Photography – With exposure objective focusing means – focusing aid – or... – Optical detail with photoelement system
Reexamination Certificate
1999-10-05
2001-02-13
Perkey, W. B. (Department: 2851)
Photography
With exposure objective focusing means, focusing aid, or...
Optical detail with photoelement system
Reexamination Certificate
active
06188846
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical system for a focus detecting device that is suitable to an optical apparatus such as a single-lens reflex camera.
FIG. 1
shows a conventional single-lens reflex camera
10
. The camera
10
is provided with a taking lens
51
as a main lens to form an image onto a film
12
. A TTL (Through-The-Lens) passive focus detecting device
21
is installed in a bottom portion of a camera body
11
of the single-lens reflex camera
10
. A part of light passed through the taking lens
51
passes through a half-mirror portion
14
of a main mirror
13
, and then the light is reflected by a sub-mirror
15
toward the focus detecting device
21
. In
FIG. 1
, x-, y- and z-directions are defined. The z-direction is parallel to an optical axis of the taking lens
51
, the x-direction is parallel to an equivalent optical axis of the taking lens
51
after reflection by the sub-mirror
15
, and the y-direction is perpendicular to both of the x- and z-direction.
The focus detecting device
21
includes a box
21
a
in which a condenser lens
31
, a mirror
25
, a pair of separator lenses
27
a
and
27
b
and a line sensor
29
are arranged. The line sensor
29
has a great number of pixels that are aligned with the y-direction. The optical path in the focus detecting device
21
is also shown in
FIG. 2
in a developed fashion. An opening
23
to define a focus detecting area is bored on the box
21
a
. The opening
23
is located at an equivalent film plane
17
. The light entered from the opening
23
is converged by the condenser lens
31
and then reflected by the mirror
25
. The separator lenses
27
a
and
27
b
divide the light reflected from the mirror
25
into two portions and form images on different areas
29
a
and
29
b
on the line sensor
29
. In
FIG. 2
, light
21
LC passes through the central area of the opening
23
and light
21
LO passes through the peripheral area of the opening
23
. Entrance pupils of the focus detecting device
21
is represented by ovals
21
P. The entrance pupils
21
P are defined as areas that are optically conjugate with the entrance pupils of the separator lenses
27
a
and
27
b
through the condenser lens
31
and the opening
23
.
Since the correlation between the positions of the images formed on the line sensor
29
represents focusing condition of the taking lens
51
with respect to the film
12
, the focusing condition can be detected by calculating output signals from the line sensor
29
.
Downsizing of the camera requires a compact design of the focus detecting device
21
, which also requires decreasing the size of the box
21
a
. On the other hand, a predetermined optical path length must be kept in order to prevent overlapping of the separated images on the line sensor
29
. If the optical path extends along a diagonal line of the box, it makes the most effective use of the confined space in the box of the focus detecting device.
FIG. 3
shows an optical system in an improved focus detecting device
22
. A condenser lens
31
is provided at an opening
23
bored to a box
22
a
. The condenser lens
31
is decentered to the opposite side of a line sensor
29
with respect to the center of the opening
23
, which deflects light
21
L incident in the focus detecting device
22
toward the opposite direction to the line sensor
29
. The deflected light is reflected by the mirror
25
to be incident on the line sensor
29
through the separator lenses
27
a
and
27
b
.
FIG. 4
shows the deflecting function of the decentered condenser lens
31
. The rectangular opening
23
is located at the peripheral portion of the condenser lens
31
as shown in
FIG. 5
when the opening
23
is viewed along the optical axis O of the condenser lens
31
.
The mirror
25
is diagonally opposite to the line sensor
29
, which makes the optical path between the mirror
25
and the line sensor
29
extend along the diagonal line of the box
22
a
. This therefore makes the most effective use of the confined space in the box
22
a
of the focus detecting device
22
.
However, the focus detecting device
22
has a disadvantage of mismatch between an exit pupil of the taking lens
51
and entrance pupils
21
P of the focus detecting device
22
due to coma. The decentered arrangement of the condenser lens
31
causes coma in the light, and particularly, the longer the distance from the optical axis O is, the larger the coma becomes. Since the coma changes a deflecting angle of the passing light, the deflecting angle of the light
21
LO passing the periphery of the opening
23
is larger than the deflecting angle of the light
21
LC passing the center of the opening
23
as shown in FIG.
6
. It is therefore, the entrance pupils
21
P of the focus detecting device
22
are curved such that the peripheral portions bend upward as shown in
FIG. 7
, which may cause the mismatch between the exit pupil
51
P of the taking lens
51
and the entrance pupils
21
P of the focus detecting device
22
.
In the example of
FIG. 7
, outside portions of the entrance pupils
21
P represented by hatching are not within the exit pupil
51
P, which results in the difference between the brightness distribution detected by the line sensor
29
and the actual brightness distribution, causing error in the focus detection. Thus, the decentered arrangement of the condenser lens
31
in the focus detecting device
22
only allows a narrow focus detection area in the center of the equivalent film plane
17
in order prevent the error in the focus detection.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical system of a focus detecting device, which is capable of keeping a wide focus detection area with a compact construction.
For the above object, according to the present invention, there is provided an optical system of a focus detecting device for detecting a focusing condition of a main lens with respect to an image taking plane, which includes:
a focus detection area arranged on an equivalent image taking plane;
a first deflecting element for deflecting the light from the main lens through the focus detection area in a first direction;
a condenser lens for converging the light deflected by the first deflecting element, the condenser lens being arranged such that the light transmits along about the meridional plane of the condenser lens;
a second deflecting element for deflecting the light through the condenser lens in a second direction opposite to the first direction; and
a light receiving element for receiving the light deflected by the second deflecting element.
With this construction, the optical path is once deflected in the first direction and then deflected in the second direction that is opposite to the first direction, which enables a diagonal arrangement of an optical path between the second deflecting element and the light receiving element in a box of the focus detecting device, making the most effective use of the confined space in the box. Further, the light travels along about the meridional plane of the condenser lens, which reduces the effect of the coma, matching the exit pupil of the main lens with the entrance pupils of the focus detecting device. As a result, the actual brightness distribution can be accurately detected by the light receiving element, even if the focus detecting device has a wide focus detection area.
The first deflecting element may be a prism and the second deflecting element may be a mirror. In such a case, the condenser lens is tilted with respect to an equivalent optical axis of the main lens on the main lens's side of the prism such that the light deflected by the prism transmits along about the meridional plane of the condenser lens.
The prism and the condenser lens may be independently arranged or these may be combined as a single condenser unit. One surface of the single condenser unit at the side of the focus detecting area may be a plane and the other surface thereof may be a curved surface.
REFERENCES:
patent
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Perkey W. B.
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