Ocular lens system

An ocular system is disclosed which is composed of at least three lens elements for constituting, in order from the eye side, at least first and second lens units. At least one of the lens elements is made of resin. Two of the lens elements are substantially cemented together. In addition, an aspherical plastic lens is effectively used in the system.

BACKGROUND OF THE INVENTION 
The present invention relates to an ocular lens for binoculars, 
astronomical telescopes and the like, which may be made in low cost with a 
wide viewing angle. 
The following three-element types of ocular lens systems for binoculars or 
the like are known; 
i) A three-lens element system for an ocular lens for binoculars (for 
example, Japanese Patent Unexamined Publication No. 51-120231); 
ii) A three-unit and three-element system for an ocular lens for 
viewfinders, which uses a plastic material as an optical material with an 
aspherical surface (for example, Japanese Patent Unexamined Publication 
Nos. 60-227215 and 63-318514); and 
iii) A two-unit and three-element system for a viewfinder ocular system, 
which uses only optical glass for lens elements (for example, Japanese 
Patent Unexamined Publication No. 61-48810). 
However, the foregoing systems suffer from the following disadvantages: 
The ocular system i) suffers from a disadvantage that a curvature of field, 
an astigmatism and the like could not sufficiently be compensated for in a 
wide viewing angle. The ocular system ii) suffers from a disadvantage that 
it would be difficult to compensate for aberrations in a wide viewing 
angle due to the generation of the transversal chromatic aberration and 
the like although the curvature of field would be small. Also, the ocular 
system ii) and iii) have a small viewing angle. It should be also noted 
that any of the systems i) to iii) could not be manufactured at a low 
cost. 
On the other hand, there have been proposed ocular systems each of which 
includes four to six lens elements in order to carry out a good aberration 
compensation covering a marginal portion of the view field (see Japanese 
Patent Examined Publication Nos. 63-6851 and 60-48014). 
However, in the types having four or more lens elements, there is a serious 
problem that the manufacturing cost therefor becomes increased and a 
physical size would be increased. In order to perform a satisfactory 
aberration compensation with the ocular system which consists of four lens 
elements, it is necessary that an optical glass having a high refractive 
index be used for each lens and a radius of curvature be shortened. As a 
result, it is difficult to carry out the machining so that the 
manufacturing cost is increased. In particular, it is very difficult to 
make an aspherical surface on a glass lens, resulting in increase of 
manufacturing cost. In order to cope with these problems, it is proposed 
to use a plastic lens element. However, in general, the refractive index 
of the plastic is low so that it would be difficult to fully compensate 
for the aberration, particularly, coma aberration and spherical 
aberration. 
SUMMARY OF THE INVENTION 
Accordingly, in view of the foregoing disadvantages inherent in the prior 
art, an object of the invention is to provide an ocular lens system which 
may be manufactured at low cost, and in which a satisfactory aberration 
compensation is effected even in a wide viewing field of 50.degree. to 
60.degree. in appearance. 
In order to meet this object, according to the present invention, there is 
provided a two-unit and three-element ocular system which is composed, in 
order from the eye side, of a positive first lens unit and a positive 
second lens unit, the first lens unit consisting of a single positive lens 
element, the second lens unit consisting of a cemented lens composed of a 
positive lens element and a negative lens element or of a negative lens 
element and a positive lens element, and at least one of the three lens 
elements being made of optical material of resin. 
The above-specified ocular lens system, preferably, meets the following 
conditions: 
EQU .vertline.r.sub.4 /r.sub.n .vertline.&lt;0.47 (1) 
EQU 0.10&lt;f/f.sub.II &lt;0.65 (2) 
where r.sub.4 is the radius of curvature of the bonded surface of the 
second lens unit, r.sub.n is the radius of cuvature of the surface other 
than the bonded surface of the negative lens element of the second lens 
unit, f is the overall focal length, and f.sub.II is the focal length of 
the second lens unit. 
It is more preferable to provide an aspherical surface, given by the 
following condition, to the ocular lens system in order to compensate for 
the curvature of field and astigmatism. 
##EQU1## 
where X is an amount of deviation from an apex of a lens in the axial 
direction, y is an amount of deviation from the apex of the lens in a 
vertical direction relative to the axial direction, C is the inverted 
number of the radius of curvature, K is a conical constant, and A.sub.4, 
A.sub.6, . . . are the aspherical suface coefficients. 
The following conditions should be met in order to facilitate the 
manufacturing of the ocular lens at low cost. 
EQU .vertline.r.sub.p /r.sub.n .vertline.&lt;1.55 (3) 
EQU n&lt;1.60 (4) 
where r.sub.p is the radius of curvature other than the bonded surface of 
the positive lens element of the second lens unit, and n is the average 
value of the refractive indexes of the three lens elements on d-line. 
Furthermore, in order to keep the various aberrations at reasonable values, 
the following condition should be met. 
EQU -1.70&lt;r.sub.2 /r.sub.1 .ltoreq.0 (5) 
where r.sub.1 and r.sub.2 are the radii of curvature of the surfaces, on 
the eye side and the image side, of the first lens unit. 
According to the present invention, since the resinous optical material is 
used for at least one of the lens elements of the two-unit and 
three-element system, it is possible to manufacture the system at low 
cost. 
According to another aspect of the present invention, there is provided an 
ocular optical system which is composed, in order from the eye side, of a 
positive first lens element, a negative meniscus second lens element, a 
positive third lens element and a negative fourth lens element. The first 
and second lens elements are made of optical glass or resinous optical 
material, and the third and fourth lens elements are made of resinous 
optical material. At least one of the lens surfaces is preferably made as 
an aspherical surface as described before. 
In the ocular system including four lens elements, the average refractive 
index condition (4) should also be met. In addition, the following 
condition should be met. 
EQU -4.0&lt;f.sub.4 /f&lt;-0.5 (6) 
where f.sub.4 is the focal length of the fourth lens element. 
According to the present invention, the resinous optical material is used 
for the ocular lens system so that it is easy to machine the resinous 
material, which is benefitial for mass-production. In addition, the 
present invention compensates for the various aberrations without 
increasing the number of the lens elements.