# Patent

## Title
Variable power optical system for stereomicroscope

## Abstract

An optical system including, in order from an object: 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 negative refractive power. The second lens group has, in order from the object, a front group having negative refractive power and a rear group. In various embodiments, the following conditional expressions are satisfied, 0.4<f2F/f2<1.6 and −3.0<q2<−0.3 where f2 denotes a focal length of the second lens group, f2F denotes a focal length of the front group, and q2 denotes a form factor of a negative lens disposed closest to the object of the front group.

## Background
CROSS-REFERENCE TO RELATED APPLICATION (S)
This is a continuation of PCT International Application No. PCT/JP2009/007001, filed on Dec. 18, 2009, which is hereby incorporated by reference. This application also claims the benefit of Japanese Patent Application No. 2008-325485, filed in Japan on Dec. 22, 2008, which is hereby incorporated by reference.


BACKGROUND
Field
Various embodiments of the present invention relate to a variable power optical system for a stereomicroscope.
TECHNICAL BACKGROUND
Description of the Related Art
Conventionally, various variable power optical systems used for stereomicroscopes have been proposed. For example, a variable power optical system, including five lens groups having, in order from an object, positive, negative, negative, positive and negative refractive power, has been proposed (e.g. see Patent Literature 1).
PATENT DOCUMENT


    PATENT DOCUMENT 1: Japanese Laid-Open Patent Publication No. 2005-91755(A)


SUMMARY OF VARIOUS EMBODIMENTS OF THE PRESENT INVENTION
For a stereomicroscope, a bright object optical system having a large numerical aperture and a variable power optical system having wide variable power range are demanded. Conventionally, however, if a variable power range of the variable power optical system is wide, a numerical aperture of the objective optical system becomes small, and if a numerical aperture of the objective optical system is large, on the other hand, the variable power range of the variable power optical system becomes narrow, and both are insufficient to satisfy demanded specifications.
With the foregoing in view, various embodiments of the present invention provide a variable power optical system for a stereomicroscope which has a wide variable power range and can correct aberrations well, while ensuring a large numerical aperture of an objective optical system.
A variable power optical system for a stereomicroscope of various embodiments of the present invention includes, in order from an object: a first lens group having positive refractive power; a second lens group having negative refractive power; an aperture stop; a third lens group having positive refractive power; and a fourth lens group having negative refractive power, wherein the second lens group includes, in order from the object, a front group having negative refractive power and a rear group having an achromatic cemented lens including a positive lens and a negative lens, the second lens group and the third lens group move in opposite directions along an optical axis at least in a part of a variable power block, and the following conditional expressions are satisfied: 0.4<f2F/f2<1.6 and −3.0<q2<−0.3 where f2 denotes a focal length of the second lens group, f2F denotes a focal length of the front group constituting the second lens group, and q2 denotes a form factor of a negative lens disposed closest to the object in the front group constituting the second lens group (the form factor q2 is defined by q2=(r22+r21)/(r22−r21), where r21 denotes a radius of curvature of an object side lens surface of the negative lens, and r22 denotes a radius of curvature of an image side lens surface of the negative lens).
Moreover, in various embodiments of the present invention, the first lens group include a negative lens disposed closest to the object, and the following conditional expression be satisfied: −0.3<f2/f1<−0.2 where f1 denotes a focal length of the first lens group, and f2 denotes a focal length of the second lens group.
Further, in various embodiments of the present invention, the positive lens constituting the cemented lens in the rear group of the second lens group satisfy the following conditional expressions: T360≧0.5 and vd2<40 where T360 denotes an internal transmittance at wavelength 360 nm without reflection loss when a thickness of an optical material to be used is 10 mm, and vd2 denotes an Abbe number of the optical material to be used.
According to various embodiments of the present invention, a variable power optical system for a stereomicroscope which has a wide variable power range and can correct aberrations well, while ensuring a large numerical aperture of an objective optical system, can be provided.
Various embodiments of the present invention are described above. However, not all embodiments of the present invention include all the features described in the above embodiments.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C are cross-sectional views depicting a configuration of a variable power optical system according to Example 1, where FIG. 1A shows a lowest magnification state, FIG. 1B shows an intermediate magnification state, and FIG. 1C shows a highest magnification state, according to various embodiments of the present invention;
FIGS. 2A to 2C are graphs showing various aberrations of the variable power optical system according to Example 1, where FIG. 2A shows the lowest magnification state, FIG. 2B shows the intermediate magnification state, and FIG. 2C shows the highest magnification state, according to various embodiments of the present invention;
FIGS. 3A to 3C are cross-sectional views depicting a configuration of a variable power optical system according to Example 2, where FIG. 3A shows a lowest magnification state, FIG. 3B shows an intermediate magnification state, and FIG. 3C shows a highest magnification state, according to various embodiments of the present invention;
FIGS. 4A to 4C are graphs showing various aberrations of the variable power optical system according to Example 2, where FIG. 4A shows the lowest magnification state, FIG. 4B shows the intermediate magnification state, and FIG. 4C shows the highest magnification state, according to various embodiments of the present invention;
FIGS. 5A to 5C are cross-sectional views depicting a configuration of a variable power optical system according to Example 3, where FIG. 5A shows a lowest magnification state, FIG. 5B shows an intermediate magnification state, and FIG. 5C shows a highest magnification state, according to various embodiments of the present invention;
FIGS. 6A to 6C are graphs showing various aberrations of the variable power optical system according to Example 3, where FIG. 6A shows the lowest magnification state, FIG. 6B shows the intermediate magnification state, and FIG. 6C shows the highest magnification state, according to various embodiments of the present invention;
FIGS. 7A to 7C are cross-sectional views depicting a configuration of a variable power optical system according to Example 4, where FIG. 7A shows a lowest magnification state, FIG. 7B shows an intermediate magnification state, and FIG. 7C shows a highest magnification state, according to various embodiments of the present invention;
FIGS. 8A to 8C are graphs showing various aberrations of the variable power optical system according to Example 4, where FIG. 8A shows the lowest magnification state, FIG. 8B shows the intermediate magnification state, and FIG. 8C shows the highest magnification state, according to various embodiments of the present invention;
FIGS. 9A to 9C are cross-sectional views depicting a configuration of a variable power optical system according to Example 5, where FIG. 9A shows a lowest magnification state, FIG. 9B shows an intermediate magnification state, and FIG. 9C shows a highest magnification state, according to various embodiments of the present invention;
FIGS. 10A to 10C are graphs showing various aberrations of the variable power optical system according to Example 5, where FIG. 10A shows the lowest magnification state, FIG. 10B shows the intermediate magnification state, and FIG. 10C shows the highest magnification state, according to various embodiments of the present invention;
FIGS. 11A to 11C are cross-sectional views depicting a configuration of a variable power optical system according to Example 6, where FIG. 11A shows a lowest magnification state, FIG. 11B shows an intermediate magnification state, and FIG. 11C shows a highest magnification state, according to various embodiments of the present invention;
FIGS. 12A to 12C are graphs showing various aberrations of the variable power optical system according to Example 6, where FIG. 12A shows the lowest magnification state, FIG. 12B shows the intermediate magnification state, and FIG. 12C shows the highest magnification state, according to various embodiments of the present invention;
FIG. 13 is a cross-sectional view depicting a configuration of an image optical system which is combined with each example, according to various embodiments of the present invention;
FIG. 14 is a diagram depicting a configuration of a stereomicroscope using the variable power optical system according to various embodiments of the present embodiment;
FIG. 15 is a diagram depicting a difference of positions of a luminous flux between during a low magnification and during a high magnification, in an objective optical system which is combined with the variable power optical system according to various embodiments of the present embodiment;
FIG. 16 is a diagram depicting a relationship of a size of an object side numerical aperture of the objective optical system which is combined with the variable power optical system according to various embodiments of the present embodiment, and a size of an effective aperture of a lens closest to the object of this variable optical system;
FIGS. 17A and 17B are diagrams depicting a relationship of an inter-optical axis distance of the variable power optical system according to various embodiments of the present embodiment, and an object side numerical aperture of the objective optical system which is combined with the optical system;
FIG. 18 is a diagram depicting an aperture stop and an entrance pupil, which is an image thereof, in the variable power optical system according to various embodiments of the present embodiment;
FIG. 19 is a diagram depicting a relationship of a position of the entrance pupil of the variable power optical system according to various embodiments of the present embodiment and the size of the effective aperture; and
FIG. 20 is a diagram depicting a relationship of a position of the entrance pupil of the variable power optical system according to various embodiments of the present embodiment and the size of the objective optical system which is combined with the optical system.



These together with other aspects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.
DETAILED DESCRIPTION OF EMBODIMENTS
Various embodiments of the present invention will now be described with reference to the drawings.
First a stereomicroscope (parallel system single objective lens type binocular microscope) using a variable power optical system according to the present embodiment will be described. As FIG. 14 shows, the parallel system stereomicroscope according to the present embodiment has, in order from an object 1, one objective optical system 2, and two variable power optical systems 3, two image optical systems 4, and two ocular optical systems 5 for a left eye and a right eye, which are disposed in parallel with the optical axis of the objective optical system 2, where lights from the object 1 are converted into parallel luminous flux by the objective optical system 2, an image 6 is formed by the image optical systems 4 via the variable power optical systems 3, and the image 6 is observed by the ocular optical system 5. In the case of the stereomicroscope having this configuration, a bumpy object can be observed with a stereoscopic sense that is similar to viewing the object directly with both eyes (stereoscopic vision), hence the distance relationship between a tool, such as a tweezers, and an object can be easily grasped when looking through the microscope. Therefore the stereomicroscope is particularly effective in a precision instrument industry, and in a field that requires fine work, such as vivisection and surgery.
Lately good aberration correction, more compactness and lighter weight as well as a wider variable power range and higher resolution are demanded for stereomicroscopes.
As FIG. 15 shows, in an objective optical system constituting a stereomicroscope, a position of a luminous flux passing through the object optical system (eccentric distance of the incident luminous flux from the optical axis of the objective optical system) differs greatly between low magnification and high magnification. Therefore if it is attempted to satisfy the need of expanding the variable power range, the difference of the luminous flux passing position further increases in the objective optical system. As a result, the objective optical system becomes larger and heavier, which is not desirable.
In order to satisfy the demand of improving resolution, increasing the object side numerical aperture of the objective optical system is required. As FIG. 16 shows, the size of the object side numerical aperture of the objective optical system during high magnification is restricted by the size of the effective aperture of a lens closest to the object (lens indicated by * in FIG. 16) constituting the variable power optical system. However as FIG. 17A shows, the two variable power optical systems are close to each other, and the only way to increase the effective aperture of the lens closest to the object in the optical system is expanding the inter-optical axis distance between these two variable power optical systems. As a result, the effective aperture of the objective optical system must be increased as shown in FIG. 17B, in other words, the optical system becomes larger and heavier, which is not desirable.
As described above, a very difficult optical design is required if it is attempted to increase the variable power range and improve the resolution using the objective optical system for a parallel system stereomicroscope.
In the present embodiment, attention is turned to a variable power optical system. Various variable power optical systems have been proposed for stereomicroscopes. Any of these variable power optical systems is an afocal optical system, and as mentioned above, the variable power optical system change the diameter of the parallel luminous flux emitted from the objective optical system so as to emit the luminous flux in parallel and relay the parallel luminous flux to the image optical system (see FIG. 14).
In the case of the variable power optical system, an aperture stop is disposed approximately at the center of the optical system, as shown in FIG. 18, and an image of the aperture stop formed by the optical system disposed in the incident side (object side) of the aperture stop is called an entrance pupil. As FIG. 19 shows, a peripheral light quantity is lost unless the effective aperture of the lens closest to the object is increased more as the position of the entrance pupil is closer to the image, that is, as the distance from the vertex on the surface of the lens closest to the object in the variable power optical system to the position of the entrance pupil is longer. Furthermore, as FIG. 20 shows, the luminous flux at the outermost angle passes through an area closer to the outer edge of the objective optical system as the position of the entrance pupil is closer to the image, hence the objective optical system becomes huge, which makes it difficult for the user to handle, and difficult to correct aberrations.
As a consequence, the variable optical system of the present embodiment is constructed as described below, whereby the position of the entrance pupil of the optical system is moved closer to the object, so that the variable power range can be increased, and aberrations can be corrected well while improving the resolution (increasing the numerical aperture of the objective optical system).
As FIGS. 1A to 1C show, the variable power optical system according to the present embodiment is used for a parallel system stereomicroscope, and has, in order from an object: a first lens group G1 having positive refractive power; a second lens group G2 having negative refractive power; an aperture stop S; a third lens group G3 having positive refractive power; and a fourth lens group G4 having negative refractive power, and the second lens group G2 further has, in order from the object, a front group G2F having negative refractive power; and a rear group G2R having an achromatic cemented lens including a positive lens and a negative lens. The second lens group G2 and the third lens group G3 move in opposite directions along the optical axis at least in a part of a variable power block.
In the above configuration, the following conditional expressions (1) and (2) are satisfied, where f2 denotes a focal length of the second lens group G2, and f2F denotes a focal length of a front group G2F constituting the second lens group G2, and q2 denotes a form factor of a negative lens L21 disposed closest to the object in the front group G2F constituting the second lens group G2 (the form factor q2 is defined by q2=(r22+r21)/(r22−R21) where r21 denotes a radius of curvature of the object side lens surface of the negative lens L21, and r22 denotes a radius of curvature of the image side lens surface of the negative lens L21).

0.4<f2F/f2<1.6  (1)

−3.0<q2<−0.3  (2)

In order to move the position of the entrance pupil closer to the object, the space between the principal points of the first lens group G1 and the second lens group G2 of the variable power optical system in the lowest magnification state must be decreased, or the position of the aperture stop S must be moved closer to the object. If the position of the aperture stop S is moved closer to the object, however, it is difficult to increase a zoom ratio of the variable power optical system since the moving distance of the second lens group G2 becomes short. Therefore in the present variable power optical system having a high zoom ratio, an appropriate arrangement of refractive power in the second lens group G2 is specified by the conditional expression (1), so that the space between the principal points of the first lens group G1 and the second lens group G2 is decreased and the position of the entrance pupil is moved closer to the object.
If the upper limit value of a conditional expression (1) is exceeded, the object side principal point of the second lens group G2 moves to the image side, and the space between the first lens group G1 and the second lens group G2 cannot be secured when the variable power optical system is in the lowest magnification sate, which is not desirable. If the lower limit value of the conditional expression (1) is not reached, on the contrary, the object side principal point of the second lens group G2 moves to the object side, and the space between the second lens group G2 and the third lens group G3 (or the aperture stop S) cannot be secured when the variable power optical system is in the highest magnification state, which is not desirable. Furthermore, it becomes difficult to both correct the curvature of field on the low magnification side and correct the spherical aberration on the high magnification side, which is not desirable.
The conditional expression (2) specifies an optimum form of the negative lens L21 which is disposed closest to the object side in the front group G2F constituting the second lens group G2. If the upper limit value of the conditional expression (2) is exceeded, the object side lens surface r21 of the negative lens L21 disposed closest to the object in the front group G2F becomes small, and it becomes difficult to correct the curvature of field and coma aberration on the low magnification side, and the spherical aberration on the high magnification side, which is not desirable. If the lower limit value of the conditional expression (2) is not reached, on the other hand, the object side principal point of the second lens group G2 moves to the image side, and the space between the first lens group G1 and the second lens group G2 cannot be secured when the variable power optical system is in the lowest magnification state, which is not desirable.
In the present embodiment, it is preferable that the first lens group G1 include a negative lens disposed closest to the object, and the following conditional expression (3) be satisfied, where f1 denotes a focal length of the first lens group G1 and f2 denotes a focal length of the second lens group G2.

−0.3<f2/f1<−0.2  (3)

The conditional expression (3) specifies a ratio of focal length between the first lens group G1 and the second lens group G2, which is optimum for this variable power optical system having a wide variable power range. If the upper limit value of the conditional expression (3) is exceeded, the refractive power of the third lens group G3 must be increased in order to increase the variable power range, and as a result, it becomes difficult to correct the spherical aberration and coma aberration in the low magnification side, which is not desirable. If the lower limit value of the conditional expression (3) is not reached, on the other hand, the refractive power of the second lens group G2 is increased, and it becomes difficult to correct the curvature of field on the low magnification side, and the spherical aberration on the high magnification side, which is not desirable. If the refractive power of the second lens group G2 is maintained in a range where the aberration correction is not affected, the total zoom length becomes too long, which is not desirable.
Lately fluorescent viewing using a stereomicroscope is becoming more important. This demand is particularly conspicuous in biology fields, such as embryology. Examples of the methods used to perform epi-illumination using a stereomicroscope are a method for illuminating an object via an illumination optical system and an objective optical system separately from a variable power optical system for viewing, a method for illuminating an object via a variable power optical system for viewing and an objective optical system, and a method for illuminating an object from outside an objective optical system.
In the case of the method for illuminating an object via an illumination optical system and an objective optical system separately from a variable power optical system for viewing, the excitation light for fluorescent illumination does not pass through the variable power optical system for viewing, hence self fluorescence is not generated within the variable power optical system for viewing, and a fluorescent image with good contrast can be viewed. Since an independent optical system is disposed for illumination, ultraviolet radiation of the excitation light can be handled more easily, compared with the method for illuminating an object via the variable power optical system for viewing and the objective optical system. In the case of this method, however, the illumination optical system is disposed separately from the variable power optical system for viewing, therefore compared with the method for illuminating an object via the variable power optical system for viewing and the objective optical system, the object side numerical aperture of the illumination light cannot be increased, and it is difficult to implement bright fluorescent illumination.
Therefore according to the variable power optical system of the present embodiment, a variable power optical system for viewing is used for fluorescent illumination in order to obtain bright fluorescent illumination, at the same time, in order to handle ultraviolet radiation of the excitation light, it is preferable that the positive lens (lens L23 in FIGS. 1A to 1C) constituting the cemented lens in the rear group G2R of the second lens group G2 satisfy the following conditional expressions (4) and (5), where T360 denotes an internal transmittance that does not include a reflection loss at wavelength 360 nm on the 10 mm thick optical material to be used, and nd denotes a refractive index at the d-line (wavelength: 587.56 nm), nC denotes a refractive index at C-line (wavelength: 656.27 nm), nF denotes a refractive index at F-line (wavelength: 486.13 nm), and vd2 denotes an Abbe number, of the optical material to be used respectively.

T360≧0.5  (4)

vd2=(nd−1)/(nF−nC)<40  (5)

Normally, in order to correct the chromatic aberration of the negative lens group, a low dispersion glass material is mixed for the negative lens and a high dispersion glass material is mixed for the positive lens, but many high dispersion glass materials have low transmittance at the short wavelength side. In the present embodiment, the positive lens constituting the cemented lens in the rear group G2R of the second lens group G2 is formed of a high dispersion glass material, and also has strong refractive power (in order to provide strong refractive power to the second lens group G2), hence the positive lens is thick, which is a cause of dropping transmittance of the entire optical system in the ultraviolet region. The conditional expressions (4) and (5) specify conditions to improve transmittance of the variable power optical system in the ultraviolet region. Just like the conditional expressions (1) to (3), if the conditional expressions (4) and (5) are satisfied, chromatic aberration can be corrected well, and this variable power optical system can be used for fluorescent illumination which can handle ultraviolet radiation. If the conditional expression (4) is not satisfied, transmittance in the ultraviolet region drops, and this variable power optical system cannot be used for the fluorescent illumination which can handle ultraviolet radiation, which is not desirable. If the conditional expression (5) is not satisfied, it becomes difficult to correct chromatic aberration well, which is not desirable.
In order to implement even better performance and specifications, it is preferable to satisfy the following conditions.
In the present embodiment, it is preferable that the fourth lens group G4 have, in order from the object, a front group G4F having a negative refractive power, and a rear group G4R including a cemented lens having a convex surface facing the object, and the cemented lens of the rear group G4R include a positive lens (lens L42 in FIGS. 1A to 1C) disposed closest to the object, and the following conditional expression (6) be satisfied, where q2 denotes a form factor of the positive lens disposed closest to the object in the rear group G4R constituting the fourth lens group G4 (the form factor q4 is defined by q4=(r42+r41)/(R42−R41) where r41 denotes a radius of curvature of the object side lens surface of the positive lens, and r42 denotes a radius of curvature of the image side lens surface of the positive lens).

|q4|≧1  (6)

In the present embodiment, if the variable power range extends to the low magnification side, a lower coma of the abaxial light is refracted considerably in the second lens group G2 and the third lens group G3 during the lowest magnification, and major coma aberration is generated. To correct this coma aberration, the form of the lens closest to the object in the cemented lens constituting the rear group G4R of the fourth lens group G4 satisfies the conditional expression (6). If the range given by conditional expression (6) is not satisfied, coma aberration on the low magnification side cannot be corrected sufficiently.
In the present embodiment, it is preferable that the positive lens (lens L23 in FIGS. 1A to 1C) in the rear group G2R of the second lens group G2 satisfy the following conditional expression (7), where ng denotes a refractive index at g-line (wavelength: 435.83 nm) of an optical material to be used, and PgF2 denotes a partial dispersion ratio.

PgF2=(ng−nF)/(nF−nC)>0.57  (7)

The conditional expression (7) specifies the condition to correct a longitudinal chromatic aberration in this variable power optical system. If the conditional expression (7) is satisfied, the longitudinal chromatic aberration at g-line to C-line can be corrected well, particularly in the high magnification range. If the range of the conditional expression (7) is not satisfied, on the other hand, aberration correction at g-line becomes difficult when the longitudinal chromatic aberration is corrected at F-line to C-line.
In the present embodiment, it is preferable that the positive lens (lens L42 in FIGS. 1A to 1C) closest to the object in the rear group G4R of the fourth lens group G4 satisfy the following conditional expressions (8) and (9), where vd4 denotes an Abbe number of an optical material to be used, and nd4 denotes a refractive index at d-line.

vd4=(nd−1)/(nF−nC)<45  (8)

nd4<1.75  (9)

The conditional expressions (8) and (9) specify the conditions to correct a lateral chromatic aberration on the low magnification side in this variable power optical system. If the conditional expressions (8) and (9) are satisfied, the lateral chromatic aberration at g-line to C-line can be corrected well, particularly in the low magnification range. If the range of the conditional expressions (8) and (9) are not satisfied, on the other hand, aberration correction at g-line becomes difficult when the lateral chromatic aberration is corrected at F-line to C-line.
EXAMPLES
Examples of the present embodiment will now be described with reference to the drawings. Table 1 to Table 6 shown below are tables listing each data in Example 1 to Example 6. In [General Data], f is a composite focal length of this variable power optical system and an image optical system which is used in combination (when d0=∞), and FNO is an F number. In [Lens Data], the surface number shows a sequence of the lens surface counted from the object side along the ray traveling direction, r is a radius of curvature of each lens surface, d is a distance on the optical axis from each optical surface to the next optical surface (or an image plane), vd is an Abbe number, and nd is a refractive index at d-line (wavelength: 587.6 nm). The radius of curvature “0.0000” indicates a plane or an aperture. The refractive index of air “1.000000” is omitted. In [Variable Distance Data], f is a composite focal length of this variable optical system, and the image optical system is used in combination, and di (i is an integer) is a variable surface distance of the i-th surface. In [Conditional expression correspondence value], values corresponding to the conditional expressions (1) to (9) (conditional expressions (1) to (5) in the case of Example 5 and Example 6) are shown.
In the tables, “mm” is normally used for the unit of focal length f, radius of curvature r, surface distance d and other lengths. However the unit is not limited to “mm”, but another appropriate unit may be used instead, since an equivalent optical performance is obtained even if an optical system is proportionally expanded or proportionally reduced.
The above description on the tables is the same for the other examples, for which this description is omitted.
Example 1
Example 1 will now be described with reference to FIGS. 1A to 1C, FIGS. 2A to 2C and Table 1. FIGS. 1A to 1C are diagrams depicting the lens according to Example 1, where FIG. 1A shows a lowest magnification state (f=50.4000), FIG. 1B shows an intermediate magnification state (f=320.0000), and FIG. 1C shows a highest magnification state (f=1260.0000). As FIGS. 1A to 1C show, the variable power optical system according to Example 1 has, in order from the object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stoop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
The first lens group G1 has, in order from the object, a cemented lens of a negative meniscus lens L11 having a convex surface facing the object and a biconvex lens L12, and a planoconvex lens L13 having a convex surface facing the object. The second lens group G2 has, in order from the object, a front group G2F which includes a biconcave lens L21, and has a negative refractive power, and a rear group G2R which includes an achromatic cemented lens of a biconcave lens L22, a biconvex lens L23 and a planoconcave lens L24 having a concave surface facing the object. The third lens group G3 has, in order from the object, a biconvex lens L31, and a cemented lens of a negative meniscus lens L32 having a convex surface facing the object and a biconvex lens L33. The fourth lens group G4 has, in order from the object, a front group G4F which includes a biconcave lens L41 and has negative refractive power, and a rear group G4R which includes a cemented lens of a biconvex lens L42, a biconcave lens L43, and a positive meniscus lens L44 having a convex surface facing the object.
In the variable power optical system according to this example having the above configuration, upon zooming from the lowest magnification state to the highest magnification state, the first lens group G1 and the fourth lens group G4 are fixed, the second lens group G2 is moved to the image side and the third lens group G3 is moved to the object side.
Table 1 below shows each data of the variable power optical system according to Example 1. The surface members 1 to 23 in Table 1 correspond to the surfaces 1 to 23 in FIGS. 1A to 1C.







TABLE 1







[General Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
50.4000~
320.0000~
126.0000



FNO
16.61~
23.73~
50.06











[Lens Data]











Surface number
r
d
νd
nd







d0


1
287.5600
1.4000
42.72
1.834810


2
51.3030
3.5000
82.56
1.497820


3
−170.2215
0.1500


4
49.6124
3.1000
68.33
1.592400


5
0.0000
d5


6
−89.0973
1.0000
35.71
1.902650


7
24.8006
1.9000


8
−37.0305
1.8000
82.56
1.497820


9
22.2208
3.3000
32.27
1.738000


10
−22.2208
1.0000
54.68
1.729157


11
0.0000
d11











12
0.0000
d12
(Aperture stop S)












13
84.9551
3.6000
82.56
1.497820


14
−84.9551
0.1500


15
56.4259
1.3000
34.71
1.720467


16
26.5640
3.1000
82.56
1.497820


17
−143.7045
d17


18
−105.2698
1.4000
82.56
1.497820


19
105.2698
0.3000


20
18.7056
3.7000
35.30
1.592700


21
−204.5470
1.4000
42.72
1.834810


22
19.6043
7.0000
70.45
1.487490


23
19.5959
7.0000










[Variable Distance Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
50.4000
320.0000
1260.0000



d0
0.0000
0.0000
0.0000



d5
2.4981
45.3527
57.1127



d11
57.5693
14.7147
2.9547



d12
66.3140
39.6074
2.9856



d17
2.9990
29.7057
66.3274











[Conditional expression correspondence value]













Conditional expression (1)
f2F/f2 = 1.034



Conditional expression (2)
q2 = −0.565



Conditional expression (3)
f2/f1 = −0.234



Conditional expression (4)
T360 = 0.74



Conditional expression (5)
νd2 = 32.3



Conditional expression (6)
|q4| = 43.02



Conditional expression (7)
PgF2 = 0.5899



Conditional expression (8)
νd4 = 35.3



Conditional expression (9)
nd4 = 1.5927










As the data in Table 1 shows, all the conditional expressions (1) to (9) are satisfied in the variable power optical system according to Example 1.
FIGS. 2A to 2C are graphs showing various aberrations of the variable power optical system according to Example 1, where FIG. 2A shows a lowest magnification state (f=50.4000), FIG. 2B shows an intermediate magnification state (f=320.000), and FIG. 2C shows a highest magnification state (f=1260.0000). In each graph showing Aberrations, Y denotes an image height (unit: mm). In the graph showing the spherical aberration, a value of an F number corresponding to the maximum aperture is shown, in the graph showing astigmatism and the graph showing distortion, the maximum value of the image height is shown respectively, and in the graph showing coma aberration, a value of each image height is shown. d shows various aberrations at d-line, g shows various aberrations at g-line, and no indication shows various aberrations at d-line. In the graph showing astigmatism, the solid line indicates the sagittal image surface, and the broken line indicates the meridional image surface. The above description on the graphs showing aberrations is the same for the other examples, for which this description is omitted.
As seen in each graph showing aberrations, in Example 1, various aberrations are corrected well in each magnification state from the highest magnification state to the lowest magnification state, indicating that this variable power optical system has a superb optical performance.
Example 2
Example 2 will now be described with reference to FIGS. 3A to 3C, FIGS. 4A to 4C and Table 2. FIGS. 3A to 3C are diagrams depicting the lens according to Example 2, where FIG. 3A shows a lowest magnification state (f=56.0000), FIG. 3B shows an intermediate magnification state (f=320.0000), and FIG. 3C shows a highest magnification state (f=1008.0000). As FIGS. 3A to 3C show, the variable power optical system according to Example 2 has, in order from the object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stoop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
The first lens group G1 has, in order from the object, a cemented lens of a negative meniscus lens L11 having a convex surface facing the object and a biconvex lens L12, and a biconvex lens L13. The second lens group G2 has, in order from the object, a front group G2F which includes a biconcave lens L21 and a planoconcave lens L22 having a concave surface facing the object, and has a negative refractive power, and a rear group G2R which includes an achromatic cemented lens of a biconvex lens L23 and a biconcave lens L24. The third lens group G3 has, in order from the object, a biconvex lens L31, and a cemented lens of a negative meniscus lens L32 having a convex surface facing the object and a biconvex lens L33. The fourth lens group G4 has, in order from the object, a front group G4F which includes a biconcave lens L41 and has negative refractive power, and a rear group G4R which includes a cemented lens of a positive meniscus lens L42 having a convex surface facing the object and a negative meniscus lens L43 having a convex surface facing the object.
In the variable power optical system according to this example having the above configuration, upon zooming from the lowest magnification state to the highest magnification state, the first lens group G1 and the fourth lens group G4 are fixed, the second lens group G2 is moved to the image side and the third lens group G3 is moved to the object side.
Table 2 shows each data of the variable power optical system according to Example 2. The surface members 1 to 23 in Table 2 correspond to the surfaces 1 to 23 in FIGS. 3A to 3C.







TABLE 2







[General Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
56.0000~
320.0000~
1260.0000



FNO
15.25~
22.29~
42.13











[Lens Data]











Surface number
r
d
νd
nd







d0


1
1096.3408
1.5000
44.79
1.744000


2
44.9980
3.7000
71.31
1.569070


3
−140.5120
0.2000


4
46.8130
3.0000
82.56
1.497820


5
−5381.3812
d5


6
−53.7185
1.5000
46.58
1.804000


7
21.8338
1.1000


8
−61.5539
1.5000
46.58
1.804000


9
0.0000
0.2000


10
26.4090
3.2000
32.27
1.738000


11
−26.4090
1.0000
68.33
1.592400


12
39.7526
d12











13
0.0000
d13
(Aperture stop S)












14
117.6950
2.0000
71.31
1.569070


15
−176.2328
0.2000


16
55.9680
1.5000
32.27
1.738000


17
24.9950
3.6000
71.31
1.569070


18
−96.3689
d18


19
−79.4787
1.5000
65.44
1.603001


20
161.8346
0.2000


21
19.3029
2.8000
35.30
1.592700


22
71.5140
4.7000
46.58
1.804000


23
18.5019
7.0000










[Variable Distance Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
56.0000
320.0000
1008.0000



d0
0.0000
0.0000
0.0000



d5
3.0603
41.1620
51.5094



d12
51.8376
13.7359
3.3885



d13
59.1889
33.3335
3.3893



d18
3.1509
29.0064
58.9505











[Conditional expression correspondence value]













Conditional expression (1)
f2F/f2 = 0.694



Conditional expression (2)
q2 = −0.422



Conditional expression (3)
f2/f1 = −0.254



Conditional expression (4)
T360 = 0.74



Conditional expression (5)
νd2 = 32.3



Conditional expression (6)
|q4| = 47.20



Conditional expression (7)
PgF2 = 0.5899



Conditional expression (8)
νd4 = 35.3



Conditional expression (9)
nd4 = 1.5927










As the data in Table 2 shows, all the conditional expressions (1) to (9) are satisfied in the variable power optical system according to Example 2.
FIGS. 4A to 4C are graphs showing various aberrations of the variable power optical system according to Example 2, where FIG. 4A shows a lowest magnification state (f=56.0000), FIG. 4B shows an intermediate magnification state (f=320.0000), and FIG. 4C shows a highest magnification state (f=1008.0000). As seen in each graph showing aberrations, in Example 2, various aberrations are corrected well in each magnification state from the highest magnification state to the lowest magnification state, indicating that this variable power optical system has a superb optical performance.
Example 3
Example 3 will now be described with reference to FIGS. 5A to 5C, FIGS. 6A to 6C and Table 3. FIGS. 5A to 5C are diagrams depicting the lens according to Example 3, where FIG. 5A shows a lowest magnification state (f=63.0000), FIG. 5B shows an intermediate magnification state (f=320.0000), and FIG. 5C shows a highest magnification state (f=1000.0000). As FIGS. 5A to 5C show, the variable power optical system according to Example 3 has, in order from the object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stoop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
The first lens group G1 has, in order from the object, a cemented lens of a negative meniscus lens L11 having a convex surface facing the object and a biconvex lens L12, and a positive meniscus lens L13 having a convex surface facing the object. The second lens group G2 has, in order from the object, a front group G2F which includes a biconcave lens L21, and has a negative refractive power, and a rear group G2R which includes an achromatic cemented lens of a biconcave lens L22 and a biconvex lens L23. The third lens group G3 has, in order from the object, a biconvex lens L31, and a cemented lens of a negative meniscus lens L32 having a convex surface facing the object and a biconvex lens L33. The fourth lens group G4 has, in order from the object, a front group G4F which includes a biconcave lens L41 and has negative refractive power, and a rear group G4R which includes a cemented lens of a positive meniscus lens L42 having a convex surface facing the object and a negative meniscus lens L43 having a convex surface facing the object.
In the variable power optical system according to this example having the above configuration, upon zooming from the lowest magnification state to the highest magnification state, the first lens group G1 and the fourth lens group G4 are fixed, the second lens group G2 is moved to the image side and the third lens group G3 is moved to the object side.
Table 3 shows each data of the variable power optical system according to Example 3. The surface members 1 to 21 in Table 3 correspond to the surfaces 1 to 21 in FIGS. 5A to 5C.







TABLE 3







[General Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
63.0000~
320.0000~
1000.0000



FNO
17.74~
25.57~
47.28











[Lens Data]











Surface number
r
d
νd
nd







d0


1
103.3187
1.0000
42.72
1.834810


2
35.8739
3.8100
82.56
1.497820


3
−71.1085
0.1500


4
28.0389
3.5040
69.89
1.518600


5
46.4803
d5


6
−51.7687
1.0000
49.61
1.772500


7
22.6357
1.5889


8
−36.4262
1.0000
57.36
1.670000


9
12.7421
2.5048
32.27
1.738000


10
−102.6546
d10











11
0.0000
d11
(Aperture stop S)












12
54.9136
2.4318
64.12
1.516800


13
−92.0011
0.1500


14
47.4764
1.0000
35.33
1.749505


15
20.6148
3.4738
82.56
1.497820


16
−74.8541
d16


17
−62.8410
1.0000
51.51
1.734000


18
133.5080
0.6950


19
21.1808
6.8052
30.13
1.698950


20
75.2274
1.0000
42.72
1.834810


21
19.8119
7.0000










[Variable Distance Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
63.0000
320.0000
1000.0000



d0
0.0000
0.0000
0.0000



d5
2.5379
32.1031
41.0906



d10
41.4895
11.9243
2.9368



d11
45.8266
26.2442
2.5168



d16
2.9876
22.5700
46.2974











[Conditional expression correspondence value]













Conditional expression (1)
f2F/f2 = 1.105



Conditional expression (2)
q2 = −0.392



Conditional expression (3)
f2/f1 = −0.251



Conditional expression (4)
T360 = 0.74



Conditional expression (5)
νd2 = 32.3



Conditional expression (6)
|q4| = 29.95



Conditional expression (7)
PgF2 = 0.5899



Conditional expression (8)
νd4 = 30.13



Conditional expression (9)
nd4 = 1.69895










As the data in Table 3 shows, all the conditional expressions (1) to (9) are satisfied in the variable power optical system according to Example 3.
FIGS. 6A to 6C are graphs showing various aberrations of the variable power optical system according to Example 3, where FIG. 6A shows a lowest magnification state (f=63.0000), FIG. 6B shows an intermediate magnification state (f=320.0000), and FIG. 6C shows a highest magnification state (f=1000.0000). As seen in each graph showing aberrations, in Example 3, various aberrations are corrected well in each magnification state from the highest magnification state to the lowest magnification state, indicating that this variable power optical system has a superb optical performance.
Example 4
Example 4 will now be described with reference to FIGS. 7A to 7C, FIGS. 8A to 8C and Table 4. FIGS. 7A to 7C are diagrams depicting the lens according to Example 4, where FIG. 7A shows a lowest magnification state (f=60.0000), FIG. 7B shows an intermediate magnification state (f=320.0000), and FIG. 7C shows a highest magnification state (f=1320.0000). As FIGS. 7A to 7C show, the variable power optical system according to Example 4 has, in order from the object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stoop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
The first lens group G1 has, in order from the object, a cemented lens of a negative meniscus lens L11 having a convex surface facing the object and a positive meniscus lens L12 having a convex surface facing the object, and a biconvex lens L13. The second lens group G2 has, in order from the object, a front group G2F which includes a biconcave lens L21, and has a negative refractive power, and a rear group G2R which includes an achromatic cemented lens of a biconcave lens L22, a biconvex lens L23 and a biconcave lens L24. The third lens group G3 has, in order from the object, a biconvex lens L31, and a cemented lens of a negative meniscus lens L32 having a convex surface facing the object and a biconvex lens L33. The fourth lens group G4 has, in order from the object, a front group G4F which includes a biconcave lens L41 and has negative refractive power, and a rear group G4R which includes a cemented lens of a positive meniscus lens L42 having a convex surface facing the object and a negative meniscus lens L43 having a convex surface facing the object.
In the variable power optical system according to this example having the above configuration, upon zooming from the lowest magnification state to the highest magnification state, the first lens group G1 and the fourth lens group G4 are fixed, the second lens group G2 is moved to the image side and the third lens group G3 is moved to the object side.
Table 4 below shows each data of the variable power optical system according to Example 4. The surface members 1 to 22 in Table 4 correspond to the surfaces 1 to 22 in FIGS. 7A to 7C.







TABLE 4







[General Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
60.0000~
320.0000~
1320.0000



FNO
17.98~
24.63~
52.51











[Lens Data]











Surface number
r
d
νd
nd







d0


1
81.5794
1.0000
42.72
1.834810


2
39.4084
3.2238
82.56
1.497820


3
287.8836
0.1500


4
50.0571
3.2788
82.56
1.497820


5
−322.0492
d5


6
−61.7273
1.0000
35.71
1.902650


7
25.1793
1.6364


8
−48.5920
1.2135
70.45
1.487490


9
19.1110
2.9464
32.27
1.738000


10
−21.6847
1.0000
57.36
1.670000


11
196.2649
d11











12
0.0000
d12
(Aperture stop S)












13
76.1594
2.2658
82.56
1.497820


14
−92.9840
0.1500


15
50.9968
5.7558
34.71
1.720467


16
23.5607
2.9688
82.56
1.497820


17
−142.8948
d17


18
−78.5974
1.6409
56.32
1.568830


19
103.7739
7.2740


20
21.4911
6.0000
35.30
1.592700


21
174.2473
6.0000
47.38
1.788000


22
19.5467
7.0000










[Variable Distance Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
60.0000
320.0000
1320.0000



d0
0.0000
0.0000
0.0000



d5
2.0203
41.1667
54.2305



d11
54.9548
15.8085
2.7446



d12
61.5439
38.5006
2.7589



d17
1.9831
25.0264
60.7681











[Conditional expression correspondence value]













Conditional expression (1)
f2F/f2 = 0.929



Conditional expression (2)
q2 = −0.421



Conditional expression (3)
f2/f1 = −0.241



Conditional expression (4)
T360 = 0.74



Conditional expression (5)
νd2 = 32.3



Conditional expression (6)
|q4| = 21.11



Conditional expression (7)
PgF2 = 0.5899



Conditional expression (8)
νd4 = 35.3



Conditional expression (9)
nd4 = 1.59270










As the data in Table 4 shows, all the conditional expressions (1) to (9) are satisfied in the variable power optical system according to Example 4.
FIGS. 8A to 8C are graphs showing various aberrations of the variable power optical system according to Example 4, where FIG. 8A shows a lowest magnification state (f=60.0000), FIG. 8B shows an intermediate magnification state (f=320.0000), and FIG. 8C shows a highest magnification state (f=1320.0000). As seen in each graph showing aberrations, in Example 4, various aberrations are corrected well in each magnification state from the highest magnification state to the lowest magnification state, indicating that this variable power optical system has a superb optical performance.
Example 5
Example 5 will now be described with reference to FIGS. 9A to 9C, FIGS. 10A to 10C and Table 5. FIGS. 9A to 9C are diagrams depicting the lens according to Example 5, where FIG. 9A shows a lowest magnification state (f=50.4000), FIG. 9B shows an intermediate magnification state (f=255.4000), and FIG. 9C shows a highest magnification state (f=806.4000). As FIGS. 9A to 9C show, the variable power optical system according to Example 5 has, in order from the object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stoop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
The first lens group G1 has, in order from the object, a cemented lens of a negative meniscus lens L11 having a convex surface facing the object and a biconvex lens L12, and a positive meniscus lens L13 having a convex surface facing the object. The second lens group G2 has, in order from the object, a front group G2F which includes a biconcave lens L21 and a biconcave lens L22, and has a negative refractive power, and a rear group G2R which includes an achromatic cemented lens of a biconvex lens L23 and a negative meniscus lens L24 having a concave surface facing the object. The third lens group G3 has, in order from the object, a biconvex lens L31, and a cemented lens of a negative meniscus lens L32 having a convex surface facing the object and a biconvex lens L33. The fourth lens group G4 has a biconcave lens L41 and a positive meniscus lens L42 having a convex surface facing the object.
In the variable power optical system according to this example having the above configuration, upon zooming from the lowest magnification state to the highest magnification state, the first lens group G1 and the fourth lens group G4 are fixed, the second lens group G2 is moved to the image side and the third lens group G3 is moved to the object side. Table 5 shows each data of the variable power optical system according to Example 5. The surface members 1 to 21 in Table 5 correspond to the surfaces 1 to 21 in FIGS. 9A to 9C.







TABLE 5







[General Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
50.4000~
255.4000~
806.4000



FNO
15.83~
22.58~
41.89











[Lens Data]











Surface number
r
d
νd
nd







d0


1
53.8702
3.0654
42.72
1.834810


2
31.6153
3.2177
82.56
1.497820


3
−228.3800
0.1500


4
28.7188
1.8757
82.56
1.497820


5
35.1807
d5


6
−115.1220
1.0000
46.58
1.804000


7
14.6475
2.3715


8
−35.7682
1.0000
46.58
1.804000


9
109.2343
0.1500


10
26.3386
2.8565
32.27
1.738000


11
−28.2457
1.0715
68.33
1.592400


12
−55.9329
d12











13
0.0000
d13
(Aperture stop S)












14
144.2578
5.0000
71.31
1.569070


15
−106.3280
2.7622


16
82.6294
5.0000
32.27
1.738000


17
29.5478
2.9283
71.31
1.569070


18
−88.9623
d18


19
−77.9499
1.0000
57.36
1.670000


20
27.0867
2.1691
35.33
1.749505


21
95.2686
8.0000










[Variable Distance Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
50.4000
255.4000
806.4000



d0
0.0000
0.0000
0.0000



d5
1.6437
40.8510
52.8087



d12
51.9085
12.7012
0.7435



d13
61.5685
35.2907
3.2384



d18
2.5104
28.7882
60.8405











[Conditional expression correspondence value]













Conditional expression (1)
f2F/f2 = 0.430



Conditional expression (2)
q2 = −0.774



Conditional expression (3)
f2/f1 = −0.257



Conditional expression (4)
T360 = 0.74



Conditional expression (5)
νd2 = 32.3










As the data in Table 5 shows, all the conditional expressions (1) to (5) are satisfied in the variable power optical system according to Example 5. FIGS. 10A to 10C are graphs showing various aberrations of the variable power optical system according to Example 5, where FIG. 10A shows a lowest magnification state (f=50.4000), FIG. 10B shows an intermediate magnification state (f=255.4000), and FIG. 10C shows a highest magnification state (f=806.4000). As seen in each graph showing aberrations, in Example 5, various aberrations are corrected well in each magnification state from the highest magnification state to the lowest magnification state, indicating that this variable power optical system has a superb optical performance.
Example 6
Example 6 will now be described with reference to FIGS. 11A to 11C, FIGS. 12A to 12C and Table 6. FIGS. 11A to 11C are diagrams depicting the lens according to Example 6, where FIG. 11A shows a lowest magnification state (f=50.4000), FIG. 11B shows an intermediate magnification state (f=252.4000), and FIG. 11C shows a highest magnification state (f=806.4000). As FIGS. 11A to 11C show, the variable power optical system according to Example 6 has, in order from the object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stoop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power.
The first lens group G1 has, in order from the object, a cemented lens of a negative meniscus lens L11 having a convex surface facing the object and a positive meniscus lens L12 having a convex surface facing the object, and a positive meniscus lens L13 having a convex surface facing the object. The second lens group G2 has, in order from the object, a front group G2F which includes a negative meniscus lens L21 having a convex surface facing the object, and has a negative refractive power, and a rear group G2R which includes an achromatic cemented lens of a biconcave lens L22 and a biconvex lens L23. The third lens group G3 has, in order from the object, a biconvex lens L31, and a cemented lens of a negative meniscus lens L32 having a convex surface facing the object and a biconvex lens L33. The fourth lens group G4 has a cemented lens of a positive meniscus lens L41 having a concave surface facing the object and a biconcave lens L42.
In the variable power optical system according to this example having the above configuration, upon zooming from the lowest magnification state to the highest magnification state, the first lens group G1 and the fourth lens group G4 are fixed, the second lens group G2 is moved to the image side and the third lens group G3 is moved to the object side. Table 6 shows each data of the variable power optical system according to Example 6. The surface members 1 to 19 in Table 6 correspond to the surfaces 1 to 19 in FIGS. 11A to 11C.







TABLE 6







[General Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
50.4000~
252.4000~
806.4000



FNO
15.69~
22.39~
42.07











[Lens Data]











Surface number
r
d
νd
nd







d0


1
134.0840
1.0000
42.72
1.834810


2
53.2279
2.6938
82.56
1.497820


3
−335.8070
0.1500


4
45.5850
2.4500
82.56
1.497820


5
210.0983
d5


6
49.1503
1.0000
46.58
1.804000


7
19.4288
5.2550


8
−24.7077
1.6091
68.33
1.592400


9
23.9940
5.0000
32.27
1.738000


10
−146.7920
d10











11
0.0000
d11
(Aperture stop S)












12
167.1732
5.0000
71.31
1.569070


13
−126.2240
6.3761


14
71.7975
5.0000
32.27
1.738000


15
30.3223
2.8264
71.31
1.569070


16
−110.3420
d16


17
−74.8231
2.0590
35.33
1.749505


18
−28.1913
1.0000
57.36
1.670000


19
127.0133
8.0000










[Variable Distance Data]













Lowest
Intermediate
Highest




magnification
magnification
magnification




state
state
state







f
50.4000
252.4000
806.4000



d0
0.0000
0.0000
0.0000



d5
1.0096
43.3310
56.5269



d10
56.2825
13.9612
0.7653



d11
64.1268
35.9252
0.7570



d16
1.0670
29.2686
64.4368











[Conditional expression correspondence value]













Conditional expression (1)
f2F/f2 = 1.536



Conditional expression (2)
q2 = −2.307



Conditional expression (3)
f2/f1 = −0.263



Conditional expression (4)
T360 = 0.74



Conditional expression (5)
νd2 = 32.3










As the data in Table 6 shows, all the conditional expressions (1) to (5) are satisfied in the variable power optical system according to Example 6. FIGS. 12A to 12C are graphs showing various aberrations of the variable power optical system according to Example 6, where FIG. 12A shows a lowest magnification state (f=50.4000), FIG. 12B shows an intermediate magnification state (f=252.4000), and FIG. 12C shows a highest magnification state (f=806.4000). As seen in each graph showing aberrations, in Example 6, various aberrations are corrected well in each magnification state from the highest magnification state to the lowest magnification state, indicating that this variable power optical system has a superb optical performance.
Now an image optical system which is used in combination with the variable power optical system according to each example will be described, with reference to FIG. 13 and Table 7. As FIG. 13 shows, the image optical system has, in order from the object, a cemented lens of a biconvex lens L11 and a negative meniscus lens L12 having a concave surface facing the object, and a plane parallel glass block L12. Table 7 shows the data values of this image optical system. In Table 7, f′ is a focal length of the entire image optical system. The rest of the description is the same as Table 1 to Table 6.








TABLE 7







[Lens Data]



f′ = 200












radius of
surface




surface number
curvature
distance
νd
nd














1
212.0000
3.0000
82.56
1.497820


2
−65.1000
2.0000
41.49
1.575010


3
−150.0000
2.0000


4
0.0000
22.0000
56.32
1.568830


5
0.0000









As shown above, the variable power region is about 16 times to 25 times, although the maximum numerical aperture of the objective optical system (f=80 mm) is 0.15 or more, that is, the variable power optical system for a parallel system stereomicroscope having good optical performance can be provided.
The embodiment was described using the configuration requirements to clearly explain various embodiments of the present invention, but needless to say, various embodiments of the present invention are not limited to this embodiment.
For example, it is sufficient if the second lens group G2 and the third lens group G3 are constructed to move in opposite directions along the optical axis in at least a part of the variable power block, and the second lens group G2 and the third lens group G3 may be constructed to move (decentering movement) so as to have components orthogonal to the optical axis in the rest of the variable power block.
DESCRIPTION OF SYMBOLS


    
    
        G1 first lens group
        G2 second lens group
        G2F front group (of the second lens group)
        G2R rear group (of the second lens group)
        G3 third lens group
        G4 fourth lens group
        G4F front group (of the fourth lens group)
        G4R rear group (of the fourth lens group)
        S aperture stop
    
    


The many features and advantages of embodiments of the present invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the embodiments that fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the inventive embodiments to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope thereof.

## Claims

1. A variable power optical system for a stereomicroscope comprising, in order from an object:
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 negative refractive power,
the second lens group including, in order from the object, a front group having negative refractive power and a rear group having an achromatic cemented lens including a positive lens and a negative lens,
the second lens group and the third lens group moving in opposite directions along an optical axis at least in a part of a variable power block, and
the following conditional expressions being satisfied:


0.4<f2F/f2≦1.105



−3.0<q2<−0.3


where f2 denotes a focal length of the second lens group, f2F denotes a focal length of the front group, q2 denotes a form factor of a negative lens disposed closest to the object in the front group, the form factor q2 is defined by q2=(r22+r21)/(r22−r21), where r21 denotes a radius of curvature of an object side lens surface of the negative lens in the front group, and r22 denotes a radius of curvature of an image side lens surface of the negative lens in the front group.

2. The variable power optical system for a stereomicroscope according to claim 1, wherein
the first lens group includes a negative lens disposed closest to the object, and
the following conditional expression is satisfied:


−0.3<f2/f1<−0.2


where f1 denotes a focal length of the first lens group, and f2 denotes a focal length of the second lens group.

3. The variable power optical system for a stereomicroscope according to claim 2, wherein
the positive lens constituting the cemented lens in the rear group of the second lens group satisfies the following conditional expressions:


T360≧0.5



vd2<40


where T360 denotes an internal transmittance at wavelength 360 nm without reflection loss when a thickness of an optical material to be used is 10 mm, and vd2 denotes an Abbe number of the optical material to be used.

4. The variable power optical system for a stereomicroscope according to claim 1, wherein
the positive lens constituting the cemented lens in the rear group of the second lens group satisfies the following conditional expressions:


T360≧0.5



vd2<40


where T360 denotes an internal transmittance at wavelength 360 nm without reflection loss when a thickness of an optical material to be used is 10 mm, and vd2 denotes an Abbe number of the optical material to be used.

5. A variable power optical system for a stereomicroscope comprising, in order from an object:
a first lens group having positive refractive power;
a second lens group having negative refractive power;
a third lens group; and
a fourth lens group,
the second lens group including, in order from the object, a front group having negative refractive power and a rear group, and
the following conditional expressions being satisfied:


0.4<f2F/f2≦1.105



−0.3<f2/f1<−0.2


where f2 denotes a focal length of the lens group, f2F denotes a focal length of the front group constituting the second lens group, and f1 denotes a focal length of the first lens group.

6. A variable power optical system according to claim 5, wherein the lens group further comprises:
a rear group having an achromatic cemented lens including a positive lens and a negative lens.

7. A stereomicroscope comprising the variable power optical system according to claim 5.