Optical: systems and elements – Lens – Eyepiece
Reexamination Certificate
2001-09-05
2002-07-23
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
Eyepiece
C359S646000, C359S643000, C359S690000, C359S432000
Reexamination Certificate
active
06424468
ABSTRACT:
This application claims the benefit of Japanese Patent applications Nos. 2000-269193, 2000-270687 and 2001-252345 which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an eyepiece lens and, more particularly, to an eyepiece lens most suitably for the use in a camera, etc.
2. Related Background Art
In an eyepiece lens for observing an image formed by an objective lens through an erect system, it is necessary to enlarge the erect system in order to obtain a higher magnification since a light beam spreads. Then, since the length of an optical path for the light beam is prolonged, the focal length of the eyepiece lens has to be increased in proportion thereto. As a result, it is impossible to achieve a higher magnification. Particularly, an eyepiece lens having a long eye relief (the distance from the eyepiece lens to the eyepoint) has this tendency conspicuously.
Moreover, as an eyepiece lens for an observing optical system of a camera, or the like, there is known a lens which is comprised of three groups including a positive lens group, a negative lens group and another positive lens group, so as to move an arbitrary lens group along the optical to correct the diopter. As such an eyepiece lens, various lenses have been proposed including one which is disclosed in Japanese Patent Post-Examination Publication No. 61-19968.
With the recent electronization of cameras, etc., various display devices such as an image pick-up device or a monitor are increasingly mounted on a camera body. As a result, the camera body has to be enlarged, and an optical finder is required to have a greater distance between the focal surface and the eyepoint correspondingly, compared with conventional ones. Also, an image formation area on an image pick-up device is made smaller than one frame of a conventional silver halide film, so that a finder magnification is also required to be greater. Further, the eyepiece lens is required to additionally have a diopter correcting function, and the like, in order to cope with the visual acuity of any photographer.
SUMMARY OF THE INVENTION
The present invention has been contrived taking the above inconveniences into consideration, and an object of the invention is to provide an eyepiece lens having an excellent aberration performance with a sufficient eye relief length and high magnification, and preferably, an eyepiece lens further having a diopter adjusting function.
In order to solve the above problems, there is provided an eyepiece lens for observing an image formed by an objective lens through an erect system, at least comprising a first lens group having a positive refracting power and a second lens group having a negative refracting power in the order from the side of an observer, and satisfying the following condition:
−3<SF1<−1 (1)
where SF1 indicates a shape factor of the first lens group, and SF indicates a shape factor of the lens group, which is defined by:
SF=
(
Rs+RE
)/(
Rs−RE
),
where Rs indicates the radius of curvature of a lens surface closest to the object side and RE indicates the radius of curvature of a lens surface closest to the observer side.
According to the present invention, with the above structure, it is possible to move the principal point of the entire eyepiece lens to the observer side, and to prevent a prolongation of the focal length of the eyepiece lens which may be caused by the prolongation of the optical path length of the erect system, thereby raising the magnification. It is also possible to secure a sufficient eye relief length since this eyepiece lens has a retro-focus type layout which has positive and negative refracting powers when seen from the object side toward the observer side.
Also according to the present invention, it is preferable to satisfy the following condition:
−3<SF1<−1 (1)
Above the upper limit of the condition (1), the lens is not preferable for improving the finder magnification. On the contrary, below the lower limit of the condition (1), it is difficult to correct a distortion and a coma. This case is not preferable also in terms of manufacturing since a fluctuation in aberration becomes great due to an error such as eccentricity. Accordingly, it is practically desirable to satisfy the condition (1) that −3<SF1<−1
In this case, the layout of the second lens group and the first lens group is of a retro-focal type having the negative and positive refracting powers when seen from the object side to the observer side, so that a sufficient length for the eye relief can be secured.
According to the present invention, it is referable to adjust the diopter by moving the first lens group having the positive refracting power or a lens component having a positive refracting power in the first lens group, along the optical axis. In this manner, it is possible to adjust the diopter while maintaining a corrected aberration state, and particularly, excellent distortion. It is also possible to adjust the diopter by moving a lens component having a negative refracting power in the second lens group along the optical axis. In this case, a lens closer to the focal surface has a greater fluctuation in aberration when the diopter is changed. As a result, it is preferable to adjust the diopter by moving a lens component which is positioned on the observer side, in the second lens group.
Also, according to the present invention, it is preferable to satisfy, in addition to the above condition (1), the following condition (2):
1<SF2<6 (2)
where SF2 indicates the shape factor of the second lens group.
If the condition (2) is satisfied and the second lens group is formed in a meniscus shape concave on the observer side, an excellent aberration state can be securely obtained. Also, since the principal point of the entire eyepiece lens is moved to the image formation surface side, a high magnification can be achieved. Below the lower limit of the condition (2), an astigmatism or a coma which is generated in the first lens group satisfying the condition (1) can not be sufficiently corrected. On the contrary, above the upper limit of the condition (2), it is difficult to correct a distortion. As a result, it is preferable to satisfy the condition (2) that 1<SF2<6.
Further, according to the present invention, in addition to satisfy the condition (1), it is preferable to constitute the second lens group having the negative refracting power with a lens group having a negative refracting power and a lens group having a positive refracting power in this order from the observer side. Then, it is possible to conduct excellent aberration correction while maintaining a wide distance between the lens group having the positive refracting power and the lens group having the negative refracting power in the second lens group. It is also possible to maintain a long distance from the focal surface to the eyepoint to cope with enlargement of a camera size while aiming cost reduction of the glass material.
It is also preferable to satisfy the following condition (3):
−2<SF2p<−1 (3)
where SF2p indicates the shape factor of the positive lens in the second lens group. When the positive lens group in the second lens group is formed into a meniscus form which is concave on the observer side, the principal point of the entire eyepiece lens is moved to the imaging surface side. By the use of this phenomenon, the focal length of the entire eyepiece lens can be reduced to thereby raise the magnification. Above the upper limit of the condition (3), it is difficult to improve the finder magnification. On the contrary, below the lower limit of the condition (3), it is difficult to correct a distortion and a coma. Accordingly, it is preferable to satisfy the condition (3) that −2 <SF2p<−1.
Also, according to the present invention, it is preferable to satisfy the following condition (5):
Dx/fe≧0.08 (5)
where D
Mouri Motohisa
Nozaki Hitoshi
Oshita Kouichi
Lester Evelyn A
Miles & Stockbridge P.C.
Nikon Corporation
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