Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
1999-12-02
2001-10-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C396S111000, C396S358000
Reexamination Certificate
active
06297909
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a focus detecting optical system that is installed in a film camera or a video camera that exposes or captures a subject image formed by an objective lens for detecting a focusing condition of the objective lens with respect to the subject to be taken.
An autofocus single lens reflex camera is provided with a focus detecting optical system.
FIG. 5
shows an optical system of a conventional single lens reflex camera. Light from a subject of a photograph is incident on an taking lens
1
, a major portion of the incident light is reflected by a main mirror
2
to form an image of the subject on a focusing screen
3
. A user observes the image of the subject via an eyepiece lens
4
and a pentagonal prism
5
that constitute a finder optical system.
The main mirror
2
is provided with a half mirror portion in the center thereof. The transmitted light through the half mirror portion of the main mirror
2
is reflected by a sub mirror
6
toward a focus detecting unit
8
.
The main mirror
2
and the sub mirror
6
move to the side out of the optical path such that an image of the subject is formed on a film
9
at the time of the exposure.
The conventional focus detecting unit
8
includes, as shown in
FIG. 6
, a field mask
8
a
, a condenser lens
8
b
, a separator mask
8
g
, a pair of separator lenses
8
c
and
8
d
, and a pair of line sensors
8
e
and
8
f
that are arranged in this order from the side of the sub mirror
6
. The field mask
8
a
is substantially coincident with an equivalent film plane F on which a primary image of the subject is formed through the taking lens
1
. The separator mask
8
g
has a pair of openings to limit the light to be incident on the separator lenses
8
c
and
8
d
. The separator lenses
8
c
and
8
d
re-form the primary image into a pair of secondary images. The line sensors
8
e
and
8
f
capture the secondary images.
The positional relationship between the secondary images formed on the line sensors
8
e
and
8
f
varies in accordance with the focusing condition of the taking lens
1
with respect to the subject. Therefore, the focusing condition can be detected on the basis of the relationship that is calculated from outputs of the line sensors
8
e
and
8
f.
However, since the conventional focus detecting unit
8
detects the focusing condition of the taking lens only at the center of the taking area, it cannot meet requirement of users to detect the focusing condition of the taking lens with respect to a subject at any point in the taking area.
In order to meet the requirement, the focus detecting optical system must be provided with area sensors as image sensors and must expand the range of the light taken into the focus detecting unit
8
. However, since the expansion of the range of the taken light with the above descried optical system increases the sizes of the sub mirror
6
and the condenser lens
8
b
, it is hard to install such a focus detecting optical system in a limited space of the single lens reflex camera.
The focus detecting optical system disclosed in the Japanese Patent Provisional Publication No. Hei 9-184965 uses area sensors to enable focus detection at any chosen point in the taking area and a concave mirror as the sub mirror to expand the range of the taken light without upsizing of the focus detecting optical system.
However, the convergent mirror is tilted with respect to the optical axis in the optical system disclosed in the publication, which causes distortions in the secondary images. This may cause a mismatch between the intended focus detecting point in the taking area and the actual detecting point on the area sensor, which may obstruct the accurate focus detection with respect to the intended subject.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved focus detecting optical system, which is capable of accurately detecting a focusing condition of an objective lens with respect to an intended subject without upsizing of the device even if an area sensor is used to enable focus detection at any chosen point in the taking area.
For the above object, according to the present invention, there is provided a focus detecting optical system, which includes a reduction lens having a positive refractive power that is located between an objective lens and an image taking plane on which a primary image of a subject is formed by the objective lens, a separation plane mirror for separating light passed through the reduction lens from an optical path toward the image taking plane, a pair of separator lenses for re-forming a primary image formed through the reduction lens and the separation plane mirror into a pair of secondary images, and a pair of image sensors for capturing the secondary images. The optical axis of the reduction lens is coaxial with the optical axis of the objective lens.
With this construction, the light passing through the objective lens is converged by the reduction lens to be incident on the separator lenses, which expands the range of the light taken by the focus detecting optical system without upsizing the separation plane mirror, the separator lenses nor the image sensors. Further, the optical axes of the objective lens and the reduction lens are coaxial to each other and the separation mirror is a plane mirror, which does not distort the secondary images.
It is preferable that the reduction lens has a refractive power to make an exit pupil of the objective lens be optically conjugate with entrance pupils of the separator lenses.
The image sensors may be area sensors in which pixels are two-dimensionally arranged. Each area sensor may consist of a plurality of line sensors whose pixels are linearly arranged. In such a case, the line sensors are arranged in parallel at predetermined intervals.
The focus detecting optical system of the present invention can be applied to a single lens reflex camera. In such a case, the objective lens is a taking lens, and the reduction lens is located on the side of the image taking plane with respect to a main mirror that reflects a light passing through the taking lens toward a finder optical system.
REFERENCES:
patent: 4841326 (1989-06-01), Koyama
patent: 5212514 (1993-05-01), Goto
patent: 5296888 (1994-03-01), Yamada
patent: 5771413 (1998-06-01), Suda
patent: 5835797 (1998-11-01), Odaka
patent: 5839001 (1998-11-01), Ohtaka et al.
patent: 9184965 (1997-07-01), None
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
O'Neiil Gary
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