Focus detecting apparatus

A focus detecting apparatus, in which a pair of bundles of rays, transmitted through different portions of an exit pupil of a taking lens, are transmitted through a common focus detecting zone formed on an imaginary focal plane of the taking lens. The focus detecting apparatus includes two image reforming optical systems which reform images of the bundles of rays, incident on the focus detecting zone in different directions and transmitted therethrough, in accordance with a change in position of the exit pupil in the optical axis direction of the taking lens. A pair of line sensors are located at predetermined positions, upon which the images of the respective bundles of rays are reformed through the respective image reforming optical systems.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a focus detecting apparatus for detecting 
a focus state of a taking lens of a camera or the like, with respect to an 
object to be photographed. 
2. Description of Related Art 
In a known focus detecting apparatus in a single lens reflex camera or the 
like, bundles of rays transmitted through different portions of an exit 
pupil of a taking lens are converged onto a pair of line sensors by an 
image reforming lens of a focus detecting optical system, so that the 
focus state of the taking lens can be detected in accordance with a 
relationship between outputs of the line sensors. The principle of the 
focus detection by the focus detecting apparatus is disclosed, for 
example, in U.S. Pat. No. 4,636,624. 
A focus detecting apparatus having an off-axis detecting area, that 
deviated from an optical axis, is a known apparatus for detecting the 
focus state of a taking lens with respect to an object located at a 
position other than the center of an image plane. 
However, if the deviation of the off-axis detecting area from the optical 
axis increases, vignetting focus detecting optical system pupil occurs 
when position or size of the exit pupil of the taking lens changes as a 
result of a lens change (i.e., when using an interchangeable lens), or 
during a zooming operation. In such cases, the area of the focus detecting 
optical system pupil, through which the bundle of rays is transmitted, is 
reduced. As a result, the applicability of such an automatic focus 
detecting system to an interchangeable lens is restricted, or it is 
possible that the automatic focus detecting system will not work at a 
specific focal length within a zooming range. 
To solve these problems, it is known to provide three image reforming 
optical systems using three bundles of rays transmitted through three 
different portions of the exit pupil of the taking lens in a focus 
detecting apparatus having an off-axis detecting area, as disclosed for 
example in Japanese Kokai patent publication No. HEI 2-58012. In this 
focus detecting apparatus , the outputs of line sensors, corresponding to 
two of three image reforming optical systems in which no vignetting 
occurs, are used to detect an focus with respect to the object within the 
image plane. Accordingly, the adverse influence of vignetting on the focus 
detecting optical system, due to the change of the exit pupil of the 
taking lens, is eliminated. 
However, in the known focus detecting apparatus, as mentioned above, the 
three image reforming optical systems must be provided in a single 
detecting area and the three line sensors must be arranged along a 
diametric direction of the taking lens. 
Furthermore, if the area to be detected is large, namely, is an image is 
long, vignetting may also occur in an area including the optical axis of 
the taking lens. The prior art mentioned above does not solve this 
problem. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a focus detecting 
apparatus in which the focus state of various taking lenses can be 
detected by only two image reforming optical systems for one detecting 
area, even if the detecting area including the optical axis contains a 
long image. 
To achieve the object mentioned above, according to the present invention, 
there is provided a focus detecting apparatus in which a pair of bundles 
of rays transmitted through different portions of an exit pupil of a 
taking lens, are transmitted through a common focus detecting zone formed 
on an imaginary focal plane of the taking lens, so that images of the 
bundles of rays are formed on predetermined positions. The focus detecting 
apparatus comprises at least two image reforming optical systems which 
reform images of the bundles of rays incident on the focus detecting zone 
from different directions, in accordance with a change in position of the 
exit pupil in an optical axis direction of the taking lens. The apparatus 
further comprises at least one pair of line sensors located at said 
predetermined positions, so that the images of the bundles of rays are 
reformed on the respective line sensors through respective image reforming 
optical systems. 
According to an aspect of the present invention, there is provided a focus 
detecting apparatus in which a pair of bundles of rays transmitted through 
different portions of an exit pupil of a taking lens are transmitted 
through a common focus detecting zone formed on an imaginary focal plane 
of a taking lens, so that images of the bundles of rays are formed on 
predetermined positions. The apparatus comprises at least two image 
reforming optical systems which reform images of the bundles of rays 
incident on the focus detecting zone from different directions, in 
accordance with a change in position of the exit pupil in an optical axis 
direction of the taking lens. The apparatus also comprises a common line 
sensor located at said predetermined positions, so that the images of the 
bundles of rays are reformed on the common line sensor through respective 
image reforming optical systems. 
According to another aspect of the present invention, a focus detecting 
apparatus comprises a first image reforming optical system which transmits 
an image formed by a taking lens on a focus detecting zone of an imaginary 
focal plane, through a first light gathering means and reforms the image 
on at least one line sensor having picture elements. The apparatus further 
comprises a second image reforming optical system which has a second light 
gathering means spaced from the first light gathering means in a direction 
that is perpendicular to the direction of alignment of the picture 
elements of the line sensor. The second image reforming optical system 
transmits an image, formed by the taking lens on the focus detecting zone 
of the imaginary focal plane, through a second light gathering means and 
reforms the image on the line sensor, wherein the bundle of rays are 
incident from a direction different from that of the bundle of rays 
incident on the first image reforming optical system. 
The present disclosure relates to subject matter contained in Japanese 
patent application. No. HEI 3 218155 (filed on May 20, 1991), which is 
expressly incorporated herein by reference in its entirety.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a first embodiment of the present invention. A focus detecting 
zone 11, which is elongated in a direction perpendicular to a radial 
direction (sagittal direction) at a position deviated from an optical axis 
Ax of a taking lens L (FIG. 1), is provided on an imaginary focal plane 
10. An image of an object is formed by the taking lens L on the focus 
detecting zone 11 to detect a focus state of the taking lens L with 
respect to an image within the focus detecting zone 11. The imaginary 
focal plane 10 is located at a position that is conjugate with an actual 
focal plane. 
As is well known, an exit pupil EPt of the taking lens L at a telephoto 
extremity is spaced farther from the imaginary focal plane 10 than at exit 
pupil EPw thereof at a wide angle extremity. Furthermore, the size of the 
exit pupil varies depending on the F-number of the taking lens. 
In view of this, there are provided two sets of image reforming optical 
systems of a focus detecting optical system corresponding to one focus 
detecting zone 11, in the first embodiment of the present invention, so 
that one set of the two image reforming optical systems may be selectively 
used in accordance with the position of the exit pupil of the taking lens. 
The first image reforming optical system receives a pair of bundles of rays 
transmitted through different portions At and Bt of the taking lens exit 
pupil EPt at a telephoto extremity and through a first condenser lens 20. 
The received pair of bundles of rays is made incident upon a pair of 
separator lenses 30 and 31, so that two separate images within the 
detecting zone 11 are formed on respective sensor portions 40A and 40B of 
a first line sensor 40. 
Similarly, the second image reforming optical system includes a second 
condenser lens 21, a pair of separator lenses 32 and 33, and a second line 
sensor 41. The second condenser lens 21 receives a pair of bundles of rays 
transmitted through different portions Aw and Bw of the taking lens exit 
pupil EPw of the taking lens at a wide angle extremity. The received pair 
of bundles of rays is made incident upon a pair of separator lenses 32 and 
33, so that two separate images are formed on respective sensor portions 
41A and 41B of the second line sensor 41. 
The line sensors 40 and 41 each include two sensor portions 40A and 40B, 
and 41A, and 41B, which receive a pair of split images, respectively. The 
focus state of the taking lens L with respect to the image within the 
detecting zone 11 can be detected by calculating the positional 
relationship of the split images formed in the respective sensor portions 
40 or 41 comprising pairs of sensors 40A, 40B or 41A, 41B, respectively. 
Namely, the outputs of the line sensors 40 or 41 are inputted to a signal 
processing circuit 50 and a calculation controlling circuit 51 which 
calculates a positional relationship of the split images, to thereby 
detect a amount of defocus of the object, based on the outputs of the line 
sensors 40 or 41. The calculation controlling circuit 51 drives a control 
motor 52 provided in a camera body B, based on the calculation results, 
and actuates a focus adjusting mechanism 54 of the taking lens L, through 
a drive coupling 53, to thereby move a focus adjusting lens 55 in the 
optical axis direction. 
The output of either line sensor 40 or 41, to be used for calculation, can 
be selected in advance. Namely, the position and size of the exit pupil of 
the taking lens L depend on the focal length and the F-number thereof. The 
focal length and the F-number are inherent data peculiar to the taking 
lens, and accordingly can be stored in a lens ROM or CPU in the 
interchangeable lens (taking lens) L. Consequently, the selection of the 
line sensor output 40 or 41 can be predetermined in accordance with the 
kind of interchangeable lens to be used, through data communication 
between the interchangeable lens L and the camera body B of a single lens 
reflex camera, by a known communicating means. 
When a zoom lens is to be used, the output of either line sensor 40 or 41 
is determined in accordance with the focal length thereof. 
Alternatively, it is possible to automatically and selectively use the 
outputs of the line sensors 40 or 41 in accordance with the quantity of 
light incident thereon. 
The condensor lenses 20 and 21 function as light gathering means which 
gather light transmitted through the focus detecting zone 11 to form 
images on the line sensors 40 and 41, respectively. The condenser lenses 
20 and 21 are located side by side in parallel and spaced from one another 
in a direction that is perpendicular to the direction of alignment of 
picture elements of the line sensors 40 and 41. The condensor lenses 20 
and 21 transmit the bundle of rays incident on the detecting zone 11 at 
different angles, depending on the change in the position of the exit 
pupil, in a substantially parallel direction. The power of the condenser 
lenses 20 and 21 gradually changes in a direction perpendicular to the 
longitudinal direction (i.e., alignment direction of the picture elements) 
of the line sensors 40 and 41. 
In the focus detecting apparatus as constructed above, if a change in the 
exit pupil of the taking lens takes place, the focus state of the taking 
lens, with respect to an object image, can be precisely detected by 
selecting one of the two image reforming optical systems in accordance 
with the position and size of the exit pupil. 
The feature that both the two image reforming optical systems can detect 
the focus state for one object located at the same position is 
advantageous, particularly when the focus detecting apparatus of the 
invention is incorporated in a camera. Namely, a focus detecting zone 
representing a range of the focus detection is usually indicated in a 
finder system of an automatic focusing camera. A photographer positions 
the object to be photographed within the focus detecting zone of the 
finder, so that the focus of the taking lens can be automatically 
adjusted. 
However, if the focus detecting zone on the imaginary focal plane is moved, 
for example, by the switching of the image reforming optical system, the 
detecting zone in the finder will no longer be coincidental with an actual 
zone to be detected, and the taking lens will be focused for an object 
other than the object the photographer intends to photograph. 
FIGS. 2 (2A, 2B and 2C) through 6 (6A and 6B) show several modifications of 
the first embodiment illustrated in FIG. 1. In FIGS. 2A, 2B and 2C, each 
of the image reforming optical systems include a line sensor and separator 
lenses, similar to FIG. 1. In the arrangement shown in FIG. 2A, the image 
reforming optical systems include respective condenser lenses 20A and 21A 
having approximately the same power. A prism 21a is provided in front of 
the condenser lens 21A. The deflection function of the prism 21a allows 
for the use of identical condenser lenses 20A and 21A to gather light. 
Note that in FIGS. 2 through 6, the direction of the alignment of the 
picture elements of the line sensors 40 and 41 is perpendicular to the 
plane of the drawings. 
In FIG. 2B, the two condenser lenses 20 and 21 are provided with different 
powers which gradually change in the direction perpendicular to the 
alignment direction of the picture elements of the line sensors 40 and 41, 
similar to FIG. 1. 
In FIG. 2C, a single aspherical condenser lens 20B, common to the two image 
reforming optical systems, is provided. The aspherical condenser lens 20B 
has a power which increases toward the peripheral edge from the center 
thereof, so that the bundle of rays transmitted through an upper portion 
of a tall image is more sharply refracted than the bundle of rays 
transmitted through the center portion of the image. The light is thereby 
emitted from the aspherical condenser lens 20B in a substantially parallel 
direction. 
FIG. 3 shows another modification of FIG. 1, in which a single condenser 
lens 20C is used. A prism 20a is provided in the optical path of the image 
reforming optical system which receives light at the wide angle extremity. 
The prism 20a enables the single condenser lens 20C, on which a bundle of 
rays is made incident from different directions, to emit a bundle of rays 
substantially in a parallel direction. 
FIGS. 4A, 4B, 5A, 5B, and 5C show additional modified examples of FIG. 1, 
in which the two image reforming optical systems have common separator 
lenses. In the arrangements shown in FIGS. 4A through 5C, one of the 
condenser lenses (condenser means) 21, 21A, 21B shown in the arrangements 
shown in FIGS. 1 through 3 has a stronger power, so that the bundle of 
rays is made incident upon the separator lenses 30, 31, common to the two 
image reforming optical systems, and the split images are formed on the 
separate line sensors 40 and 41. Namely, the separator lenses 32 and 33 
are not provided in the arrangement shown in FIGS. 4A through 5C. 
In FIG. 4A, there are two condenser lenses 20D and 21D having different 
powers. In FIG. 4B, the light gathering means of one of the image 
reforming optical systems, that gathers light in the telephoto mode, 
includes a composite optical system having a condenser lens 20E and a 
prism 20a, and the light gathering means of the other image reforming 
optical system includes a single condenser lens 21E, respectively. 
In FIG. 5A, one of the condenser lenses 20F and 21F that gathers light in 
the wide angle mode, i.e., the condenser lens 21F, has an inclined optical 
axis and a prism 21a, which is provided behind the condenser lens 21F. In 
FIG. 5B, there is a single aspherical condenser lens 20G common to the two 
image reforming optical systems. The single aspherical condenser lens 20G 
has a power which gradually increases from the center towards the 
peripheral edge thereof. In FIG. 5C, there is a single condenser lens 20H 
common to the two image reforming optical systems and a prism 20a, which 
is provided in the optical path of one of the two image reforming optical 
systems that gathers light in the wide angle mode. 
In FIGS. 6A and 6B, there is a single line sensor 40 that is common to the 
two image reforming optical systems. To allow for the common use of the 
single line sensor 40, it is necessary to intercept the optical path of 
the image reforming optical systems which is not being used, to thereby 
detect only the bundle of rays of the image reforming optical system which 
is to be used, in accordance with the position of the exit pupil of the 
taking lens. 
To this end, in the arrangement illustrated in FIG. 6A, there is a single 
condenser lens 20J and a prism 20a that is provided in the optical path of 
the image reforming optical system that gathers light in the wide angle 
mode. A light intercepting plate 50 is movable in directions shown by an 
arrow in FIG. 6A to selectively intercept only the optical path of the 
image reforming optical system that is not used for detection. 
In FIG. 6B, a second prism 20b is provided in front of the separator lenses 
32 or 33 of the image reforming optical system that gathers light in the 
wide angle mode, in addition to the components in the arrangement shown in 
FIG. 6A. 
As can be seen from the above discussion, according to the present 
invention, the common use of the separator lenses or the line sensor 
contributes to a reduction in the number of necessary components and a 
simplification in the construction of the apparatus. 
Although the above discussion has been directed to a single detecting zone 
11, provided at a position that deviates from the optical axis, for the 
purpose of simplifying the explanation, it is possible to provide two 
additional detecting zones 11A and 11B (FIG. 1) at a position that is 
symmetrical to the detecting zone 11 with respect to the optical axis and 
a position on the optical axis between the aforementioned two detecting 
zones 11 and 11A. This alternative is a preferred arrangement for its 
practicability, since a more appropriate detecting zone, corresponding to 
the object to be focused, can be selected from among the three detecting 
zones to detect the focus state of the taking lens. 
FIG. 7 shows another embodiment of the present invention. In this 
embodiment, detecting zone 12 is located at the center of an imaginary 
focal plane 10, i.e., at a position that includes including the optical 
axis Ax of the taking lens. The two image reforming optical systems are 
selectively used for the detecting zone 12. 
A bundle of rays, emitted from different portions Aw and Bw of exit pupil 
EPw of the taking lens at a wide angle extremity and transmitted through a 
detecting zone 12, is gathered and deflected by the first condenser lens 
22 and is imaged on portions 42A and 42B of line sensor 42 by separator 
lenses 34 and 35. 
On the other hand, a bundle of rays, emitted from different portions At and 
Bt of exit pupil EPt of the taking lens at a telephoto extremity and 
transmitted through the detecting zone 12, is gathered and deflected by 
second condenser lens 23 and is imaged on portions 43A and 43B of line 
sensor 43 by separator lenses 34 and 35. The object image, which is the 
same as that formed on the detecting zone 12, is split and formed on the 
line sensors 42 or 43. 
When the elongated detecting zone 12 is symmetrical with respect to the 
optical axis, as shown in FIG. 7, and if longitudinal length L of the 
detecting zone is small, the latter can be easily influenced by a change 
in the exit pupil of the taking lens. However, if the longitudinal L is 
large, the bundle of rays, transmitted through the exit pupil EPt at the 
telephoto extremity, does not overlap the bundle of rays transmitted 
through the exit pupil EPw at the wide angle extremity at the upper 
portion of the object image, as shown in FIG. 8. Consequently, if a single 
condenser lens 23, which can effectively gather light in the wide angle 
mode, were used, the bundle of rays corresponding to the upper portion of 
the object image can not be gathered in the wide angle mode by the 
condenser lens 23. 
To solve this problem, the condenser lens system is made of a composite 
optical system having a strong condenser lens 22 and a weak condenser lens 
23 in combination, in the embodiment illustrated in FIGS. 7 and 8. 
Consequently, in the telephoto mode, the image reforming optical system 
comprised of the weak condenser lens 23 and the common separator lenses 
34, 35 is used to form an image on the line sensor 43. Conversely, in the 
wide angle mode, the image reforming optical system comprised of the 
strong condenser lens 22 and the common separator lenses 34, 35 is used to 
form an image on the line sensor 42. 
As can be understood from the above discussion, according to the present 
invention, the image reforming optical systems are selectively used in 
accordance with a change in the position and size of the exit pupil of the 
taking lens. Accordingly, vignetting of light does not occur, and a 
sufficient quantity of light can be received by the line sensor or 
sensors. Thus, the focus state of the taking lens can be precisely 
detected.