Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-10-18
2004-08-10
Lester, Evelyn A. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S678000, C359S708000, C359S720000, C359S728000, C359S729000, C359S365000, C359S432000
Reexamination Certificate
active
06775073
ABSTRACT:
This application claims benefit of Japanese Application No. 2000-319256 filed in Japan Oct. 19, 2000, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a real image type zoom finder comprising an image-inversion optical system, and more specifically to a zoom finder that is used on cameras, video cameras or the like to erect the inverted image of an object formed by an objective optical system, thereby observing the image in an erected form through an image-inversion optical system.
Finder optical systems used on compact cameras, etc. are provided separately from phototaking optical systems unlike those used on single-lens reflex cameras. Such finder optical systems are generally broken down into a virtual image type finder and a real image type finder. The virtual image type finder is found to be unsuitable for use on compact cameras, because its construction renders it difficult to have a good sight of a field frame and especially because the diameter of a lens arrangement becomes large when the finder is designed in a zoom mode. At present, accordingly, the so-called real image type finder is generally used, wherein a primary real image formed by an objective optical system is erected by an image-inverting optical system for observation through an eyepiece optical system.
In recent years, there is a growing demand for compact cameras to have high zoom ratios and reduced size. However, it is still difficult to reduce the size of this real image type finder. An associated phototaking optical system, too, has a similar problem. Now, this problem is resolved to a certain degree by contriving the construction of a frame in such a way that zooming groups can be compactly received or collapsed therein when the camera is not in use. In view of design, however, it is difficult to construct finder optical systems in such a manner, and so it is particularly difficult to reduce the size of the finder optical systems.
So far, the size of the finder optical system has been reduced by contriving or designing the group arrangement of an objective optical system in such a way as to make the telephoto ratio low. However, this method of reducing the telephoto ratio is found to have some limitation in view of performance.
Another possible approach is to reduce the size of the finder optical system by modifying the construction of an eyepiece optical system. That is, the objective optical system is constructed in such a way that the size of an intermediate image is reduced while the necessary angle of view is kept, and the scaling factor of the eyepiece optical system is increased, accordingly. With this approach, the size of the finder optical system may be reduced because while the same specifications are kept, the focal lengths of the objective and eyepiece optical systems can be shortened.
However, a problem with this approach is the magnitude of an eye point. For instance, when light from the intermediate image is incident on the eyepiece optical system at an infinite entrance pupil position (=parallel light) as shown in
FIG. 31
, an exit pupil position, i.e., the eye point has the same distance as a focal length, as can be seen from the paraxial theory. In other words, if the focal length f of the eyepiece optical system is shortened, the eye point will in principle become short. For instance, this will in turn offer a shading problem to an observer with spectacles on, because the periphery of a screen is shaded.
Thus, the method of slimming down the finder optical system by shortening the focal length of the eyepiece optical system is practically difficult to carry out, because of another problem that the eye point becomes short.
Outside of compactness, on the other hand, there is another need of a recently growing camera user base. The primary object of compact cameras produced for mass markets is compactness, and ease of use, and so they are inferior to professional-or high amateur-grade single-lens reflex cameras in terms of some specifications. The angle of field is among these demerits. With finder optical systems for compact cameras proposed so far in the art, it is impossible to view an image under observation on an enlarged scale because the field of view is more limited than that of the single-lens reflex camera for lack of any sufficient angle of field.
One possible method for increasing this angle of field is to increase the size of an image formed by an objective optical system. However, this method is unsuitable for compact cameras, because the increased image immediately leads to an increase in the size of the whole optical system.
Another possible method is to increase the scaling factor of an eyepiece optical system by reducing the focal length thereof while the size of an intermediate image is maintained. With this arrangement, the magnification may be increased without any increase in the size of the optical system. However, this method, too, have some problems, in which there is the magnitude of an optical path length. For a real image type finder designed to form a real image once in an optical path, it is required to locate an image-inverting optical system (for instance, a prism or mirror) so as to make correction for an inverted image. If the focal length of the eyepiece optical system is merely reduced to this end, it is then difficult to locate the image-inverting optical system in an optical path, because the space for receiving it becomes unacceptably small.
Thus, some limitation is imposed on the method of making the field of view wide by reducing the focal length of the eyepiece optical system, because of the problem that the optical path length becomes short.
Thus, the needs of a variety of user bases cannot be fully addressed because of some limitations on the construction of the eyepiece optical system in real image type finders. Never until now is there any substantial attempt to make improvements in the construction of an eyepiece optical system.
Among recently proposed approaches, on the other hand, there is one proposal wherein the reflecting surface of an image-inverting optical system in a real image type finder, i.e., the reflecting surface of a prism or mirror forming part of the image-inverting optical system is formed of a curved surface to give power thereto. Since the reflecting surface of an image-inverting optical system is generally decentered with respect to an optical axis, giving power to that surface causes rotationally asymmetric decentration aberration. In principle, such decentration aberration cannot possibly be corrected only by use of a rotationally symmetric surface. According to recently proposed approaches, this decentration aberration is corrected by using a rotationally asymmetric surface to improve performance, as referred to below.
JP-A 11-242165 discloses a real image type zoom finder wherein the reflecting surface of a prism on the eyepiece side is formed of a rotationally asymmetric surface. In Example 1, 2 and 5, the eyepiece optical system is made up of a prism and a refracting lens, and in Example 3 and 4 the eyepiece optical system is made up of a prism alone. However, the refracting exit surfaces of both prisms have strong positive power.
JP-A 10-197796 shows in Numerical Example 6 that rotationally asymmetric curved surfaces are used for the refracting surface and reflecting surface of a prism on the eyepiece side of a finder optical system. However, the half angle of view on the wide-angle end is barely about 15°.
JP-A 11-84476 shows a real image type zoom finder wherein the reflecting surface of a prism on the eyepiece side is constructed of a rotationally asymmetric surface. The entrance and exit optical axes of an eyepiece optical system are parallel with each other in the same direction. Image inversion relies on a relay method for forming images twice in an optical path. Finder magnifications are 0.33, 0.34, and 0.28. Throughout the examples, single-focus systems are given.
JP-A 10-260357 shows that the reflecting surface o
Lester Evelyn A.
Olympus Corporation
Pillsbury & Winthrop LLP
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