Photography – With exposure objective focusing means – focusing aid – or... – Reliability of focus/distance signal
Reexamination Certificate
2001-08-24
2003-10-28
Adams, Russell (Department: 2851)
Photography
With exposure objective focusing means, focusing aid, or...
Reliability of focus/distance signal
C396S111000
Reexamination Certificate
active
06640054
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement in a focus detecting device that has a light receiving means composed of a plurality of photoelectric converters.
2. Description of the Related Art
A camera having a main mirror and a sub mirror designed to be movable has been proposed in the past. In the camera, the main mirror has a semi-transparent member that routes light, which has passed through a photography lens, to a viewfinder optical system. The sub mirror routes the light, which has passed through the semi-transparent member of the main mirror, to a focus detecting device. For focus detection or observation through a viewfinder, the main mirror and sub mirror are advanced to predetermined positions on a photographic light path, and brought to a standstill at the positions. For photography, the main mirror and sub mirror are withdrawn to recede from the photographic light path.
In this type of camera, when the number of movements made by the main mirror and sub mirror increases, members involved in driving the main mirror and sub mirror are worn down. This brings about a shift in the positions at which the main mirror and sub mirror come to a standstill during focus detection. Consequently, a change in an optical length for the light to be routed to the focus detecting device leads to a deviation of an in-focus position of an actual image plane from an in-focus position detected by the focus detecting device.
Japanese Unexamined Patent Publication No. 9-54243 has proposed a camera in which a relationship between the number of movements made by an optical member and a correction value by which a focus detection signal is corrected is stored in a memory in advance. During focus detection, the focus detection signal is corrected based on the number of movements detected at that time and the stored relationship between the number of movements and the correction value.
FIG. 25
shows the configuration of a major portion of a camera disclosed in the Japanese Unexamined Patent Publication No. 9-54243.
Referring to
FIG. 25
, there are shown a photography lens
101
, a ray axis
101
a
of the photography lens, and an image recording medium
102
on which an object image that has passed through the photography lens
101
is projected. A main mirror
103
has a semi-transparent member that can be moved to recede from a photographic light path during photography. The main mirror
103
routes part of the light, which has passed through the photography lens
101
, to a viewfinder optical system composed of a focusing screen
104
, a pentagonal prism
105
, and an eyepiece
106
. On the other hand, the remaining light that has passed through the semi-transparent member of the main mirror
103
is reflected downward from a sub mirror
107
that can be moved while being interlocked with the main mirror
103
. The light is then routed to a focus detecting device
108
of a known phase difference type composed of an image sensor, a pair of secondary image formation lenses, an aperture stop, and a field lens. The image sensor consists of a pair of photoelectric converter trains. The aperture stop has a pair of apertures.
Now, the principles of phase-difference type focus detection will be described in conjunction with FIG.
26
. Components bearing the same reference numerals as the components shown in
FIG. 25
have the same abilities as the components shown therein. Moreover, the main mirror
103
and sub mirror
107
are omitted from
FIG. 26
for the brevity's sake. The field lens
115
, the aperture stop
116
having a pair of apertures, the pair of secondary image formation lenses
117
, and the image sensor
118
composed of a pair of photoelectric converter trains are developed along the ray axis
101
a
of the photography lens.
Light emanating from a point on the ray axis
101
a
passes through the photography lens
101
, and then converges on a primary image plane that is optically conjugate to the image recording medium
102
. After passing through the field lens
115
, aperture stop
116
, and secondary image formation lenses
117
, the resultant light rays converge on the image sensors
118
with a certain distance between them. The field lens
115
is positioned so that the field lens
115
will converge an exit pupil
101
b
of the photography lens
101
and entrance pupils of the pair of secondary image formation lenses
117
, that is, images formed near the aperture stop
116
. The field lens
115
splits the exit pupil
101
b
of the photography lens
101
vertically in
FIG. 26
to route the resultant images to the pair of apertures of the aperture stop
116
.
Owing to the foregoing components, for example, the photography lens
101
is thrust leftward in FIG.
26
. This causes light to converge on a plane on the left-hand side of the image recording medium
102
. Consequently, the pair of images on the image sensor
118
is displaced in directions of arrows. The image sensor
118
detects the relative displacements of the pair of images, whereby the photography lens
101
can be focused. The same applies to a case where the photography lens
101
is thrust rightward in FIG.
26
. Moreover, the same applies to an object point other than the point on the ray axis
101
a
of the photography lens
101
.
The focus detecting device
108
designed based on the foregoing principles is used to detect the focus of the photography lens
101
.
Referring back to
FIG. 25
, a microcomputer
109
controls actions to be performed in the camera. A CPU
109
a
, a ROM
109
b
, a RAM
109
c
, and an electrically erasable programmable ROM (EEPROM)
109
d
are incorporated in the microcomputer
109
. Programs concerning focus detection and others are stored in the ROM
109
b
. A focus detection circuit
110
is connected to the image sensor
118
. A mirror driving means
111
moves the main mirror
103
off the photographic light path. A mirror drive circuit
112
drives the mirror driving means
111
. A lens driving means
113
is used to focus the photography lens
101
. A lens drive circuit
114
drives the lens driving means
113
.
A relationship between the number of movements made by the main mirror
103
and a correction value by which a focus detection signal is corrected, which has been acquired from experimental data in advance, is stored in the EEPROM
109
d
incorporated in the microcomputer
109
. The number of movements made by the main mirror
103
up to the present is stored in the RAM
109
b
, while the relationship between the number of movements and the correction value by which a focus detection signal is corrected which is stored in the EEPROM
109
d
. Based on the number of movements made by the main mirror
103
and the relationship between the number of movements and the correction value, a focus detection signal produced by the focus detecting device
108
and focus detection circuit
110
is corrected according to the focus detection program stored in the ROM
109
c
. Consequently, when the number of movements made by the main mirror
103
and sub mirror
107
increases, a member that holds the main mirror
103
and sub mirror
107
so that the mirrors can be moved is worn down. This brings about a shift in positions at which the main mirror and sub mirror come to a standstill during focus detection. The shift in positions causes an optical length for a focus detection optical system to change. Consequently, an in-focus position detected by the focus detecting device
108
and focus detection circuit
110
deviates from an in-focus position on the image recording medium
102
. However, since the focus detection signal is corrected based on the number of movements made by the main mirror
103
, the focus of the photography lens can be detected highly precisely.
As a means for sensing an abnormal movement made by the main mirror
103
or sub mirror
107
, it is generally known to sense the completion of a movement made by a mechanism using a contact switch or to detect the moved state of the mechanism using
Tokura Go
Yamasaki Ryo
Robin Blecker & Daley
Smith Arthur A
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