JP2008122640A - Microscope objective lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microscope objective lens which has a numerical aperture of the extent of 0.65 and has a color aberration corrected in a wide wavelength region ranging from ultraviolet to near-infrared (405 nm to 1,064 nm). <P>SOLUTION: The microscope objective lens is composed of a positive first lens group G1, a positive second lens group G2 and a negative third lens group G3 in the order from the object side, wherein an achromatic lens, in which a positive lens and a negative lens are joined, is disposed on each of G1 to G3 at least by one, a diffraction optical face is formed on at least one face of an optical element constituting G2 and, when a partial dispersion ratio of glass material and absolute value of a difference of the Abbe number of positive lens and negative lens of an achromatic lens of G1 are ΔP1, Δvd1, a partial dispersion ratio of glass material and absolute value of a difference of the Abbe number of positive lens and negative lens of an achromatic lens of G2 are ΔP2, Δvd2, a focal distance of the whole system is f and a focal distance of G1 is f1, the microscope objective lens satisfies the following relations: ΔP1>0.04, ΔP2>0.04, Δvd1>20.0, Δvd2>25.0 and 1.0<f1/f<1.6. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a microscope objective lens.

  Conventionally, in an objective lens used in a biological microscope, correction of chromatic aberration is generally only in the visible range. However, in addition to the conventional fluorescence observation using ultraviolet light, in recent years, since it is applied to optical tweezers and multiphoton excitation is performed in the near infrared, a microscope objective in which chromatic aberration from ultraviolet to near infrared is well corrected. There is an increasing market demand for lenses. For this reason, a chromatic aberration correction range extending from 400 nm to 1000 nm has been developed (see, for example, Patent Document 1).

JP 2006-133248 A

  However, since a large amount of anomalous dispersion glass is required to widen the chromatic aberration correction range, there is a problem that the cost becomes very high.

  The present invention has been made in view of such problems, has a magnification of about 40 times, a numerical aperture of about 0.65, and a wide wavelength range from ultraviolet to near infrared (405 nm to 1064 nm). An object of the present invention is to provide a microscope objective lens in which chromatic aberration is corrected.

In order to solve the above problems, a microscope objective lens according to the present invention includes, in order from the object side, a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a negative refractive power. And a third lens group. At this time, the first lens group has at least one achromatic lens formed by bonding a positive lens and a negative lens, and the second lens group is formed by bonding a positive lens and a negative lens. An achromatic lens in which a diffractive optical surface is formed on at least one surface of an optical element constituting the second lens group, and the third lens group is formed by joining a positive lens and a negative lens. It is comprised including at least one. Then, the refractive index for the d-line is nd, the refractive index for the light with a wavelength of 1064 nm is n1064, the refractive index for the h-line is nh, and the partial dispersion ratio P of the d-line and h-line with respect to the light with a wavelength of 1064 nm is
P = (nd−n1064) / (nh−n1064)
, The absolute value ΔP1 of the difference between the partial dispersion ratio of the positive lens glass material and the partial dispersion ratio of the negative lens glass material included in the first lens group, and included in the second lens group. The absolute value ΔP2 of the difference between the partial dispersion ratio of the glass material of the positive lens of the achromatic lens and the partial dispersion ratio of the glass material of the negative lens is expressed by the following equation:
ΔP1> 0.04, ΔP2> 0.04 (1)
Satisfied. In addition, the absolute value Δνd1 of the difference between the Abbe number of the positive lens glass material and the negative lens glass material of the achromatic lens included in the first lens group, and the achromatic lens included in the second lens group The absolute value Δνd2 of the difference between the Abbe number of the positive lens glass material and the negative lens glass material is
Δνd1> 20.0, Δνd2> 20.0 (2)
Satisfied. Furthermore, the focal length f of the entire system and the focal length f1 of the first lens group are expressed by the following equations.
1.0 <f1 / f <1.6 (3)
Satisfied.

In such a microscope objective lens according to the present invention, the absolute value ΔP3 of the difference between the partial dispersion ratio of the positive lens glass material and the negative lens glass material included in the third lens group is the following. formula
ΔP3 <0.04 (4)
Is preferably satisfied.

  Further, the diffractive optical surface is composed of a diffractive optical element in which two optical members formed of different resin materials are joined and a diffraction grating is formed on the joined surface, and the second lens group is made of optical glass, It is preferable to have a diffractive optical element joined in the order of a diffractive optical element and optical glass.

  When the microscope objective lens according to the present invention is configured as described above, it has a magnification of about 40 times, a numerical aperture of about 0.65, and good chromatic aberration in a wide wavelength range from ultraviolet to near infrared (405 nm to 1064 nm). By using a diffractive optical element, the microscope objective lens corrected to the above can be configured with less expensive optical materials such as ED glass and fluorite, and flare can be reduced.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. First, the configuration of the microscope objective lens according to the present invention will be described with reference to FIG. The microscope objective lens includes, in order from the object side (the cover plate C side), a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a first lens group having a negative refractive power. It is composed of three lens groups G3. In such a microscope objective lens, the first lens group G1 includes at least one achromatic lens (lenses L3 and L4 in FIG. 1) formed by cementing a positive lens and a negative lens.

  The second lens group G2 has at least one achromatic lens (lenses L5 and L6 in FIG. 1) formed by cementing a positive lens and a negative lens, and constitutes the second lens group G2. A diffractive optical surface is formed on any surface of the optical element. For example, in the microscope objective lens shown in FIG. 1, a diffractive optical element in which an optical glass L7 and two optical members L8 and L9 formed from different resin materials are bonded and a diffraction grating D is formed on the bonded surface. And a diffractive optical element in which the optical glass L10 is joined in this order.

  Further, the third lens group G3 has a negative refracting power as a whole, and includes at least one achromatic lens (lenses L11 and L12 in FIG. 1) formed by joining a positive lens and a negative lens. The

  The diffractive optical element (diffractive optical surface) has a negative dispersion value (Abbe number = −3.453), a large dispersion, and a strong anomalous dispersion (partial dispersion ratio = 0.2956). Has chromatic aberration correction capability. The Abbe number of the optical glass is usually about 30 to 80, but the Abbe number of the diffractive optical element has a negative value. In other words, in the diffractive optical element, the longer the wavelength of light, the larger the bending. Therefore, it is possible to correct chromatic aberration that cannot be achieved with ordinary optical glass. That is, the microscope objective lens according to the present embodiment is configured with a small number of lenses by using a diffractive optical element having a negative dispersion characteristic formed of a resin without using many expensive optical materials such as ED glass and fluorite. To do. The advantage that the diffractive optical elements L8 and L9 are made of resin is that a diffraction grating surface can be formed by molding and ultraviolet curing more easily than ordinary optical glass.

  The conditions for configuring the microscope objective lens according to the present embodiment will be described below. This microscope objective lens is a positive lens (in FIG. 1) of the achromatic lens included in the first lens group G1, where P is the partial dispersion ratio of d-line (588 nm) and h-line (405 nm) to a 1064 nm ray. The absolute value of the difference in the partial dispersion ratio P between the glass material of the lens L4) and the glass material of the negative lens (lens L3 in FIG. 1) is ΔP1, and the positive lens of the achromatic lens included in the second lens group G2 (FIG. 1). When the absolute value ΔP2 of the difference in the partial dispersion ratio P between the glass material of the lens L6) and the glass material of the negative lens (lens L5 in FIG. 1) is satisfied, the following conditional expression (1) is satisfied. Yes.

              ΔP1> 0.04, ΔP2> 0.04 (1)

  The partial dispersion ratio P of the d-line and h-line with respect to the 1064 nm ray is expressed as follows when the refractive index for the d-line is nd, the refractive index for the light with a wavelength of 1064 nm is n1064, and the refractive index for the h-line is nh. It is defined as an expression.

            P = (nd−n1064) / (nh−n1064)

  Further, in this microscope objective lens, the absolute value of the difference between the glass material Abbe number of the positive lens and the glass material of the negative lens included in the first lens group G1 is Δνd1, and the second lens group G2 When the absolute value of the difference between the Abbe number of the positive lens glass material and the negative lens glass material included in the lens is Δνd2, the following conditional expression (2) is satisfied. .

              Δνd1> 20.0, Δνd2> 20.0 (2)

  Furthermore, this microscope objective lens is configured to satisfy the following conditional expression (3), where f is the focal length of the entire system and f1 is the focal length of the first lens group G1.

                  1.0 <f1 / f <1.6 (3)

  Conditional expression (1) defines chromatic aberration characteristics of an achromatic lens used in combination with a diffractive optical element. The achromatism in the case of using a diffractive optical element needs to be selected differently from the normal selection of glass. In particular, when correcting the secondary spectrum, the difference in partial dispersion ratio between the positive lens and the negative lens constituting the achromatic lens is usually selected to be close to 0, but this is different when a diffractive optical element is used. In other words, diffractive optical elements have an Abbe number different from that of ordinary optical glass, and chromatic aberration generated on the diffractive optical surface is linear with respect to the wavelength. It is necessary to align with the chromatic aberration characteristics of the optical element. Therefore, the achromatic lens used for the microscope objective lens according to the present embodiment needs to satisfy the conditional expression (1). If the range defined by the conditional expression (1) is not satisfied, a secondary spectrum remains even if a diffractive optical element is used.

  On the other hand, it is necessary to correct the chromatic aberration on the primary axis simultaneously with the secondary spectrum, but in order to set the grating pitch of the diffractive optical element (diffraction grating D) to a manufacturable range, to this diffractive optical element It is necessary to reduce the burden of achromatism. For this reason, the achromatic lens used in combination with this diffractive optical element must mainly be responsible for primary chromatic aberration correction. Conditional expression (2) is a condition for causing the achromatic lens used in the microscope objective lens according to the present embodiment to perform primary chromatic aberration correction as described above, and is defined by this conditional expression (2). Outside the above range, the burden of chromatic aberration correction on the diffractive optical element increases, the grating pitch becomes fine, making manufacturing difficult, and complete correction of chromatic aberration becomes impossible.

  Conditional expression (3) relates to correction of spherical aberration. If the lower limit of conditional expression (3) is not reached, higher-order bending of spherical aberration occurs, making correction difficult. On the contrary, if the upper limit of conditional expression (3) is exceeded, the correction burden of spherical aberration in the second lens group G2 increases, and it becomes impossible to achieve correction of spherical aberration simultaneously with chromatic aberration.

  In the microscope objective lens according to the present embodiment, the glass material of the positive lens (lens L11 in FIG. 1) and the glass material of the negative lens (lens L12 in FIG. 1) included in the third lens group G3. When the absolute value of the difference in the partial dispersion ratio P is ΔP3, it is preferable that the following conditional expression (4) is satisfied.

                            ΔP3 <0.04 (4)

  Conditional expression (4) relates to correction of chromatic aberration of magnification. In the microscope objective lens according to the present example, achromaticity by the diffractive optical element is mainly used for correcting axial chromatic aberration, and thus cannot contribute to correcting chromatic aberration of magnification. Therefore, the correction of the secondary spectrum of magnification can be achieved by selecting the glass material constituting the achromatic lens of the third lens group G3 so as to satisfy the conditional expression (4). If the range defined by the conditional expression (4) is not satisfied, a secondary spectrum of the color of magnification will remain.

  In the following, two examples of the microscope objective lens according to the present invention are shown. In each example, the phase difference of the diffractive optical surface formed in the second lens group G2 is the normal refractive index and an aspheric surface to be described later. It calculated by the ultrahigh refractive index method performed using Formula (5). The ultrahigh refractive index method uses a certain equivalent relationship between the aspherical shape and the grating pitch of the diffractive optical surface. In this embodiment, the diffractive optical surface is represented by data of the ultrahigh refractive index method. That is, it is shown from an aspherical expression (5) described later and its coefficient. In the present embodiment, d-line, g-line, C-line, and F-line are selected as aberration characteristic calculation targets. The wavelength of these d-line, C-line, F-line, and g-line used in this example and the value of the refractive index to be used for the calculation of the ultra-high refractive index method set for each spectral line. Shown in Table 1 below.

(Table 1)
Wavelength Refractive index (by ultra-high refractive index method)
d-line 587.562nm 10001
C line 656.273nm 11170.4255
F line 486.133nm 8274.7311
g-line 435.835nm 7418.6853

In each embodiment, the height of the aspheric surface in the direction perpendicular to the optical axis is y, and the distance (sag amount) along the optical axis from the tangential plane of the apex of each aspheric surface to each aspheric surface at height y. Is S (y), r is the radius of curvature of the reference sphere (vertex radius of curvature), κ is the conic coefficient, and C _n is the nth-order aspheric coefficient, it is expressed by the following equation (5). At this time, the paraxial radius of curvature R is expressed by the following formula (6).

S (y) = (y ² / r) / {1+ (1−κ · y ² / r ² ) ^1/2 }
+ C ₂ · y ² + C ₄ · y ⁴ + C ₆ · y ⁶ + C ₈ · y ⁸ + C ₁₀ · y ¹⁰ + (5)
R = 1 / (1 / r + 2C ₂ ) (6)

  In each example, the lens surface on which the diffractive optical surface is formed is marked with an asterisk (*) on the right side of the surface number in the table. The aspherical expression (5) indicates the performance of the diffractive optical surface. The specifications are shown.

  Further, the microscope objective lens in each example is of the infinity correction type, has the configuration shown in FIG. 2, and is used together with the imaging lens having the specifications shown in Table 2. In Table 2, the first column m is the number of each optical surface from the object side, and corresponds to the surface numbers 1 to 6 shown in FIG. The second column r is the radius of curvature of each optical surface, the third column d is the distance on the optical axis from each optical surface to the next optical surface, the fourth column nd is the refractive index with respect to the d-line, and the fifth column A column νd indicates the Abbe number with respect to the d line. The description of the specification table is the same in the following embodiments.

(Table 2)
m r d nd νd
1 75.043 5.1 1.623 57.0
2 -75.043 2 1.750 35.2
3 1600.580 7.5
4 50.256 5.1 1.668 42.0
5 -84.541 1.8 1.613 44.4
6 36.911 5.5

  This imaging lens is composed of, in order from the object side, a cemented lens in which a biconvex lens L21 and a biconcave lens L22 are cemented, and a cemented lens in which a biconvex lens L23 and a biconcave lens L24 are cemented.

(First embodiment)
FIG. 1 used in the above description shows a first embodiment of the microscope objective lens according to the present invention. As described above, this microscope objective lens has, in order from the object side, that is, the cover plate C side, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a negative refractive power. And a third lens group G3. The first lens group G1 includes a positive meniscus lens L1 having a concave surface facing the object side, a positive meniscus lens L2 having a concave surface facing the object side, and a cemented lens (achromatic lens) formed by cementing a biconcave lens L3 and a biconvex lens L4. ). The second lens group G2 is formed of a resin material different from the cemented lens (achromatic lens) obtained by cementing the biconcave lens L5 and the biconvex lens L6 and the flat optical glass L7 as described above. In addition, two flat optical members L8 and L9 are joined, and a diffractive optical element in which a diffraction grating D is formed on the joint surface and a flat optical glass L10 are joined in this order. The Furthermore, the third lens group G3 includes a cemented lens (achromatic lens) in which a biconvex lens L11 and a biconcave lens L12 are cemented.

  Table 3 shows the specifications of the microscope objective lens according to the first example shown in FIG. In Table 3, NA represents the numerical aperture, β represents the magnification, and d0 represents the distance on the optical axis from the cover glass C to the apex of the first lens (lens L1). The numbers of the optical surfaces shown in the first column m (* on the right indicate the lens surfaces formed as diffractive optical surfaces) correspond to the surface numbers 1 to 19 shown in FIG. In the second column r, in the case of a diffractive optical surface, the radius of curvature of the spherical surface serving as a reference for the base aspherical surface is shown. The table also shows values corresponding to the conditional expressions (1) to (4), that is, the condition corresponding values. The description of the above table is the same in other embodiments.

  In addition, “mm” is generally used as the unit of the radius of curvature r, the surface interval d and other lengths published in all the following specifications unless otherwise specified, but the optical system is proportionally enlarged or reduced. Since the same optical performance can be obtained, the unit is not limited to “mm”, and other appropriate units can be used.

  In the first embodiment, the cover glass C has a thickness of 0.17 mm, a refractive index of 1.522 for the d-line, and an Abbe number of 58.8 for the d-line.

(Table 3)
f = 5
NA = 0.65
β = 40
d0 = 0.74

m r d nd νd
1 -3.709 6.15 1.773 49.6
2 -5.636 0.20
3 -134.630 3.65 1.603 65.4
4 -10.330 0.20
5 -179.716 1.10 1.744 44.8
6 13.120 4.70 1.487 70.4
7 -12.099 0.60
8 -1678.660 1.20 1.757 31.6
9 11.150 3.65 1.603 65.4
10 -25.286 0.20
11 ∞ 1.50 1.517 64.1
12 ∞ 0.20 1.557 50.2
13 * ∞ 0 10001.000 -3.5
14 ∞ 0.20 1.528 34.7
15 ∞ 2.50 1.517 64.1
16 ∞ 20.50
17 17.729 3.85 1.755 27.5
18 -10.900 1.55 1.720 34.7
19 10.499

Aspherical data 13th surface κ = 1 C ₂ = −4.03226 × 10 ⁻⁸ C ₄ = −2.58495 × 10 ⁻¹¹
C ₆ = -1.57650 × 10 ⁻¹² C ₈ = −2.20057 × 10 ⁻¹⁴ C ₁₀ = 0.0

Condition corresponding value (1) ΔP1 = 0.051 ΔP2 = 0.056
(2) Δνd1 = 25.6 Δνd2 = 33.8
(3) f1 / f = 1.5
(4) ΔP3 = 0.023

  Thus, it can be seen that all the conditional expressions (1) to (4) are satisfied in the first embodiment. FIG. 3 shows various aberration diagrams of spherical aberration, lateral chromatic aberration, and coma aberration for the d-line, g-line, C-line, F-line, h-line, and light beam having a wavelength of 1064 nm in the first embodiment. In these graphs, d represents the d-line, g represents the g-line, C represents the C-line, F represents the F-line, h represents the h-line, and # represents a light beam having a wavelength of 1064 nm. The spherical aberration diagram shows the numerical aperture value with respect to the maximum aperture, and the coma aberration diagram shows the image height Y when it is 12.5 mm, when it is 8 mm, and when it is 0 mm. The explanation of the above aberration diagrams is the same in other examples. As is apparent from the respective aberration diagrams shown in FIG. 3, it can be seen that in the first example, various aberrations are satisfactorily corrected and excellent imaging performance is ensured.

(Second embodiment)
Next, a microscope objective lens shown in FIG. 4 will be described as a second embodiment. The microscope objective lens shown in FIG. 4 also has, in order from the cover glass C, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a third lens having a negative refractive power. A lens group G3 is included. The first lens group G1 includes a positive meniscus lens L31 having a concave surface facing the object side, a cemented lens (achromatic lens) in which a negative meniscus lens L32 having a convex surface facing the object side and a biconvex lens L33, and an object side. It is composed of a cemented lens (achromatic lens) in which a negative meniscus lens L34 having a convex surface facing to the surface and a biconvex lens L35 are cemented. The second lens group G2 is made of a resin material different from a cemented lens (achromatic lens) in which a negative meniscus lens L36 having a convex surface facing the object side and a biconvex lens L37 are cemented, and a flat optical glass L38. A diffractive optical element in which two flat optical members L39 and L40 formed from the above are joined, a diffractive optical element having a diffraction grating D formed on the joint surface thereof, and a flat optical glass L41 are joined in this order. Consists of Further, the third lens group G3 includes a cemented lens (achromatic lens) in which a biconvex lens L42 and a biconcave lens L43 are cemented.

  The first lens group G1 of the microscope objective lens according to the second example includes two achromatic lenses, that is, an achromatic lens in which a lens L32 and a lens L33 are cemented, a lens L34, and a lens L35. Shows a case where the lens is configured to have a cemented achromatic lens.

  Table 4 shows the specifications of the microscope objective lens according to the second example shown in FIG. In addition, the surface number shown in Table 4 corresponds with the surface numbers 1-20 shown in FIG. The specifications of the cover glass C in the second embodiment are the same as those in the first embodiment.

(Table 4)
f = 5
NA = 0.70
β = 40
d0 = 0.65

m r d nd νd
1 -2.871 4.83 1.804 46.6
2 -4.028 0.95
3 4172.412 1.44 1.717 47.9
4 20.128 3.52 1.487 70.4
5 -15.621 0.62
6 69.285 1.29 1.744 44.8
7 15.541 3.41 1.487 70.4
8 -10.264 0.20
9 167.608 0.89 1.795 28.5
10 10.770 3.29 1.603 65.4
11 -42.057 0.16
12 ∞ 1.50 1.517 64.1
13 ∞ 0.20 1.557 50.2
14 * ∞ 0.00 10001.000 -3.5
15 ∞ 0.20 1.528 34.7
16 ∞ 1.50 1.517 64.1
17 ∞ 17.97
18 11.270 3.48 1.755 27.5
19 -43.823 2.35 1.720 34.7
20 7.670

Aspheric data 14th surface κ = 1 C ₂ = -4.46058 × 10 ^-8 C ₄ = -8.90295 × 10 ^-11
C ₆ = 4.86183 × 10 ^-13 C ₈ = -3.97085 × 10 ^-14 C ₁₀ = 0.0

Condition corresponding value (1) ΔP1 = 0.046 (lenses L32 and L33)
ΔP1 = 0.051 (Lens L34, L35)
ΔP2 ＝ 0.063
(2) Δνd1 = 22.5 (lenses L32 and L33)
Δνd1 = 25.6 (Lens L34, L35)
Δνd2 = 36.9
(3) f1 / f = 1.4
(4) ΔP3 = 0.023

  As described above, in the second embodiment, the conditional expressions (1) to (4) are all satisfied, and any of the two achromatic lenses included in the first lens group G1 is conditional expression (1). It can be seen that (2) is satisfied. FIG. 5 shows various aberration diagrams of spherical aberration, lateral chromatic aberration, and coma aberration of the microscope objective lens according to the second example. As is apparent from the respective aberration diagrams, it is understood that the aberration is corrected well and excellent imaging performance is ensured also in the second embodiment.

It is a lens block diagram of the microscope objective lens which concerns on 1st Example of this invention. It is a figure which shows the lens structure of the imaging lens used with the microscope objective lens which concerns on this invention. FIG. 6 is a diagram illustrating various aberrations of the microscope objective lens according to the first example. It is a lens block diagram of the microscope objective lens which concerns on 2nd Example of this invention. FIG. 6 is various aberration diagrams of the microscope objective lens according to the second example.

Explanation of symbols

G1 First lens group G2 Second lens group G3 Third lens group D Diffraction grating (diffractive optical surface)

Claims (3)

In order from the object side, the first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power,
The first lens group includes at least one achromatic lens formed by bonding a positive lens and a negative lens;
The second lens group has at least one achromatic lens formed by joining a positive lens and a negative lens, and a diffractive optical surface is formed on at least one surface of an optical element constituting the second lens group. ,
The third lens group includes at least one achromatic lens formed by bonding a positive lens and a negative lens;
The refractive index for the d-line is nd, the refractive index for the light beam having a wavelength of 1064 nm is n1064, the refractive index for the h-line beam is nh, and the partial dispersion ratio P of the d-line and h-line for the light beam with a wavelength of 1064 nm
P = (nd−n1064) / (nh−n1064)
When defined in
The absolute value ΔP1 of the difference between the partial dispersion ratio of the glass material of the positive lens and the partial dispersion ratio of the glass material of the negative lens of the achromatic lens included in the first lens group, and included in the second lens group The absolute value ΔP2 of the difference between the partial dispersion ratio of the glass material of the positive lens and the partial dispersion ratio of the glass material of the negative lens of the achromatic lens is given by
ΔP1> 0.04, ΔP2> 0.04 (1)
Satisfied,
The absolute value Δνd1 of the difference between the Abbe number of the glass material of the positive lens and the Abbe number of the glass material of the negative lens of the achromatic lens included in the first lens group, and the color included in the second lens group The absolute value Δνd2 of the difference between the Abbe number of the glass material of the positive lens and the Abbe number of the glass material of the negative lens of the eraser lens is given by
Δνd1> 20.0, Δνd2> 20.0 (2)
Satisfied,
The focal length f of the entire system and the focal length f1 of the first lens group are given by
1.0 <f1 / f <1.6 (3)
Satisfying microscope objective lens. The absolute value ΔP3 of the difference between the glass material partial dispersion ratio of the positive lens and the glass material of the negative lens of the achromatic lens included in the third lens group is expressed by the following equation:
ΔP3 <0.04 (4)
The microscope objective lens according to claim 1, wherein: The diffractive optical surface is composed of a diffractive optical element in which two optical members formed from different resin materials are bonded and a diffraction grating is formed on the bonded surface,
The microscope objective lens according to claim 1 or 2, wherein the second lens group includes a diffractive optical element joined in the order of optical glass, the diffractive optical element, and optical glass.

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