Ultra-wide field eyepiece for microscopes

The invention provides an ultra-wide field eyepiece for microscopes, in which such off-axial aberrations as the curvature of field, astigmatism and coma are well-enough corrected at a field number as large, as 26.5, and in which the flatness of the image surface is degraded even when the entrance pupil position varies, and which comprises, in order from the entrance side, a first lens unit G1 of negative refracting power, a second lens unit G2 of positive refracting power, a third lens unit G3 of negative refracting power, a fourth lens unit G4 of positive refracting power and a fifth lens unit G5 of positive refracting power with the surface having a stronger curvature facing the entrance side, said third to fifth lens units being located in the rear of an intermediate image plane.

BACKGROUND OF THE INVENTION 
The present invention relates to an ultra-wide field eyepiece for 
microscopes. 
An eyepiece is a sort of loupe, and so has often a simple structure that 
generally comprises a combination of convex lenses. For this reason, the 
Petzval's sum assumes a positive value so large that some curvature of 
field often remains with an increased distortion. In an optical microscope 
system, the curvature of field is set off by inclining images formed by 
the objective and eyepiece in opposite directions, thereby achieving a 
flat image. However, the inclinations of the images are not well 
corrected. In order to ward off this, reliance is generally placed on some 
procedure in which the mean image surface of the meridional and sagittal 
image surfaces is made upright-enough to eliminate the curvature of field. 
So far, this procedure, for instance, has been embodied by a lens system 
set forth in Japanese Provisional Patent Publication No. 60-57315. To be 
more specific, this system is an external focus type of eyepiece that 
comprises four lens units, say, in order from the incident side, a 
positive lens, a negative lens, a positive lens and a positive lens. This 
eyepiece has a field number as wide as 20 with well-corrected curvature of 
field, coma, distortion and other aberrations. 
Another conventional example of the lens system whose astigmatism is again 
reduced by erecting the mean image surface is disclosed in Japanese 
Provisional Patent Publication No. 2-222914. This lens system is an 
external-focus type of eyepiece that comprises, in order from the incident 
side, a first lens unit of positive or negative refracting power and a 
second lens unit of positive refracting power. As in the case of the first 
example, this lens has a field number as large as 20 with well-corrected 
aberrations. 
Conventional examples similar in lens arrangement to the present invention 
are set forth in Japanese Provisional Patent Publication No. 3-87805 and 
Japanese Provisional Patent Publication No. 49-69148. First, the lens 
arrangement described in Japanese Provisional Patent Publication No. 
3-87805 is directed to an eyepiece lens for telescopes that comprises, in 
order from the incident side, a diverging lens element, a converging lens 
element and a multi-lens type of converging element located through an 
intermediate image plane. Japanese Provisional Patent Publication No. 
49-69148, on the other hand, relates to an internal focus type of eyepiece 
for microscopes that comprises five lens units, say, in order from the 
incident side, a positive lens and a negative lens, a positive lens, a 
positive lens and a positive lens, the last four lenses located in the 
rear of an intermediate image plane 
While the eyepiece of Japanese Provisional Patent Publication No. 60-57315 
is excellent in the flatness of the image surface, some astigmatism 
remains in the vicinity of the maximum image height having a field number 
of 20. Moreover, it is of course that the flatness of the image surface 
gets worse as the field number increases. 
The lens system of Japanese Provisional Patent Publication No. 2-222914 is 
improved in terms of astigmatism, but leaves coma intact. Moreover, the 
flatness of the image surface gets worse as the field number increases. 
The lens system of Japanese Provisional Patent Publication No. 3-87805 
cannot stand comparison with an eyepiece for microscopes, because it is an 
eyepiece for telescopes and so is different from the eyepiece for 
microscopes in terms of to what degree aberrations are to be corrected. 
In addition, the eyepiece for telescopes of Japanese Provisional Patent 
Publication No. 49-69148 is very unfavorable in terms of coma in the 
vicinity of the field of view. 
As described above, there has not been an eyepiece that is improved in 
terms of such off-axial aberrations as the curvature of field, astigmatism 
and coma in an ultra-wide field level that refers to a field of view 
having a field number of the order of 26.5 until the present invention. 
Even though a large inclination of the image formed by an eyepiece is set 
off by the inclination of the image formed by an objective, such a large 
inclination of the image cannot be well corrected and so is likely to 
remain. Further, when the meridional image surface is erected so as to 
correct the inclination of the image, the meridional surface separates 
from the sagittal surface, leaving some considerable astigmatism intact 
and giving rise to an image of inferior quality. Still further, when an 
eyepiece is used in combination with an objective having a large numerical 
aperture on the exit side, it is impossible to obtain an image of good 
quality if coma cannot be corrected completely at a place having a large 
numerical aperture. 
When coma remains in the eyepiece, the flatness of the image surface gets 
worse due to the fact that the exit pupil position varies for each 
objective or that even with a single objective, the exit pupil position 
varies by the insertion of an intermediate tube in the rear of the 
objective. 
SUMMARY OF THE INVENTION 
In view of such situations as mentioned above, an object of the invention 
is to provide an ultra-wide field eyepiece for microscope which overcomes 
the defects of the prior art mentioned above, and so is well-enough 
corrected in terms of off-axial aberrations, e.g., the curvature of field, 
astigmatism and coma in an ultra-wide field having a field number of the 
order of 26.5 and in which the flatness of the image surface is not 
deteriorated even by a variation in the entrance pupil position that is 
caused by an objective or the insertion of an intermediate tube. 
According to the present invention, the object mentioned above is achieved 
by the provision of an ultra-wide field eyepiece for microscopes 
comprising a first lens unit of negative refracting power and a second 
lens unit of positive refracting power which are located on the entrance 
side of an intermediate image plane, a third lens unit of negative 
refracting power, a fourth lens unit of positive refracting power and a 
fifth lens unit with the surface having a stronger curvature facing the 
entrance side, which are located on the exit side of said intermediate 
image plane. 
In the following description, why the arrangement mentioned above is 
adopted and how it works will be explained. 
In order to reduce the curvature of field and astigmatism, it is required 
to reduce the Petzval's sum. To this end, a negative lens is incorporated 
in the third lens unit. In other words, the negative lens is located from 
the objective image position (intermediate image position) toward the eye 
side to raise the height of ray. The raised height of ray is gradually 
lowered by the positive refracting power of the fourth lens unit. Finally, 
the light is converged on the eyepoint by the fifth lens unit of positive 
refracting power, with the surface having a stronger curvature facing the 
incident side. 
The first and second lens units are located in front of the objective image 
position so as to correct coma well-enough. In the case of the external 
focus type of lens arrangement or, in other words, in case lens units are 
located from the objective image position toward the eye alone, the 
optical system is made so asymmetric that it is difficult to correct coma 
well in an ultra-wide field. Thus, the coma can be well corrected by the 
incorporation of negative and positive refracting powers in the first and 
second lens units, respectively. The upper marginal light ray in front of 
the objective image position is converted to the lower marginal light ray 
in the rear of the objective image position. The lower marginal light ray 
in front of the objective image, by contrast, is converted to the upper 
marginal light ray in the rear of the objective image. The coma of the 
eyepiece as a whole can be well corrected in a well-balanced state by 
making use of the design that the heights of the upper and lower rays with 
respect to the principal ray are reversed around the objective image 
position or, more specifically, locating the two, negative and positive, 
lens units. 
In order to correct coma more satisfactorily, it is desired that a negative 
meniscus lens convex on the incident side be incorporated in the first 
lens unit and a positive lens convex on the eye side in the second lens 
unit. More preferably, this positive lens should be a positive meniscus 
lens. The off-axial luminous flux incident on the first lens unit is 
lowered in height by the thickness of the first lens unit, thereby 
allowing the negative power of the rear surface of the first lens unit to 
serve effectively. The luminous flux can then be smoothly directed to the 
eye lens through the positive lens of the second lens unit that is convex 
on the eye side. 
Now let f.sub.12 and f denote the composite focal length of the first and 
second lens unit and the focal length of the total system, respectively. 
Then, if the following condition (1): 
EQU .vertline.f/f.sub.12 .vertline.&lt;0.3 (1) 
is satisfied, the coma of the eyepiece as a whole will be easy to correct. 
This means that the composite focal length of the first and second lens 
units is increased or, in other words, the composite refracting power of 
the first and second lens units is reduced. To put it another way, the 
correction of coma is achieved by reducing the Petzval's sum of the 
eyepiece as a whole by regulating the power distributed to the third, 
fourth and fifth lens units, thereby giving refracting powers to the first 
and second lens units in such a way that the power distribution of the 
eyepiece is not adversely affected. If there is a departure from the 
condition (1), the whole power distribution is greatly off balance and the 
balance of offaxial aberrations deteriorates. 
When f.sub.3 is let stand for the focal length of the third lens unit 
contributing greatly to the whole Petzval's sum, the following condition 
(2): 
EQU .vertline.f/f.sub.3 .vertline.&lt;1 (2) 
should be met, because it is then possible to reduce the Petzval's sum, 
thereby placing the whole aberrations in a well-balanced state. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises the features of construction, 
combinations of elements, and arrangement of parts will be exemplified in 
the construction hereinafter set forth, and the scope of the invention 
will be indicated in the claims.