# Patent

## Title
Afocal zoom lens for microscopes

## Abstract

An afocal zoom lens for microscopes includes five lens units having, in order from the object side, positive, negative, negative, positive, and negative refracting powers. When the magnification of the afocal zoom lens is changed in the range from a low-magnification position to a high-magnification position, a first lens unit and a fifth lens unit are fixed, a second lens unit is moved toward the image side, and a fourth lens unit is moved toward the object side. The afocal zoom lens satisfies the following condition:

0≦F2/F3≦2.5

where the focal length of the second lens unit is represented by F2 and the focal length of a third lens unit is represented by F3. The third lens unit is constructed with at least one negative lens with a concave surface facing the object side.

## Background
This application claims priority to Japanese Patent Application No. 2003-325101 filed 17 Sep. 2003, the content of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an afocal zoom lens for microscopes.
2. Description of Related Art
Conventional afocal zoom lenses for microscopes are proposed, for example, by Japanese Patent Publication No. Hei 0677104 and Japanese Patent Kokai Nos. Hei 11-95009 and 2001-154092.
SUMMARY OF THE INVENTION
The afocal zoom lens for microscopes according to the present invention includes five lens units having, in order from the object side, positive, negative, negative, positive, and negative refracting powers. The afocal zoom lens is constructed so that when the magnification of the afocal zoom lens is changed in the range from a low-magnification position to a high-magnification position, a first lens unit and a fifth lens unit are fixed, a second lens unit is moved toward the image side, and a fourth lens unit is moved toward the object side.
When the focal length of the second lens unit is represented by F2 and the focal length of a third lens unit is represented by F3, the afocal zoom lens for microscopes according to the present invention satisfies the following condition:

0≦F2/F3≦2.5  (1)

The afocal zoom lens for microscopes according to the present invention is such that the third lens unit is constructed with at least one negative lens with a concave surface facing the object side.
According to the present invention, the afocal zoom lens, in contrast with the conventional afocal zoom lens of a four-lens-unit arrangement, is capable of reducing the overall length and achieving a high variable magnification ratio exceeding 15 and correction for aberration in the whole variable magnification range.
According to the present invention, the third lens unit is constructed as a moving lens unit, and thereby the performance of correction for aberration is further improved in the whole variable magnification range. At the same time, the shift of the position of an exit pupil caused when the magnification is changed in the range from the low-magnification position to the high-magnification is suppressed and a reduction of the amount of marginal light can be obviated, so that a systematic property of the optical system is improved.
These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B, and 1C are sectional views showing optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of a first embodiment of the afocal zoom lens according to the present invention;
FIGS. 2A, 2B, and 2C are diagrams showing aberration characteristics at the low-magnification position of the afocal zoom lens in the first embodiment;
FIGS. 2D, 2E, and 2F are diagrams showing aberration characteristics at the middle position of the afocal zoom lens in the first embodiment;
FIGS. 2G, 2H, and 21 are diagrams showing aberration characteristics at the high-magnification position of the afocal zoom lens in the first embodiment;
FIGS. 3A, 3B, and 3C are sectional views showing optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of a second embodiment of the afocal zoom lens according to the present invention;
FIGS. 4A, 4B, and 4C are diagrams showing aberration characteristics at the low-magnification position of the afocal zoom lens in the second embodiment;
FIGS. 4D, 4E, and 4F are diagrams showing aberration characteristics at the middle position of the afocal zoom lens in the second embodiment;
FIGS. 4G, 4H, and 4I are diagrams showing aberration characteristics at the high-magnification position of the afocal zoom lens in the second embodiment;
FIGS. 5A, 5B, and 5C are sectional views showing optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of a third embodiment of the afocal zoom lens according to the present invention;
FIGS. 6A, 6B, and 6C are diagrams showing aberration characteristics at the low-magnification position of the afocal zoom lens in the third embodiment;
FIGS. 6D, 6E, and 6F are diagrams showing aberration characteristics at the middle position of the afocal zoom lens in the third embodiment;
FIGS. 6G, 6H, and 6I are diagrams showing aberration characteristics at the high-magnification position of the afocal zoom lens in the third embodiment;
FIGS. 7A, 7B, and 7C are sectional views showing optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of a fourth embodiment of the afocal zoom lens according to the present invention;
FIGS. 8A, 8B, and 8C are diagrams showing aberration characteristics at the low-magnification position of the afocal zoom lens in the fourth embodiment;
FIGS. 8D, 8E, and 8F are diagrams showing aberration characteristics at the middle position of the afocal zoom lens in the fourth embodiment;
FIGS. 8G, 8H, and 8I are diagrams showing aberration characteristics at the high-magnification position of the afocal zoom lens in the fourth embodiment;
FIGS. 9A, 9B, and 9C are sectional views showing optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of a fifth embodiment of the afocal zoom lens according to the present invention;
FIGS. 10A, 10B, and 10C are diagrams showing aberration characteristics at the low-magnification position of the afocal zoom lens in the fifth embodiment;
FIGS. 10D, 10E, and 10F are diagrams showing aberration characteristics at the middle position of the afocal zoom lens in the fifth embodiment;
FIGS. 10G, 10H, and 10I are diagrams showing aberration characteristics at the high-magnification position of the afocal zoom lens in the fifth embodiment;
FIG. 11 is a sectional view showing an example of an arrangement, developed along the optical axis, of an imaging lens combined with the afocal zoom lens of each of the above embodiments; and
FIGS. 12A and 12B are conceptual views showing the entire arrangements of stereomicroscope optical systems, each having the afocal zoom lens of each embodiment and the imaging lens of FIG. 11.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before undertaking the description of the embodiments, the function and effect of the present invention will be explained.
The afocal zoom lens, as in the present invention, includes five lens units having, in order from the object side, positive, negative, negative, positive, and negative refracting powers. When the magnification is changed in the range from the low-magnification position to the high-magnification position, the first lens unit and the fifth lens unit remain fixed, the second lens unit is moved from the object side toward the image side, and the fourth lens unit is moved from the image side toward the object side.
In contrast to the conventional afocal zoom lens of the four-lens-unit arrangement in which the second lens unit has a variable magnification function, the afocal zoom lens of the present invention is such that two lens units, the second and third lens units, have the variable magnification functions, not the negative refracting power of the second lens unit alone. According to this arrangement of the present invention, the negative refracting power of the variable magnification function is strengthened and thereby a high variable magnification ratio can be obtained. Moreover, the five-lens-unit arrangement is constructed so that the variable magnification functions are shared between two negative refracting powers of the second and third lens units, and thereby the high magnification ratio exceeding 15 and a compact design can be achieved simultaneously. Also, the third lens unit may be fixed when the magnification is changed, but when it is designed to move, the performance of correction for aberration is further improved in the whole variable magnification range. At the same time, the shift of the position of the exit pupil caused when the magnification is changed in the range from the low-magnification position to the high-magnification position is suppressed and a reduction of the amount of marginal light, due to eclipse, can be obviated, so that the systematic property of the optical system is improved.
Condition (1) is provided for the purpose of constructing the afocal zoom lens so that aberration can be properly corrected in the whole zoom range.
Below the lower limit of Condition (1), the variable magnification ratio cannot be increased and correction for aberration at the high-magnification position becomes difficult.
Beyond the upper limit of Condition (1), the refracting power of the third lens unit is so high that correction for aberration at the low-magnification position becomes difficult. Alternatively, the overall length of the afocal zoom lens is increased, which is unfavorable.
In the afocal zoom lens for microscopes according to the present invention, when the focal length of the first lens unit is denoted by F1, a combined focal length of the second and third lens units at the low-magnification position is denoted by F23L, and a combined focal length of the second and third lens units at the high-magnification position is denoted by F23H, the focal zoom lens satisfies one of the following conditions:
Since the second lens unit is moved along the optical axis when the magnification is changed, the combined focal length of the second and third lens units varies with the low-magnification position and the high-magnification position.

1≦F23L/F23H≦3  (2)

3≦|F1/F23L|≦15  (3)

When Condition (2) is satisfied, the combined focal length of the second and third lens units at the low-magnification position becomes shorter than that of the second and third lens units at the high-magnification position. Thus, the negative refracting power of the variable magnification function is strengthened at the low-magnification position so that the focal length can be made shorter at the low-magnification position.
In addition, since the negative refracting powers of the second and third lens units at the high-magnification position and the low-magnification position are optimized in a good balance, it becomes possible to further correct aberrations at the low- and high-magnification positions.
When Condition (3) is satisfied, the first lens unit and the second lens unit do not interfere with each other at the low-magnification position, and off-axis aberration produced at the low-magnification position and spherical aberration at the high-magnification position can be favorably corrected.
Below the lower limit of Condition (2), the negative refracting power of the third lens unit is weakened and the refracting power of the second lens unit is strengthened. Consequently, the performance of correction for aberration is degraded at the low-magnification position, or the overall length of the afocal zoom lens is increased.
Beyond the upper limit of Condition (2), the negative refracting power of the second lens unit or the third lens unit is so high that correction for aberration in the whole zoom range becomes difficult, which is unfavorable.
Below the lower limit of Condition (3), it becomes difficult to obtain the variable magnification ratio exceeding 15, or the first lens unit and the second lens unit interfere with each other at the low-magnification position. This is unfavorable.
Beyond the upper limit of Condition (3), the performance of correction for off-axis aberration at the low-magnification position or for axial aberration at the high-magnification position is degraded, which is undesirable.
In the afocal zoom lens of the present invention, when a distance from the object-side surface of the first lens unit to the object-side surface of the third lens unit at the low-magnification position is represented by D3L and the overall length of the afocal zoom lens is represented by D0, it is more desirable to satisfy the following condition:

0.3≦D3L/D0≦0.7  (4)

When Condition (4) is satisfied, the amount of movement of each of the second and fourth lens units moved in the magnification change is optimized. At the same time, the fourth lens unit ceases to interfere with the fifth lens unit at the low-magnification position, and each of the second lens unit and the fourth lens unit ceases to interfere with the third lens unit at the high-magnification position. In this way, the high variable magnification ratio and a compact design of the afocal zoom lens can be achieved.
Beyond the upper limit of Condition (4), the amount of movement of the fourth lens unit is reduced to lower the variable magnification ratio, or the fourth lens unit interferes with the fifth lens unit. Alternatively, the refracting power of the fourth lens unit is increased and correction for spherical aberration becomes difficult at the low-magnification position.
Below the lower limit of Condition (4), the amount of movement of the second lens unit is reduced to lower the variable magnification ratio, or the second lens unit interferes with the first lens unit or the third lens unit. Alternatively, the refracting power of the second lens unit is strengthened, and off-axis aberration at the low-magnification position and spherical aberration at the high-magnification position are deteriorated, which is undesirable.
When the third lens unit, as in the afocal zoom lens of the present invention, is constructed with at least one negative lens with a concave surface facing the object side, astigmatism and coma can be effectively corrected in the whole zoom range.
In the optical arrangement mentioned above, when the focal length of the third lens unit is denoted by F3, the radius of curvature of the concave surface facing the object side is denoted by RG3, the refractive index of the negative lens is denoted by ng3, and the variable magnification ratio is denoted by Z, it is desirable to satisfy the following conditions:

7≦RG3/F3·Z≦30  (5)

1.65≦ng3  (6)

When Condition (5) is satisfied, a proper bending shape of each surface of the third lens unit corresponding to the variable magnification ratio exceeding 10 is given and coma can be favorably corrected in the whole zoom range.
When Condition (6) is satisfied, the Petzval sum is suppressed, which is favorable for correcting curvature of field.
Beyond the upper limit of Condition (5), the radius of curvature of the image-side surface of the negative lens in the third lens unit becomes small, and spherical aberration and come, produced by the negative lens in the third lens unit are extremely increased in the range from the low-magnification position to the middle position. This is unfavorable.
Below the lower limit of Condition (5), the radius of curvature of the concave surface facing the object side, of the negative lens in the third lens unit becomes small, and spherical aberration and coma, produced by the negative lens in the third lens unit are extremely increased in the whole zoom range. This is unfavorable.
Below the lower limit of Condition (6), curvature of field is deteriorated and the radius of curvature of the concave surface facing the object side becomes so small that off-axis aberration at the low-magnification position and spherical aberration produced in the range from the middle position to the high-magnification position are deteriorated. This is unfavorable.
Additionally, in the afocal zoom lens of the present invention, it is desirable that an aperture stop is interposed between the second lens unit and the fourth lens unit, because eclipse caused in each zoom region can be properly obviated.
EMBODIMENTS
Subsequently, the embodiments of the present invention will be described with reference to the drawings.
The afocal zoom lens of the present invention is used in combination with an imaging lens shown in FIG. 11 and Numerical data 6 to be described later. The focal length of the imaging lens is 211.675 mm and a prism optical system is located on the image side of the imaging lens.
The afocal zoom lens of the present invention, as show in FIG. 12A, is capable of constructing a stereomicroscope optical system of a Galilean type (a parallel optical system) by combining an objective lens 2, an imaging lens optical system 4, and an eyepiece 5. Alternatively, as shown in FIG. 12B, a microscope zoom optical system can be constructed by combining the objective lens 2 of an infinity-corrected optical system, an afocal zoom lens 3 of the present invention, the imaging lens optical system 4, and the eyepiece 5.
First Embodiment
FIGS. 1A, 1B, and 1C show optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of the first embodiment of the afocal zoom lens according to the present invention. FIGS. 2A–2C, 2D–2F, and 2G–2I show aberration characteristics at the low-magnification, middle, and high-magnification positions, respectively, of the afocal zoom lens in the first embodiment.
The afocal zoom lens of the first embodiment comprises, in order from the object side, a first lens unit G1 with positive refracting power, a second lens unit G2 with negative refracting power, a third lens unit G3 with negative refracting power, an aperture stop S, a fourth lens unit G4 with positive refracting power, and a fifth lens unit G5 with negative refracting power.
The first lens unit G1 includes, in order from the object side, a cemented lens component of a biconvex lens L11 and a negative meniscus lens L12 with a concave surface facing the object side, and a plano-convex lens L13 whose object-side surface is convex and whose image-side surface is flat.
The second lens unit G2 includes, in order from the object side, a cemented lens component of a biconvex lens L21 and a biconcave lens L22, and a biconcave lens L23.
The third lens unit G3 is constructed with a biconcave lens L31.
The fourth lens unit G4 includes a biconvex lens L41 and a cemented lens component of a negative meniscus lens L42 with a convex surface facing the object side and a biconvex lens L43.
The fifth lens unit G5 is constructed with a cemented lens component of a positive meniscus lens L51 with a concave surface facing the object side and a biconcave lens L52.
When the magnification is changed in the range from the low-magnification position to the high-magnification position, the first lens unit G1, the third lens unit G3, the aperture stop S, and the fifth lens unit G5 remain fixed, the second lens unit G2 is moved from the object side toward the image side, and the fourth lens unit G4 is moved from the image side toward the object side.
Subsequently, numerical data of optical members constituting the afocal zoom lens of the first embodiment are shown below.
Also, in the numerical data of the first embodiment, r1, r2, . . . denote radii of curvature of surfaces of individual lenses; d1, d2, . . . denote thicknesses of individual lenses or air spacings between them; nd1, nd2, . . . denote refractive indices of individual lenses at the d line; νd1, νd2, . . . denote Abbe's numbers of individual lenses; F1, F2, . . . denote focal lengths of individual lenses; FL denotes of the focal length of the entire system; and NA denotes a numerical aperture.
These symbols are also used for the numerical data of the other embodiments to be described later.










Numerical data 1


















r1 = 67.1073
d1 = 4.15
nd1 = 1.497
νd1 = 81.54


r2 = −48.4555
d2 = 2
nd2 = 1.71736
νd2 = 29.52


r3 = −182.742
d3 = 0.25


r4 = 42.6536
d4 = 2.9
nd4 = 1.56907
νd4 = 71.3


r5 = ∞
d5 = D5


r6 = 24.9018
d6 = 4.05
nd6 = 1.76182
νd6 = 26.52


r7 = −27.9083
d7 = 1.5
nd7 = 1.72916
νd7 = 54.68


r8 = 15.5066
d8 = 2.0814


r9 = −37.4483
d9 = 1.8
nd9 = 1.755
νd9 = 52.32


r10 = 33.1991
d10 = D10


r11 = −23.7456
d11 = 1.83
nd11 = 1.7847
νd11 = 26.29


r12 = 103.6318
d12 = 1


r13 = ∞ (Aperture stop)
d13 = D13


r14 = 162.471
d14 = 2.6
nd14 = 1.603
νd14 = 65.44


r15 = −42.4593
d15 = 0.2


r16 = 47.3698
d16 = 1.6
nd16 = 1.834
νd16 = 37.16


r17 = 24.9751
d17 = 3.1
nd17 = 1.497
νd17 = 81.54


r18 = −53.5454
d18 = D18


r19 = −55.8574
d19 = 1.6
nd19 = 1.7552
νd19 = 27.51


r20 = −31.8024
d20 = 1.5
nd20 = 1.741
νd20 = 52.64


r21 = 746.0709










Zoom data














Low-magnification
Middle
High-magnification





FL
54 mm
360 mm
864 mm


NA
0.03195
0.02533
0.01273


Overall length
95 mm
 95 mm
 95 mm


Exit end
−359.0
−238.7
−220.6


position


D5
0.367
20.459
24.018


D10
25.963
5.871
2.313


D13
36.010
21.627
4.948


D18
0.498
14.881
31.560










Variable magnification ratio: 16







Focal lengths













F1 =
50.55



F2 =
−19.57



F3 =
−24.47



F4 =
32.47



F5 =
−71.00



F1/F3 =
−2.07



F2/F3 =
0.80



F4/F3 =
−1.33



F5/F3 =
2.90



F23L =
−6.91



F23H =
−10.49



F23L/F23H =
1.52



|F1/F23L| =
7.32



D3L/D0 =
0.47



(RG3/F3) · Z =
15.53



ng3 =
1.7847










Second Embodiment
FIGS. 3A, 3B, and 3C show optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of the second embodiment of the afocal zoom lens according to the present invention. FIGS. 4A–4C, 4D–4F, and 4G–4I show aberration characteristics at the low-magnification, middle, and high-magnification positions, respectively, of the afocal zoom lens in the second embodiment.
The afocal zoom lens of the second embodiment comprises, in order from the object side, the first lens unit G1 with positive refracting power, the second lens unit G2 with negative refracting power, the third lens unit G3 with negative refracting power, the aperture stop S, the fourth lens unit G4 with positive refracting power, and the fifth lens unit G5 with negative refracting power.
The first lens unit G1 includes, in order from the object side, the cemented lens component of the biconvex lens L11 and the negative meniscus lens L12 with the concave surface facing the object side, and the plano-convex lens L13 whose object-side surface is convex and whose image-side surface is flat.
The second lens unit G2 includes, in order from the object side, the cemented lens component of the biconvex lens L21 and the biconcave lens L22, and the biconcave lens L23.
The third lens unit G3 is constructed with the biconcave lens L31.
The fourth lens unit G4 includes the biconvex lens L41 and the cemented lens component of the negative meniscus lens L42 with the convex surface facing the object side and the biconvex lens L43.
The fifth lens unit G5 is constructed with the cemented lens component of the positive meniscus lens L51 with the concave surface facing the object side and the biconcave lens L52.
When the magnification is changed in the range from the low-magnification position to the high-magnification position, the first lens unit G1 and the fifth lens unit G5 remain fixed, the second lens unit G2 is moved from the object side toward the image side, the third lens unit G3 is moved, together with the aperture stop S, from the image side toward the object side, and the fourth lens unit G4 is moved from the image side toward the object side. The third lens unit G3 is moved when the magnification is changed, and thereby an optical system suppressing the shift of the position of the exit pupil is realized.
Subsequently, numerical data of optical members constituting the afocal zoom lens of the second embodiment are shown below.










Numerical data 2


















r1 = 78.1429
d1 = 3.986
nd1 = 1.497
νd1 = 81.54


r2 = −50.641
d2 = 2
nd2 = 1.71736
νd2 = 29.52


r3 = −177.066
d3 = 0.25


r4 = 45.3027
d4 = 2.5638
nd4 = 1.56907
νd4 = 71.3


r5 = ∞
d5 = D5


r6 = 46.1166
d6 = 2.7744
nd6 = 1.7847
νd6 = 26.29


r7 = −23.4179
d7 = 1.5103
nd7 = 1.741
νd7 = 52.64


r8 = 21.0177
d8 = 1.8346


r9 = −31.1044
d9 = 1.8
nd9 = 1.755
νd9 = 52.32


r10 = 43.9016
d10 = D10


r11 = −23.4911
d11 = 1.8
nd11 = 1.7725
νd11 = 49.6


r12 = 2180.678
d12 = 2.0345


r13 = ∞ (Aperture stop)
d13 = D13


r14 = 224.1202
d14 = 2.1284
nd14 = 1.56907
νd14 = 71.3


r15 = −35.8633
d15 = 0.2


r16 = 44.0038
d16 = 1.5835
nd16 = 1.834
νd16 = 37.16


r17 = 22.9718
d17 = 3.4756
nd17 = 1.497
νd17 = 81.54


r18 = −48.68
d18 = D18


r19 = −78.5752
d19 = 1.6
nd19 = 1.76182
νd19 = 26.52


r20 = −46.7222
d20 = 1.6
nd20 = 1.72916
νd20 = 54.68


r21 = 2058.414










Zoom data














Low-magnification
Middle
High-magnification





FL
  54 mm
 360 mm
 648 mm


NA
0.032
0.025
0.01852


Overall length
85.5 mm
85.5 mm
85.5 mm


Exit end position
−231.4
−223.0
−216.9


D5
0.400
26.062
29.523


D10
36.291
4.634
0.999


D13
17.386
11.982
3.570


D18
0.282
11.380
20.267










Variable magnification ratio: 12







Focal lengths













F1 =
54.48



F2 =
−17.72



F3 =
−30.07



F4 =
30.92



F5 =
−106.95



F1/F3 =
−1.81



F2/F3 =
0.59



F4/F3 =
−1.03



F5/F3 =
3.56



F23L =
−6.30



F23H =
−10.82



F23L/F23H =
1.72



|F1/F23L| =
8.64



D3L/D0 =
0.62



(RG3/F3) · Z =
9.37



ng3 =
1.7725










Third Embodiment
FIGS. 5A, 5B, and 5C show optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of the third embodiment of the afocal zoom lens according to the present invention. FIGS. 6A–6C, 6D–6F, and 6G–6I show aberration characteristics at the low-magnification, middle, and high-magnification positions, respectively, of the afocal zoom lens in the third embodiment.
The afocal zoom lens of the third embodiment comprises, in order from the object side, the first lens unit G1 with positive refracting power, the second lens unit G2 with negative refracting power, the third lens unit G3 with negative refracting power, the aperture stop S, the fourth lens unit G4 with positive refracting power, and the fifth lens unit G5 with negative refracting power.
The first lens unit G1 includes, in order from the object side, the cemented lens component of the biconvex lens L11 and the negative meniscus lens L12 with the concave surface facing the object side, and the plano-convex lens L13 whose object-side surface is convex and whose image-side surface is flat.
The second lens unit G2 includes, in order from the object side, the cemented lens component of the biconvex lens L21 and the biconcave lens L22, and the biconcave lens L23.
The third lens unit G3 is constructed with the biconcave lens L31.
The fourth lens unit G4 includes the biconvex lens L4, and the cemented lens component of the negative meniscus lens L42 with the convex surface facing the object side and the biconvex lens L43.
The fifth lens unit G5 is constructed with the cemented lens component of the positive meniscus lens L51 with the concave surface facing the object side and the biconcave lens L52.
When the magnification is changed in the range from the low-magnification position to the high-magnification position, the first lens unit G1, the aperture stop S, and the fifth lens unit G5 remain fixed, the second lens unit G2 is moved from the object side toward the image side, the third lens unit G3 is moved from the object side toward the image side so that spacing between the second lens unit G2 and the third lens unit G3 is once narrowed and then somewhat widened, and the fourth lens unit G4 is moved from the image side toward the object side. The third lens unit G3 is moved when the magnification is changed, and thereby the optical system suppressing the shift of the position of the exit pupil is realized.
Subsequently, numerical data of optical members constituting the afocal zoom lens of the third embodiment are shown below.










Numerical data 3


















r1 = 80.2951
d1 = 3.7959
nd1 = 1.497
νd1 = 81.54


r2 = −55.7625
d2 = 2.6
nd2 = 1.72825
νd2 = 28.46


r3 = −185.593
d3 = 0.25


r4 = 55.2449
d4 = 2.5513
nd4 = 1.56907
νd4 = 71.3


r5 = ∞
d5 = D5


r6 = 34.9024
d6 = 3.2865
nd6 = 1.7847
νd6 = 26.29


r7 = −26.0188
d7 = 1.4898
nd7 = 1.741
νd7 = 52.64


r8 = 21.4048
d8 = 1.7059


r9 = −37.9087
d9 = 1.7856
nd9 = 1.741
νd9 = 52.64


r10 = 24.7416
d10 = D10


r11 = −24.9718
d11 = 1.8
nd11 = 1.741
νd11 = 52.64


r12 = 631.4463
d12 = D12


r13 = ∞ (Aperture stop)
d13 = D13


r14 = 78.5147
d14 = 2.6378
nd14 = 1.56907
νd14 = 71.3


r15 = −44.4687
d15 = 0.2


r16 = 32.7895
d16 = 1.65
nd16 = 1.834
νd16 = 37.16


r17 = 18.486
d17 = 4.0922
nd17 = 1.497
νd17 = 81.54


r18 = −42.954
d18 = D18


r19 = −44.0524
d19 = 1.8628
nd19 = 1.7552
νd19 = 27.51


r20 = −24.3713
d20 = 1.6
nd20 = 1.741
νd20 = 52.64


r21 = 89.3433










Zoom data














Low-magnification
Middle
High-magnification





FL
 45 mm
360 mm
1250 mm


NA
0.032
0.025
0.00096


Overall length
100 mm
100 mm
 100 mm


Exit end position
−237.1
−233.5
−211.0


D5
0.392
33.819
36.692


D10
26.206
2.041
4.789


D12
15.901
6.638
1.018


D13
25.007
23.710
0.925


D18
1.187
2.484
25.269










Variable magnification ratio: 27.8







Focal lengths













F1 =
62.31



F2 =
−17.75



F3 =
−32.38



F4 =
26.49



F5 =
−39.8076



F1/F3 =
−1.92



F2/F3 =
0.55



F4/F3 =
−0.82



F5/F3 =
1.23



F23L =
−7.56



F23H =
−10.53



F23L/F23H =
1.39



|F1/F23L| =
8.24



D3L/D0 =
0.44



(RG3/F3) · Z =
21.44



ng3 =
1.741










Fourth Embodiment
FIGS. 7A, 7B, and 7C show optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of the fourth embodiment of the afocal zoom lens according to the present invention. FIGS. 8A–8C, 8D–8F, and 8G–8I show aberration characteristics at the low-magnification, middle, and high-magnification positions, respectively, of the afocal zoom lens in the fourth embodiment.
The afocal zoom lens of the fourth embodiment comprises, in order from the object side, the first lens unit G1 with positive refracting power, the second lens unit G2 with negative refracting power, the third lens unit G3 with negative refracting power, the aperture stop S, the fourth lens unit G4 with positive refracting power, and the fifth lens unit G5 with negative refracting power.
The first lens unit G1 includes, in order from the object side, the cemented lens component of the biconvex lens L11 and the negative meniscus lens L12 with the concave surface facing the object side, and the plano-convex lens L13 whose object-side surface is convex and whose image-side surface is flat.
The second lens unit G2 includes, in order from the object side, the cemented lens component of the biconvex lens L21 and the biconcave lens L22, and the biconcave lens L23.
The third lens unit G3 is constructed with a plano-concave lens L31′ whose object-side surface is concave and whose image-side surface is flat.
The fourth lens unit G4 includes the biconvex lens L41 and the cemented lens component of the negative meniscus lens L42 with the convex surface facing the object side and the biconvex lens L43.
The fifth lens unit G5 is constructed with the cemented lens component of the positive meniscus lens L51 with the concave surface facing the object side and the biconcave lens L52.
When the magnification is changed in the range from the low-magnification position to the high-magnification position, the first lens unit G1, the third lens unit G3, the aperture stop S, and the fifth lens unit G5 remain fixed, the second lens unit G2 is moved from the object side toward the image side, and the fourth lens unit G4 is moved from the image side toward the object side.
Subsequently, numerical data of optical members constituting the afocal zoom lens of the fourth embodiment are shown below.










Numerical data 4


















r1 = 51.4154
d1 = 4.0083
nd1 = 1.43875
νd1 = 94.93


r2 = −46.1254
d2 = 2
nd2 = 1.68893
νd2 = 31.07


r3 = −139.727
d3 = 0.25


r4 = 52.2809
d4 = 2.1703
nd4 = 1.603
νd4 = 65.44


r5 = ∞
d5 = D5


r6 = 63.3929
d6 = 3.5049
nd6 = 1.7847
νd6 = 26.29


r7 = −19.3362
d7 = 1.5103
nd7 = 1.741
νd7 = 52.64


r8 = 16.1879
d8 = 1.3419


r9 = −18.7999
d9 = 1.8
nd9 = 1.755
νd9 = 52.32


r10 = 95.7422
d10 = D10


r11 = −213.202
d11 = 1.8
nd11 = 1.834
νd11 = 37.16


r12 = ∞
d12 = 1


r13 = ∞ (Aperture stop)
d13 = D13


r14 = 277.7207
d14 = 1.8148
nd14 = 1.56907
νd14 = 71.3


r15 = −60.8805
d15 = 0.2


r16 = 71.4519
d16 = 0.968
nd16 = 1.834
νd16 = 37.16


r17 = 38.4312
d17 = 2.686
nd17 = 1.43875
νd17 = 94.93


r18 = −65.9166
d18 = D18


r19 = −123.656
d19 = 1.5
nd19 = 1.76182
νd19 = 26.52


r20 = −73.6322
d20 = 1.4
nd20 = 1.741
νd20 = 52.64


r21 = 2395.896










Zoom data














Low-magnification
Middle
High-magnification





FL
54 mm
360 mm
864 mm


NA
0.03
0.025
0.01273


Overall length
97 mm
 97 mm
 97 mm


Exit end position
−259.6
−253.9
−228.1


D5
0.368
27.168
29.776


D10
29.982
3.182
0.574


D13
29.478
27.569
−0.255


D18
9.218
11.126
38.951










Variable magnification ratio: 16.0







Focal lengths













F1 =
52.02



F2 =
−12.78



F3 =
−255.64



F4 =
51.91



F5 =
−161.579



F1/F3 =
−0.20



F2/F3 =
0.05



F4/F3 =
−0.20



F5/F3 =
0.63



F23L =
−10.92



F23H =
−12.10



F23L/F23H =
1.11



|F1/F23L| =
4.77



D3L/D0 =
0.48



(RG3/F3) · Z =
13.34



ng3 =
1.834










Fifth Embodiment
FIGS. 9A, 9B, and 9C show optical arrangements, developed along the optical axis, at low-magnification, middle, and high-magnification positions, respectively, of the fifth embodiment of the afocal zoom lens according to the present invention. FIGS. 10A–10C, 10D–10F, and 10G–10I show aberration characteristics at the low-magnification, middle, and high-magnification positions, respectively, of the afocal zoom lens in the fifth embodiment.
The afocal zoom lens of the fifth embodiment comprises, in order from the object side, the first lens unit G1 with positive refracting power, the second lens unit G2 with negative refracting power, the third lens unit G3 with negative refracting power, the aperture stop S, the fourth lens unit G4 with positive refracting power, and the fifth lens unit G5 with negative refracting power.
The first lens unit G1 includes, in order from the object side, the cemented lens component of the biconvex lens L11 and the negative meniscus lens L12 with the concave surface facing the object side, and the plano-convex lens L13 whose object-side surface is convex and whose image-side surface is flat.
The second lens unit G2 includes, in order from the object side, the cemented lens component of the biconvex lens L21 and the biconcave lens L22, and the biconcave lens L23.
The third lens unit G3 includes the biconcave lens L31 and a positive meniscus lens L32 with a convex surface facing the object side.
The fourth lens unit G4 includes the biconvex lens L41 and the cemented lens component of the negative meniscus lens L42 with the convex surface facing the object side and the biconvex lens L43.
The fifth lens unit G5 is constructed with a cemented lens component of the positive meniscus lens L51 with the concave surface facing the object side and a negative meniscus lens L52′ with a concave surface facing the object side.
When the magnification is changed in the range from the low-magnification position to the high-magnification position, the first lens unit G1, the third lens unit G3, the aperture stop S, and the fifth lens unit G5 remain fixed, the second lens unit G2 is moved from the object side toward the image side, and the fourth lens unit G4 is moved from the image side toward the object side.
Subsequently, numerical data of optical members constituting the afocal zoom lens of the fifth embodiment are shown below.










Numerical data 5


















r1 = 62.9444
d1 = 3.7346
nd1 = 1.43875
νd1 = 94.93


r2 = −46.3547
d2 = 1.8217
nd2 = 1.68893
νd2 = 31.07


r3 = −180.974
d3 = 0.25


r4 = 33.7155
d4 = 2.8449
nd4 = 1.56907
νd4 = 71.3


r5 = ∞
d5 = D5


r6 = 52.1293
d6 = 3.2408
nd6 = 1.7847
νd6 = 26.29


r7 = −29.0932
d7 = 1.3025
nd7 = 1.741
νd7 = 52.64


r8 = 19.2144
d8 = 2.1443


r9 = −60.3387
d9 = 1.8
nd9 = 1.755
νd9 = 52.32


r10 = 556.9835
d10 = D10


r11 = −15.2124
d11 = 1.4
nd11 = 1.788
νd11 = 47.37


r12 = 22.7388
d12 = 1.4606
nd12 = 1.68893
νd12 = 31.07


r13 = 130.3733
d13 = 3


r14 = ∞ (Aperture stop)
d14 = D14


r15 = 107.7266
d15 = 2.2666
nd15 = 1.56907
νd15 = 71.3


r16 = −36.2893
d16 = 0.2


r17 = 40.7491
d17 = 1.6709
nd17 = 1.834
νd17 = 37.16


r18 = 23.1439
d18 = 2.7546
nd18 = 1.43875
νd18 = 94.93


r19 = −97.7421
d19 = D19


r20 = −55.2552
d20 = 1.6731
nd20 = 1.68893
νd20 = 31.07


r21 = −28.8866
d21 = 1.3157
nd21 = 1.741
νd21 = 52.64


r22 = −579.261










Zoom data














Low-magnification
Middle
High-magnification





FL
54 mm
360 mm
864 mm


NA
0.032
0.025
0.01273


Overall length
98 mm
 98 mm
 98 mm


Exit end position
−422.8
−239.5
−222.9


D5
0.404
19.610
23.433


D10
23.819
4.614
0.791


D14
40.413
22.512
4.303


D19
0.483
18.385
36.593










Variable magnification ratio: 16.0







Focal lengths













F1 =
45.62



F2 =
−28.99



F3 =
−16.11



F4 =
34.62



F5 =
−77.0173



F1/F3 =
−2.83



F2/F3 =
1.80



F4/F3 =
−2.15



F5/F3 =
10.03



F23L =
−6.65



F23H =
−9.89



F23L/F23H =
1.49



|F1/F23L| =
6.86



D3L/D0 =
0.42



(RG3/F3) · Z =
15.11



ng3 =
1.788










Subsequently, a list of preset data in the embodiments is shown in Table 1.













TABLE 1







1st embodiment
2nd embodiment
3rd embodiment
4th embodiment
5th embodiment





















Low-magnification position
54
54
45
54
54


Middle position
360
360
360
360
360


High-magnification position
864
648
1250
864
864


Variable magnification ratio
16
12
27.8
16
16


Overall length
95
85.5
100
97
98


3rd lens unit
Fixed
Moved
Moved
Fixed
Fixed


Aperture stop
Fixed
Moved with the
Fixed
Fixed
Fixed




3rd lens unit


Exit pupil (low)
−539
−231.4
−237.1
−259.6
−422.8


Exit pupil (middle)
−238.7
−223
−233.5
−253.9
−239.5


Exit pupil (high)
−220.6
−216.9
−211
−228.1
−222.9


F1/F3
−2.07
−1.81
−1.92
−0.20
−2.83


F2/F3
0.80
0.59
0.55
0.05
1.80


F4/F3
−1.33
−1.03
−0.82
−0.20
−2.15


F5/F3
2.90
3.56
1.23
0.63
10.03


F1
50.55
54.48
62.31
52.02
45.62


F2
−19.57
−17.72
−17.75
−12.78
−28.99


F3
−24.47
−30.07
−32.38
−255.64
−16.11


F4
32.47
30.92
26.49
51.91
34.62


F5
−71.00
−106.95
−39.8076
−161.5789
−77.0173


F23L
−6.91
−6.30
−7.56
−10.92
−6.65


F23H
−10.49
−10.82
−10.53
−12.10
−9.89


F23L/F23H
1.52
1.72
1.39
1.11
1.49


|F1/F23L|
7.32
8.64
8.24
4.77
6.86


D3L/D0
0.47
0.62
0.44
0.48
0.42


RG3/F3 · Z
15.53
9.37
21.44
13.34
15.11


ng3
1.7847
1.7725
1.741
1.834
1.788









FIG. 11 show an example of an arrangement, developed along the optical axis, of an imaging lens combined with the afocal zoom lens of each of the above embodiments.
The imaging lens of this example includes, in order from the object side, a positive meniscus lens Lk1 with a convex surface facing the object side and a cemented lens component of a positive meniscus lens Lk2 and a negative meniscus lens Lk3 with a convex surface facing the object side.
The afocal zoom lens of each embodiment is combined with the imaging lens as an imaging lens optical system by placing a prism optical system (not shown) on the image side of the imaging lens.
Next, numerical data of optical members constituting the imaging lens optical system of FIG. 11 are shown below. Also, the numerical data contain the data of the prism optical system placed on the image side of the imaging lens.










Numerical data 6


FL 211.675 mm


















r1 = 37.966
d1 = 3
nd1 = 1.51742
νd1 = 52.43


r2 = 134.562
d2 = 2.08


r3 = 31.192
d3 = 3.5
nd3 = 1.48749
νd3 = 70.23


r4 = 340.249
d4 = 2
nd4 = 1.58267
νd4 = 46.42


r5 = 24.76
d5 = 22


r6 = ∞
d6 = 100
nd6 = 1.51633
νd6 = 64.14


r7 = ∞
d7 = 0


r8 = ∞
d8 = 65
nd8 = 1.56883
νd8 = 56.36


r9 = ∞
d9 = 53.2061


r10 = ∞ (Image plane)
d10 = 0

## Claims

1. An afocal zoom lens for microscopes, comprising five lens units having, in order from an object side, positive, negative, negative, positive, and negative refracting powers, wherein when a magnification of the afocal zoom lens is changed in a range from a low-magnification position to a high-magnification position, a first lens unit and a fifth lens unit are fixed, a second lens unit is moved toward the image side, and a fourth lens unit is moved toward the object side.

2. An afocal zoom lens for microscopes according to claim 1, satisfying the following condition:


0≦F2/F3≦2.5


where F2 is a focal length of the second lens unit and F3 is a focal length of a third lens unit.

3. An afocal zoom lens for microscopes according to claim 1 or 2, wherein the third lens unit is constructed with at least one negative lens with a concave surface facing the object side.

4. An afocal zoom lens for microscopes according to claim 1, satisfying the following condition:


1≦F23L/F23H≦3


where F23L is a combined focal length of the second lens unit and the third lens unit at the low-magnification position and F23H is a combined focal length of the second lens unit and the third lens unit at the high-magnification position.

5. An afocal zoom lens for microscopes according to claim 1, satisfying the following condition:


3≦|F1/F23L|≦15


where F1 is a focal length of the first lens unit and F23L is a combined focal length of the second lens unit and the third lens unit at the low-magnification position.

6. An afocal zoom lens for microscopes according to claim 1, satisfying the following condition:


0.3≦D3L/D≦0.7


where D3L is a distance from an object-side surface of the first lens unit to an object-side surface of the third lens unit at the low-magnification position and DO is an overall length of the afocal zoom lens.

7. An afocal zoom lens for microscopes according to any one of claims 4–6, satisfying the following conditions:


7≦RG3/F3×Z≦30



1.65≦ng3


where F3 is a focal length of the third lens unit, RG3 is a radius of curvature of a concave surface facing the object side, of a negative lens, ng3 is a refractive index of the negative lens, and Z is a variable magnification ratio.

8. An afocal zoom lens for microscopes according to claim 1 or 2, wherein an aperture stop is interposed between the second lens unit and the fourth lens unit.

9. An afocal zoom lens for microscopes according to claim 3, satisfying the following conditions:


7≦RG3F3×Z≦30



1.65≦ng3


where F3 is a focal length of the third lens unit, RG3 is a radius of curvature of a concave surface facing the object side, of a negative lens, ng3 is a refractive index of the negative lens, and Z is a variable magnification ration.