Optical: systems and elements – Compound lens system – Telescope
Reexamination Certificate
2000-08-22
2004-08-17
Nguyen, Thong Q. (Department: 2872)
Optical: systems and elements
Compound lens system
Telescope
C359S831000, C359S834000
Reexamination Certificate
active
06778324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a viewfinder optical system, and more particularly to a viewfinder optical system suitable for use in an optical apparatus, such as a video camera, a digital camera or the like, arranged to enable a viewfinder image (object image) formed as an inverted real image by an objective lens to be observed as a non-inverted erecting viewfinder image by utilizing an image inverting unit as appropriately set.
2. Description of Related Art
Heretofore, in viewfinder systems for optical apparatuses, such as photographic cameras and video cameras, there have been proposed a variety of viewfinder optical systems of the real image type in which a real image formed on a primary image forming plane by an objective lens is converted into an erecting image and the erecting image is viewed through an eyepiece lens. Such a viewfinder optical system of the real image type makes it easier to reduce the size of the whole optical system than a viewfinder optical system of the virtual image type. Therefore, these days, the viewfinder optical system of the real image type is widely used in cameras having zoom lenses.
A viewfinder optical system of the real image type using a Porro prism for forming a non-inverted erecting image has such a tendency that a part of the Porro prism protrudes in the height (up-and-down) direction and the width (right-and-left) direction of an optical apparatus according to the external form of the Porro prism, thereby enlarging the whole viewfinder system. In order to shorten the total lens length of a viewfinder optical system according to a decrease in size and thickness of the whole camera, in Japanese Laid-Open Patent Application No. Hei 6-167739 (corresponding to U.S. Pat. No. 5,640,632), there is disclosed a small-sized viewfinder optical system in which an optical path leading to a primary image forming plane on which an object image is formed by an objective lens is bent by a reflecting surface and the primary image forming plane is thus formed inside an image inverting unit.
FIG. 30
is a sectional view showing essential parts of a conventional viewfinder optical system using a prism and a roof prism for bending an optical path leading to a primary image forming plane. In
FIG. 30
, reference character OL denotes an objective lens, and reference character P denotes a prism for forming a non-inverted erecting image. The prism P is composed of a first prism P
1
and a second prism (roof prism) P
2
. Reference character S denotes a field frame, which is disposed within a narrow space across which an exit surface
13
of the first prism P
1
and an entrance surface
21
of the second prism P
2
are opposed to each other. A viewfinder image formed as an inverted real image by the objective lens OL is formed in the vicinity of the field frame S through the first prism P
1
. Reference character EL denotes an eyepiece lens, which is used for observing a non-inverted erecting viewfinder image into which the inverted real viewfinder image formed in the vicinity of the field frame S is converted through the second prism P
2
.
In the viewfinder optical system shown in
FIG. 30
, if it is designed to increase an angle of field, it is necessary to enlarge the second prism P
2
, so that there is a tendency for the size of the camera to increase in the thickness (depth) direction thereof. Meanwhile, in the viewfinder optical system, the focal length “fe” of the eyepiece lens corresponds to the length from the image forming position to the eyepiece lens. Then, assuming that the focal length of the objective lens is denoted by “fo”, the viewfinder magnification &ggr; is expressed by the following equation:
&ggr;=
fo/fe
Accordingly, if the second prism P
2
is enlarged so as to increase an angle of field, an optical path from the image forming position to the eyepiece lens EL becomes longer.
Thus, the focal length “fe” of the eyepiece lens EL becomes longer to decrease the viewfinder magnification &ggr;, so that it becomes difficult to observe a good viewfinder image.
With regard to an image inverting unit which is small in size and is capable of enlarging an angle of view and a viewfinder magnification, for example, in Japanese Laid-Open Patent Application No. Hei 8-179400 and Japanese Laid-Open Patent Application No. Hei 10-206933, there is disclosed a viewfinder optical system in which two prisms are disposed with an air gap put at a minute interval therebetween.
FIGS. 31 and 32
show the basic construction of a viewfinder optical system of the real image type using a prism and a roof prism for bending an optical path leading to a primary image forming plane, which construction is similar to that disclosed in Japanese Laid-Open Patent Application No. Hei 10-206933. In
FIGS. 31 and 32
, reference character OL denotes an objective lens, and reference character P denotes a prism for forming a non-inverted erecting image. The prism P is composed of a first prism P
11
and a second prism (roof prism) P
12
, and an exit surface
111
of the first prism P
1
and an entrance surface
121
of the second prism P
2
are disposed, in parallel, with a minute air gap “d” put therebetween. Reference character S denotes a field frame, which is disposed in the vicinity of an exit surface
123
of the second prism P
2
(on a primary image forming plane). A viewfinder image formed as an inverted real image by the objective lens OL is converted, through a roof reflecting surface
122
of the second prism P
12
, into a non-inverted erecting image, which is formed in the vicinity of the field frame S. Reference character EL denotes an eyepiece lens, which is used for observing the non-inverted erecting viewfinder image formed in the vicinity of the field frame S through the second prism P
12
.
In the construction shown in
FIGS. 31 and 32
, a light flux coming from the objective lens OL passes through the exit surface
111
of the first prism P
11
and the entrance surface
121
of the second prism P
12
and is then image-inverted and reflected once toward the object side by the roof reflecting surface
122
. The reflected light flux is further totally-reflected by the entrance surface
121
of the second prism P
12
, so that a viewfinder image is formed on the primary image forming plane in the vicinity of the exit surface
123
of the second prism P
12
. A reflecting member M
1
is arranged to reflect a light flux coming from the primary image forming plane to lead the reflected light flux to the eyepiece lens EL.
In the conventional viewfinder optical system shown in
FIGS. 31 and 32
, the exit surface
111
of the first prism P
11
and the entrance surface
121
of the second prism P
12
are decentered with respect to the optical axis of the objective lens OL or the eyepiece lens EL. Further, in order to cause a light flux reflected from the roof reflecting surface
122
of the second prism P
12
to be totally reflected by the entrance surface
121
of the second prism P
12
, i.e., in order to utilize the entrance surface
121
of the second prism P
12
both for transmission and reflection, the exit surface
111
of the first prism P
11
and the entrance surface
121
of the second prism P
12
are disposed, almost in parallel, with the minute air gap “d” put therebetween, as shown in FIG.
32
.
Therefore, as shown in
FIG. 32
, rays of light indicated by solid lines have the respective angles of refraction which differ according to the positions at which the rays pass through the exit surface
111
of the first prism P
11
or according to the angles of incidence of the rays on the exit surface
111
of the first prism P
11
. Accordingly, an optical path length possible within the minute air gap “d” becomes, for example, the length a
1
or a
2
(a
1
<a
2
). Therefore, astigmatism, coma, etc., become varying with right and left sides of a view field, so that it becomes difficult to observe a good viewfinder image.
In addition, as rays of light indicated by dashed lines, the surface reflection occurs betwe
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Thong Q.
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