Microscope objective

A microscope objective comprising a first, second, third, fourth and fifth lens components wherein the first lens component is a positive meniscus lens or biconvex lens, the second lens component is a negative lens, the third lens component is a cemented doublet, the fourth lens component is a cemented doublet, and the fifth lens component is a positive cemented doublet, the microscope objective having an extremely low magnification, being arranged to be parfocal with microscope objectives with high magnifications, and having aberrations corrected favorably.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a microscope objective and, more 
particularly, to an anchromat-class microscope objective with the 
magnification about 2.5.times.. 
(b) Description of the Prior Art 
Generally, known microscope objectives can be classified into the following 
two types, i.e., the limited distance type and infinite distance type. The 
limited distance type microscope objective is designed so that the image 
of the object is formed by the objective itself. The infinite distance 
type microscope objective is designed so that the rays, which come from 
the object and enter the objective, go out as parallel rays and is 
arranged that said parallel rays are imaged as an image of the object by 
means of an imaging lens arranged in rear of the objective. In either type 
of microscope objectives, there is almost no known microscope objective 
which is designed for a low magnfication about 2.5.times.. Especially, it 
may be said that there is no known microscope objective which is designed 
for a low magnification about 2.5.times. and, at the same time, which is 
parfocal with microscope objectives with other magnifications. For 
example, the objective disclosed in Japanese published unexamined patent 
application No. 46963/76 is designed for the magnification of 1.times. and 
2.times.. However, for said objective, the quality of image such as 
flatness of image is guaranteed only for the range of height of image up 
to 9 mm and, therefore, the quality of image assured by said objective is 
incomparably worse than that of the microscope objective according to the 
present invention for which the quality of image is guaranteed for a wide 
field range, i.e., the height of image of 13 mm or more. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide an 
achromat-class microscope objective of infinite distance type with the 
magnification about 2.5.times. which has large N.A., i.e., 0.07, and which 
enables to obtain an image with extremely favourable flatness of image 
over a wide field range. 
The microscope objective according to the present invention comprises a 
first, second, third, fourth and fifth lens components in the order from 
the object side as shown in FIG. 1, said first lens component comprising a 
positive lens L.sub.1 which is arranged as a meniscus lens convex toward 
the object side or as a biconvex lens, said second lens component 
comprising a negative lens L.sub.2, said third lens component comprising a 
positive or negative cemented doublet which consists of a positive lens 
L.sub.3 and a negative lens L.sub.4, said fourth lens component comprising 
a positive or negative cemented doublet which consists of a positive lens 
L.sub.5 and a negative lens L.sub.6, said fifth lens component comprising 
a positive cemented doublet which consists of a negative lens L.sub.7 and 
a positive lens L.sub.8, the microscope objective according to the present 
invention being further arranged to fulfill the following conditions: 
##EQU1## 
wherein reference symbol f represents the focal length of the lens system 
as a whole, reference symbol D represents the distance from the surface on 
the image side of the first lens component to the surface on the object 
side of the third lens component, reference symbols n.sub.1 and n.sub.2 
respectively represent refractive indices of the first lens component 
L.sub.1 and the second lens component L.sub.2, reference symbols 
.nu..sub.1 and .nu..sub.2 respectively represent Abbe's numbers of the 
first lens component L.sub.1 and the second lens component L.sub.2, and 
reference symbols .nu..sub.5 and .nu..sub.6 respectively represent Abbe's 
numbers of the positive lens L.sub.5 and negative lens L.sub.6 which 
constitute the fourth lens component. 
As a microscope objective of infinite distance type is used in combination 
with an imaging lens, the rearmost lens component therein is always 
arranged to have positive refractive power. Besides, the quality of image 
is determined by combining the objective with an imaging lens. Therefore, 
when designing the objective, it is necessary to correct aberrations of 
the objective by always taking the imaging lens into consideration, for 
example, the objective should be designed so that it causes adequate 
aberrations in order to cancel aberrations to be caused by the imaging 
lens. 
The condition (1) is to decide the basic composition of the objective 
according to the present invention and is established in order to obtain a 
uniform image over a wide field by keeping the working distance 
sufficiently long. If, in the condition (1), the value of D becomes larger 
than the upper limit thereof, rays in the marginal portion of the field 
cannot be satisfactorily led to the image surface and, consequently, the 
intensity of light in the marginal portion becomes insufficient. When it 
is attempted to prevent the above, the overall length of the objective 
becomes long and it becomes difficult to make the objective parfocal with 
objectives with high magnifications. If the value of D becomes smaller 
than the lower limit of the condition (1), curvature of field becomes 
unfavourable. When it is tried to correct curvature of field by means of 
bending of respective lens surfaces, it becomes impossible to ensure a 
sufficiently long working distance. When it is attempted to make the 
working distance long here, it becomes impossible to satisfactorily lead 
the rays in the marginal portion of the field to the image surface and, 
therefore, the intensity of light in the marginal portion becomes 
insufficient. 
The condition (2) is established in order to correct curvature of field and 
distortion. If the value of (n.sub.1 +n.sub.2)/2 becomes smaller than the 
lower limit of the condition (2), curvature of field is aggravated and 
distortion becomes large. When it is tried to correct these aberrations by 
means of bending of respective lens surfaces, it becomes difficult to 
correct spherical aberration. 
The conditions (3) and (4) are established in order to correct spherical 
aberration in well balanced state over the whole field, i.e., to well 
balance chromatic aberration of spherical aberration, lateral chromatic 
aberration, and chromatic aberration of coma. If, in the condition (3), 
the value of (.nu..sub.1 +.nu..sub.2)/2 becomes smaller than the lower 
limit thereof, lateral chromatic aberration becomes large. If, in the 
condition (4), the value of .nu..sub.5 -.nu..sub.6 becomes smaller than 
the lower limit thereof, chromatic aberration of spherical aberration and 
chromatic aberration of coma become large. If the conditions (3) and/or 
(4) are not fulfilled and it is attempted to correct chromatic aberration, 
which is thereby aggravated, by means of bending of respective lens 
surfaces, other aberrations such as spherical aberration and curvature of 
field will be aggravated. 
As described so far, the microscope objective which fulfills the 
above-mentioned conditions (1) through (4) is such lens system that 
enables to obtain a favourable image over a wide field, in other words, to 
attain the object of the present invention. However, when said objective 
is arranged to further fulfill the conditions (5) through (8) shown below, 
it is possible to obtain a more favourable objective. 
(5) 0.1f&lt;r.sub.1 &lt;0.5f 
(6) -0.3f&lt;r.sub.13 &lt;-0.1f 
(7) 1.6&lt;n.sub.1 
(8) 15&lt;.nu..sub.4 -.nu..sub.3 
In the conditions shown in the above, reference symbol r.sub.1 represents 
the radius of curvature of the surface on the object side of the first 
lens component L.sub.1, reference symbol r.sub.13 represents the radius of 
curvature of the surface on the image side of the fifth lens component 
(surface on the image side of the positive lens L.sub.8), reference symbol 
n.sub.1 represents the refractive index of the first lens component 
L.sub.1, and reference symbols .nu..sub.3 and .nu..sub.4 respectively 
represent Abbe's numbers of the positive lens L.sub.3 and negative lens 
L.sub.4 which constitute the third lens component. 
The condition (5) is established in order to correct distortion in well 
balanced state over the whole field. If the radius of curvature r.sub.1 of 
the surface on the object side of the first lens component L.sub.1 becomes 
larger than the upper limit of the condition (5), curvature of field 
becomes large. When it is tried to correct the above by means of bending 
of other surfaces, distortion becomes large. If r.sub.1 becomes smaller 
than the lower limit of the condition (5), distortion becomes large. 
The condition (6) is established in order to correct spherical aberration, 
curvature of field, distortion, etc. in well balanced state. If the radius 
of curvature r.sub.13 of the surface on the image side of the lens L.sub.8 
becomes larger than the upper limit of the condition (6), spherical 
aberration will be undercorrected. When it is tried to correct the 
above-mentioned spherical aberration by means of bending of other 
surfaces, curvature of field and coma will be aggravated. If r.sub.13 
becomes smaller than the lower limit of the condition (6), distortion 
becomes large and, moreover, spherical aberration will be overcorrected. 
When it is tried to correct these aberrations by means of bending of other 
surfaces, curvature of field and coma will be aggravated. 
The condition (7) is established in order to correct curvature of field and 
distortion in well balanced state. If the refractive index n.sub.1 of the 
first lens component becomes smaller than the lower limit of the condition 
(7), curvature of field will be aggravated. When it is tried to correct it 
by means of bending of respective surfaces, distortion becomes large. 
The condition (8) is established in order to correct lateral chromatic 
aberration favourably and, at the same time, to correct chromatic 
aberration of spherical aberration and chromatic aberration of coma in 
well balanced state. If the value of .nu..sub.4 -.nu..sub.3 becomes 
smaller than the lower limit of the condition (8), it becomes difficult to 
correct lateral chromatic aberration or balance between chromatic 
aberration of spherical aberration and chromatic aberration of coma 
becomes unfavourable when it is tried to correct lateral chromatic 
aberration favourably. 
When the microscope objective according to the present invention is 
arranged to further fulfill the conditions (9) through (11) shown below in 
addition to the above-mentioned conditions, it is possible to obtain a 
microscope objective which assures still more favourable quality of image. 
(9) 0&lt;r.sub.7 &lt;0.3f 
(10) -0.2f&lt;r.sub.10 &lt;-0.1f 
(11) 10&lt;.nu..sub.8 -.nu..sub.7 
In the conditions shown in the above, reference symbol r.sub.7 represents 
the radius of curvature of the surface on the image side of the third lens 
component (surface on the image side of the negative lens L.sub.4), 
reference symbol r.sub.10 represents the radius of curvature of the 
surface on the image side of the fourth lens component (surface on the 
image side of the negative lens L.sub.6), and reference symbols .nu..sub.7 
and .nu..sub.8 respectively represent Abbe's numbers of the negative lens 
L.sub.7 and positive lens L.sub.8 constituting the fifth lens component. 
Out of these conditions, the condition (9) is established in order to 
correct coma. If the radius of curvature r.sub.7 of the surface on the 
image side of the negative lens L.sub.4 becomes larger than the upper 
limit of the condition (9), spherical aberration will be undercorrected 
and symmetry of coma will become unfavourable. If r.sub.7 becomes smaller 
than the lower limit of the condition (9), spherical aberration will be 
overcorrected and symmetry of coma will become unfavourable. When it is 
tried to correct the above-mentioned aberrations by means of bending of 
other surfaces, curvature of field will become unfavourable and distortion 
will become large. 
The condition (10) is established in order to correct spherical aberration, 
curvature of field and coma in well balanced state. When the radius of 
curvature r.sub.10 of the surface on the image side of the negative lens 
L.sub.6 becomes larger than the upper limit of the condition (10), i.e., 
-0.1f, spherical aberration will be undercorrected. When it is tried to 
correct this aberration by means of bending of other surfaces, symmetry of 
coma will become unfavourable. If r.sub.10 becomes smaller than the lower 
limit of the condition (10), i.e., -0.2f, distortion will become large and 
spherical aberration will be obercorrected. When it is tried to correct 
them by means of bending of other surfaces, curvature of field will become 
unfavourable and symmetry of coma will also become unfavourable. 
The condition (11) is established in order to correct chromatic aberration 
of spherical aberration and lateral chromatic aberration. If the value of 
.nu..sub.8 -.nu..sub.7 becomes smaller than the lower limit of the 
condition (11), it is impossible to correct the above-mentioned chromatic 
aberrations in well balanced state. When it is tried to correct chromatic 
aberration of spherical aberration by means of bending of respective 
surfaces, lateral chromatic aberratopm will curve largely and this is not 
preferable.