="equation">-1<

β

.sub.4W <

1.

4. A zoom lens according to claim 3, wherein

when a zooming is performed, said first lens group is fixed with respect to the image surface, and given the zoom ratio as Z and the magnification of said fourth lens group at the wise angle end as β

_4W, said fourth lens group is structured to satisfy the following condition;

space="preserve" listing-type="equation">β

.sub.4W <

1.8-0.8Z.

5. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

given the lens group spaces on the axis between said first lens group and said second lens group at the wide angle end and telescopic end as d_12W and d_12T respectively and the back focuses at the wide angle end and telescopic end as Bf_W and Bf_T respectively, said first lens group and said second lens group are structured to satisfy the following conditions;

space="preserve" listing-type="equation">0<

(d.sub.12T -d.sub.12W)/(Bf.sub.W -Bf.sub.T)≦

1, Bf.sub.W >

Bf.sub.T ;

and wherein

said first lens group and said third lens group are fixed with respect to the image surface, and said second lens group and said fourth lens group are structured to be driven respectively to the image side when a variable power takes place from the wide angle end to the telescopic end so that the spatial interval on the axis between the lens surface of said second lens group, which is the closest to the image side, and the surface of said fourth lens group, which is the closest to the object side, is increased.

6. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said first lens group is fixed with respect to the image surface, and said second lens group and said fourth lens group are driven respectively to the image side when a variable power takes place from the wide angle end to the telescopic end, and said third lens group is structured to be movable when a variable power takes place from the wide angle end to the telescopic end so as to decrease the space between said second lens group and said third lens group and to increase the space between the lens groups of said fourth lens group and the third lens group.

7. A zoom lens according to claim 6, wherein given the lens group spaces on the axis between said first lens group and said second lens group at the wide angle end and telescopic end as d_12W and d_12T respectively and the back focuses at the wide angle end and telescopic end as Bf_W and Bf_T respectively, said first lens group and said second lens group are structured to satisfy the following conditions:

space="preserve" listing-type="equation">0<

(d.sub.12T -d.sub.12W)/(Bf.sub.W -Bf.sub.T)≦

1, Bf.sub.W >

Bf.sub.T.

8. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said first lens group and said third lens group are driven respectively to the object side, and said second lens group is structured to be movable so as to increase the space between the lens groups of said first lens group and the second lens group and to decrease the space between said third lens group and said second lens group.

9. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said zoom lens is structured in accordance with the data stated in the following table;

space="preserve" listing-type="tabular">______________________________________ No r d ν

.sub.d n.sub.d ______________________________________ 1 105.5724 6.0000 67.87 1.593189 2 -60.2393 2.5000 35.19 1.749501 3 -168.7267 d.sub.3 1.000000 4 -116.6589 3.5000 23.01 1.860741 5 -44.6231 1.6000 58.90 1.518230 6 200.6085 2.4000 1.000000 7 -188.2477 1.6000 55.60 1.696800 8 50.9322 d.sub.8 1.000000 9 150.9884 6.5000 60.14 1.620409 10 -130.0730 3.0000 23.01 1.860741 11 -265.8200 d.sub.11 1.000000 12 193.7822 8.0000 67.87 1.593189 13 -81.7527 3.0000 23.01 1.860741 14 -121.7309 B.f. 1.000000 ______________________________________ Face-to-Face Distance f = 200 300 400 ______________________________________ d.sub.3 44.15413 68.83672 74.38978 d.sub.8 33.79640 9.11381 3.56075 d.sub.11 30.66645 101.04194 150.90573 B.f. 159.48675 89.11127 39.24747 Magnification -1.562 -1.345 -0.808 of Pupil (d.sub.12T - d.sub.12W)/(Bf.sub.W - Bf.sub.T) = 0.251 β

.sub.2W = -1.54 β

.sub.4W = -0.1 ______________________________________

where the number on the left-hand side represents the order from the object side;

r, the curvature radius of the lens plane;

d, the space between the lens surface;

ν

_d, the Abbe'"'"'s number;

n_d, the refractive index at d line (λ

=587.6 n);

f, the focal length of the total system;

d_12W, the space on the axis between the lens groups of the first lens group and the second lens group at the wide angle end;

d_12T, the space on the axis between the lens groups of the first lens group and the second lens group at the telescopic end;

Bf_W, the back focus of the zoom lens at the wide angle end;

Bf_T, the back focus of the zoom lens at the telescopic end;

β

_2W, the magnification of the second lens group at the wide angle end; and

β

_4W, the magnification of the fourth lens group at the wide angle end.

10. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said zoom lens is structured in accordance with the data stated in the following table;

space="preserve" listing-type="tabular">______________________________________ No r d ν

.sub.d n.sub.d ______________________________________ 1 105.5724 6.0000 67.87 1.593189 2 -60.2393 2.5000 35.19 1.749501 3 -168.7267 d.sub.3 1.000000 4 -116.6589 3.5000 23.01 1.860741 5 -44.6231 1.6000 58.90 1.518230 6 200.6085 2.4000 1.000000 7 -188.2477 1.6000 55.60 1.696800 8 50.9322 d.sub.8 1.000000 9 150.9884 6.5000 60.14 1.620409 10 -130.0730 3.0000 23.01 1.860741 11 -265.8200 d.sub.11 1.000000 12 193.7822 8.0000 67.87 1.593189 13 -81.7527 3.0000 23.01 1.860741 14 -121.7309 B.f. 1.000000 ______________________________________ Face-to-Face Distance f = 200 300 400 ______________________________________ d.sub.3 44.15413 70.94020 74.73168 d.sub.8 33.79640 10.09640 6.79648 d.sub.11 30.66645 100.04722 149.62641 B.f. 159.48675 78.01988 36.95930 Magnification -1.562 -1.049 -0.783 of Pupil (d.sub.12T - d.sub.12W)/(Bf.sub.W - Bf.sub.T) = 0.249 β

.sub.2W = -1.54 β

.sub.4W = -0.1 ______________________________________

where the number on the left-hand side represents the order from the object side;

r, the curvature radius of the lens plane;

d, the space between the lens surfaces;

ν

_d, the Abbe'"'"'s number;

n_d, the refractive index at d line (λ

=587.6 n);

f, the focal length of the total system;

d_12W, the space on the axis between the lens groups of the first lens group and the second lens group at the wide angle end;

d_12T, the space on the axis between the lens groups of the first lens group and the second lens group at the telescopic end;

Bf_W, the back focus of the zoom lens at the wide angle end;

Bf_T, the back focus of the zoom lens at the telescopic end;

β

_2W, the magnification of the second lens group at the wide angle end; and

β

_4W, the magnification of the fourth lens group at the wide angle end.

11. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said zoom lens is structured in accordance with the data stated in the following table;

space="preserve" listing-type="tabular">______________________________________ No r d ν

.sub.d n.sub.d ______________________________________ 1 105.5724 6.0000 67.87 1.593189 2 -60.2393 2.5000 35.19 1.749501 3 -168.7267 d.sub.3 1.000000 4 -116.6589 3.5000 23.01 1.860741 5 -44.6231 1.6000 58.90 1.518230 6 200.6085 2.4000 1.000000 7 -188.2477 1.6000 55.60 1.696800 8 75.9411 d.sub.8 1.000000 9 200.0000 6.5000 60.14 1.620409 10 -130.0730 3.0000 23.01 1.860741 11 -369.7622 d.sub.11 1.000000 12 193.7822 8.0000 67.87 1.593189 13 -81.7527 3.0000 23.01 1.860741 14 -122.1353 B.f. 1.000000 ______________________________________ Face-to-Face Distance f = 200 300 400 ______________________________________ d.sub.3 46.98934 68.07221 69.31558 d.sub.8 38.81539 18.81539 3.81539 d.sub.11 30.30581 106.85922 161.42866 B.f. 125.67279 48.03649 7.22368 Magnification -1.549 -1.031 -0.775 of Pupil (d.sub.12T - d.sub.12W)/(Bf.sub.W - Bf.sub.T) = 0.188 β

.sub.2W = -4.33 β

.sub.4W = 0.133 ______________________________________

where the number on the left-hand side represents the order from the object side;

r, the curvature radius of the lens plane;

d, the space between the lens surfaces;

ν

_d, the Abbe'"'"'s number;

n_d, the refractive index at d line (λ

=587.6 n);

f, the focal length of the total system;

d_12W, the space on the axis between the lens groups of the first lens group and the second lens group at the wide angle end;

d_12T, the space on the axis between the lens groups of the first lens group and the second lens group at the telescopic end;

Bf_W, the back focus of the zoom lens at the wide angle end;

Bf_T, the back focus of the zoom lens at the telescopic end;

β

_2W, the magnification of the second lens group at the wide angle end;

β

_4W, the magnification of the fourth lens group at the wide angle end.

12. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said zoom lens is structured in accordance with the data stated in the following table;

space="preserve" listing-type="tabular">______________________________________ No r d ν

.sub.d n.sub.d ______________________________________ 1 105.5724 6.0000 67.87 1.593189 2 -60.2393 2.5000 35.19 1.749501 3 -168.7267 d.sub.3 1.000000 4 -116.6589 3.5000 23.01 1.860741 5 -44.6231 1.6000 58.90 1.518230 6 200.6085 2.4000 1.000000 7 -188.2477 1.6000 55.60 1.696800 8 37.5395 d.sub.8 1.000000 9 150.9884 6.5000 60.14 1.620409 10 -130.0730 3.0000 23.01 1.860741 11 -173.8523 d.sub.11 1.000000 12 193.7822 8.0000 67.87 1.593189 13 -81.7527 3.0000 23.01 1.860741 14 -113.6289 B.f. 1.000000 ______________________________________ Face-to-Face Distance f = 200 300 400 ______________________________________ d.sub.3 46.39362 74.96337 80.18432 d.sub.8 28.15501 16.15501 4.65501 d.sub.11 31.38065 92.71984 136.52271 B.f. 175.56714 97.65820 60.14336 Magnification -1.463 -0.984 -0.741 of Pupil (d.sub.12T - d.sub.12W)/(Bf.sub.W - Bf.sub.T) = 0.293 β

.sub.2W = -1 β

.sub.4W = -0.297 ______________________________________

where the number on the left-hand side represents the order from the object side;

r, the curvature radius of the lens plane;

d, the space between the lens surfaces;

ν

_d, the Abbe'"'"'s number;

n_d, the refractive index at d line (λ

=587.6 n);

f, the focal length of the total system;

d_12W, the space on the axis between the lens groups of the first lens group and the second lens group at the wide angle end;

d_12T, the space on the axis between the lens groups of the first and the second lens group at the telescopic end;

Bf_W, the back focus of the zoom lens at the wide angle end;

Bf_T, the back focus of the zoom lens at the telescopic end;

β

_2W, the magnification of the second lens group at the wide angle end; and

β

_4W, the magnification of the fourth lens group at the wide angle end.

13. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said zoom lens is structured in accordance with the data stated in the following table;

space="preserve" listing-type="tabular">______________________________________ No r d ν

.sub.d n.sub.d ______________________________________ 1 105.5724 6.0000 67.87 1.593189 2 -60.2393 2.5000 35.19 1.749501 3 -168.7267 d.sub.3 1.000000 4 -116.6589 3.5000 23.01 1.860741 5 -44.6231 1.6000 58.90 1.518230 6 200.6085 2.4000 1.000000 7 -188.2477 1.6000 55.60 1.696800 8 50.9322 d.sub.8 1.000000 9 150.9884 6.5000 60.14 1.620409 10 -130.0730 3.0000 23.01 1.860741 11 -265.8208 d.sub.11 1.000000 12 193.7822 8.0000 67.87 1.593189 13 -81.7527 3.0000 23.01 1.860741 14 -121.7309 B.f. 1.000000 ______________________________________ Face-to-Face Distance f = 200 300 400 ______________________________________ d.sub.3 44.15413 68.80962 72.48519 d.sub.8 33.79640 17.79640 6.79640 d.sub.11 30.66645 110.38910 163.46280 B.f. 159.48675 86.80325 50.05353 Magnification -1.562 -0.961 -0.690 of Pupil (d.sub.12T - d.sub.12W)/(Bf.sub.W - Bf.sub.T) = 0.259 β

.sub.2W = -1.54 β

.sub.4W = -0.1 ______________________________________

where the number on the left-hand side represents the order from the object side;

r, the curvature radius of the lens plane;

d, the space between the lens surfaces;

ν

_d, the Abbe'"'"'s number;

n_d, the refractive index at d line (λ

=587.6 n);

f, the focal length of the total system;

d_12W, the space on the axis between the lens groups of the first lens group and the second lens group at the wide angle end;

d_12T, the space an the axis between the lens groups of the first and the second lens group at the telescopic end;

Bf_W, the back focus of the zoom lens at the wide angle end;

Bf_T, the back focus of the zoom lens at the telescopic end;

β

_2W, the magnification of the second lens group at the wide angle end; and

β

_4W, the magnification of the fourth lens group at the wide angle end.

14. A zoom lens of a four-group structure, comprising the following sequentially from the object side:

a first lens group having positive refracting power;

a second lens group having negative refracting power, which is capable of being driven along the optical axis to increase the space between said first lens group and said second lens group when a variable power takes place from the wide angle end to the telescopic end;

a third lens group having positive refracting power; and

a fourth lens group having positive refracting power, which is capable of being driven to the image side when a variable power takes place from the wide angle end to the telescopic end to increase the space between said third lens group and said fourth lens group;

wherein

said zoom lens is structured in accordance with the data stated in the following table;

space="preserve" listing-type="tabular">______________________________________ No r d ν

.sub.d n.sub.d ______________________________________ 1 105.5724 6.0000 67.87 1.593189 2 -60.2393 2.5000 35.19 1.749501 3 -168.7267 d.sub.3 1.000000 4 -116.6589 3.5000 23.01 1.860741 5 -44.6231 1.6000 58.90 1.518230 6 200.6085 2.4000 1.000000 7 -188.2477 1.6000 55.60 1.696800 8 50.9322 d.sub.8 1.000000 9 150.9884 6.5000 60.14 1.620409 10 -130.0730 3.0000 23.01 1.860741 11 -236.0883 d.sub.11 1.000000 12 193.7822 8.0000 67.87 1.593189 13 -81.7527 3.0000 23.01 1.860741 14 -124.1565 B.f. 1.000000 ______________________________________ Face-to-Face Distance f = 200 300 400 ______________________________________ d.sub.3 44.17310 70.90362 74.24344 d.sub.8 31.23069 17.65277 5.58476 d.sub.11 31.01067 100.78910 150.47905 B.f. 155.19381 72.26278 31.30183 Magnification -1.661 -1.108 -0.831 of Pupil (d.sub.12T - d.sub.12W)/(Bf.sub.W - Bf.sub.T) = 0.243 β

.sub.2W = -1.54 β

.sub.4W = -0.0497 ______________________________________

where the number on the left-hand side represents the order from the object side;

r, the curvature radius of the lens plane;

d, the space between the lens surfaces;

ν

_d, the Abbe'"'"'s number;

n_d, the refractive index at d line (λ

=587.6 n);

f, the focal length of the total system;

d_12W, the space on the axis between the lens groups of the first lens group and the second lens group at the wide angle end;

d_12T, the space an the axis between the lens groups of the first and the second lens group at the telescopic end;

Bf_W, the back focus of the zoom lens at the wide angle end;

Bf_T, the back focus of the zoom lens at the telescopic end;

β

_2W, the magnification of the second lens group at the wide angle end; and

β

_4W, the magnification of the fourth lens group at the wide angle end.

15. A microscope optical system for observing a specimen comprising an objective optical system including a first objective lens system for condensing light from the specimen and a second objective lens system for collecting the light passing through said first objective lens system to form an image of the specimen, said second objective lens system including a zoom optical system which changes a magnification of the image of the specimen while correcting a change of a position of the image of the specimen and a change of a position of a pupil of said objective optical system.

16. A microscope optical system according to claim 15, wherein said zoom optical system contains a plurality of moving lens groups which move along the optical axis.

17. A microscope optical system according to claim 15, wherein said zoom optical system includes a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power, and wherein when the zooming is performed from the wide angle end to the telephoto end, said zoom optical system changes in order that an air spacing between said first lens group and said second lens group increases, thus resulting in increasing an air spacing between said third lens group and said fourth lens group.

18. A microscope optical system according to claim 17, wherein when an axial distance between said first lens group and said second lens group at the wide angle end and at the telephoto end are designated by d_12W and d_12T, respectively, and backfocus at the wide angle end and at the telephoto end are designated by Bf_W and Bf_T, respectively, the microscope optical system satisfies the following conditions:

space="preserve" listing-type="equation">0<

(d.sub.12T -d.sub.12W)/(Bf.sub.W -Bf.sub.T)≦

1, Bf.sub.W >

Bf.sub.T.

19. A microscope optical system according to claim 18, wherein when the zooming is performed, said second and fourth lens groups move along the optical axis.

20. A microscope optical system according to claim 18, wherein when the zooming is performed, said second, third and fourth lens groups move along the optical axis.

21. A microscope optical system according to claim 18, wherein when a magnification of said second lens group at the wide angle end is designated by β

_2W, the microscope optical system satisfies the condition;

space="preserve" listing-type="equation">β

.sub.2W >

1 or β

.sub.2W ≦

-1.

22. A microscope optical system according to claim 21, wherein when a magnification of said fourth lens group at the wide angle end is designated by β

_4W, the microscope optical system satisfies the condition;

space="preserve" listing-type="equation">-1<

β

.sub.4W <

1.

23. A microscope optical system according to claim 22, wherein when the zooming is performed, said first lens group is fixed with respect to an image surface, and wherein when zoom ratio is designated by Z, and a magnification of said fourth lens group at the wide angle end is designated by β

_4W, the microscope optical system satisfies the condition;

space="preserve" listing-type="equation">β

.sub.4W <

1.8-0.8 Z.

24. A zoom lens comprising a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power;

wherein zooming is performed by changing a distance between said first lens group and said second lens group and by changing a distance between said third lens group and said fourth lens group; and

wherein when axial distances between said first and second lens groups at the wide angle end and at the telephoto end are designated by d_12W and d_12T, respectively, and backfocus at the wide angle end and the telephoto end is designated by Bf_W and Bf_T, respectively, the zoom lens satisfies the conditions;

space="preserve" listing-type="equation">0<

(d.sub.12T -d.sub.12W)/(Bf.sub.W -Bf.sub.T)≦

1, Bf.sub.W >

Bf.sub.T.

25. A zoom lens according to claim 24, wherein when a magnification of said second lens group at the wide angle end is designated by β

_2W, the zoom lens satisfies the condition;

space="preserve" listing-type="equation">β

.sub.2W >

1 or β

.sub.2W ≦

-1.

26. A zoom lens according to claim 25, wherein when a magnification of said fourth lens group at the wide angle end is designated by β

_4W, the zoom lens satisfies the condition;

space="preserve" listing-type="equation">-1<

β

.sub.4W <

1.

27. A zoom lens according to claim 26, wherein when the zooming is performed, said first lens group is fixed with respect to an image surface, and wherein when zoom ratio is designated by Z, and a magnification of said fourth lens group at the wide angle end is designated by β

_4W, the zoom lens satisfies the condition;

space="preserve" listing-type="equation">β

.sub.4W <

1.8-0.8 Z.