Device for Detecting Focus at Different Areas of an Image Field

1. Field of the Invention

The present invention relates to a focus state detecting device of divided-pupil phase difference method, adapted for use in a camera or the like, and more particularly to a technology, in a focus state detecting device capable of distance measurement in plural distance measuring areas in an image field, for enabling independent optical adjustment in each distance measuring area thereby enabling precise distance measurement in all the distance measuring areas.

2. Related Background Art

The focus state detecting device of divided-pupil phase difference system, adapted for use in a camera or the like, is disclosed for example in the Japanese Patent Laid-open Application No. 2-120712. FIG. 8 schematically shows the principal part of the focus state detecting device described in said patent application. Said device is provided, as shown in FIG. 8, with a field mask 31 having a cross-shaped aperture 31-1 at the approximate center of the image field of an unrepresented objective lens and vertically oblong apertures 31-2, 31-3 formed in the lateral areas, a field lens 32, a diaphragm 33, a secondary optical system 34 having four pairs of secondary imaging lenses, and a photosensor 35 having four pairs of sensor arrays.

The device shown in FIG. 8 is so designed as to be capable of distance measurements in three areas in the image field, and, in the central area, the components are provided in so-called cross type arrangement in which the mutual movement of the light amount distributions of two object images on the sensor plane takes place in the vertical and lateral directions. Such arrangement enables distance measurement in the central and lateral areas of the image field, and, particularly in the central area, it allows satisfactory focus state detection even for an object showing change in the light amount distribution only in the horizontal or vertical direction, by means of the cross-shaped sensors 35-1a, 35-1b, 35-1c and 35-1d.

However, in such focus state detecting devices of divided-pupil phase difference system, including those having plural distance measuring areas as explained above, critical is the relative positional precision between the line, connecting the field mask positioned in the vicinity of the primary image plane and the peak point of face of the corresponding re-imaging lens, and the light-receiving area of the photosensor. For example, in a device having a cross-shaped distance measuring area at the center as shown in FIG. 8, the field mask is shaped as a cross as indicated by 31-1, and the light beam transmitted by such cross-shaped aperture is further transmitted by the field lens 32, then defined by the aperture of the diaphragm 33 and focused by the re-imaging lens 34 onto the sensor plane.

FIG. 9 shows the mode of imaging on the sensor plane, wherein reference numbers 35-1a, 35-1b, 35-1c and 35-1d indicate the light-receiving arrays of the photosensor 35, as in FIG. 8, while reference numbers 36-1a, 36-1b, 36-1c and 36-1d indicate image areas of the central aperture 31-1 of the field mask 31. FIG. 9 shows a state in which the field mask 31, the re-imaging lens 34 and the light-receiving arrays of the photosensor 35 are mutually adjusted in an ideal state.

However, for example if the centers of the field mask 31, re-imaging lens 34 and photosensor 35 are mutually aligned but the photosensor 35 alone is angularly displaced in a plane perpendicular to the optical axis, the images of the field mask 31 formed by the re-imaging lens 34 and the sensor arrays of the photosensor 35 are mutually positioned as shown in FIG. 10, wherein proper focus state detection is not possible because the horizontally paired arrays 35-1c, 35-1d or the vertically paired arrays 35-1a, 35-1b used for focus state detection respectively receive different parts of the image of the field mask. In such case, the photosensor 35 has to be adjusted, by rotation, to the positional relationship shown in FIG. 9.

Also if the field mask 31 and the photosensor 35 are mutually aligned but the re-imaging lens 34 is angularly displaced, the images of the aperture 31-1 of the field mask 31 appear as shown in FIG. 11, and, also in this case, the paired sensor arrays respectively receive different parts of the image. It is therefore necessary to adjust, by rotation, the photosensor 35 to the state shown in FIG. 12, whereby the paired sensor arrays receive the same parts of the image.

Thus, if only one distance measuring area is provided in the image field, there can be executed the rotational adjustment of the photosensor in the situation shown in FIG. 10 or 11, regardless of whether the aperture of the field mask is cross-shaped or not. However, if three distance-measuring areas are provided in the image field, and the re-imaging lens and the photosensor are integrally constructed in three optical systems corresponding to said distance-measuring areas, as in the device disclosed in the Japanese Patent Laid-open Application No. 2-120712 and shown in FIG. 8, the above-mentioned rotation of the photosensor can achieve adjustment in one of three distance-measuring areas but not in the other two areas, because the photosensor can no longer be moved further. In the conventional focus state detecting device with plural distance-measuring areas, therefore, it has been impossible to effect optimum optical adjustment for all the distance-measuring areas, and, for this reason, to achieve distance measurement of high precision in each of the plural distance-measuring areas.

In consideration of the foregoing, an object of the present invention is, in the focus state detecting device of divided-pupil phase difference system with plural distance measuring areas, to enable optimum adjustment in each of the distance measuring areas, thereby allowing precise focus state detection in the plural distance measuring areas.

The above-mentioned object can be attained, according to the present invention, by a focus state detecting device for forming plural light amount distributions relative to the object image, utilizing light beams transmitted by different areas of the pupil of an objective lens, by means of optical means including a re-imaging lens positioned at the image plane side of said objective lens, then determining the relative positional relationship of said plural light amount distributions by means of photosensor means, and determining the focus state of said objective lens utilizing the signal from said photosensor means, said focus state detecting device comprising said photosensor means respectively corresponding to plural detection areas in the image field in order to effect focus state detection in said plural detection areas, wherein at least a part of said photosensor means is constructed as a unit independent from other photosensor means.

Also among the paths of the light beams coming from different areas of the pupil of said objective lens and leading to respectively corresponding photosensor means, at least one may be deflected in a direction different from those of other light paths.

In the above-mentioned configuration, each of the distance measuring areas of the focus state detecting device can be independently adjusted in optically optimum state, since at least a part of plural photosensor means is constructed as a unit independent from others. Thus optimum focus state detection is rendered possible in each of the plural detection areas. Also owing to the fact that at least a part of the photosensor means corresponding to the plural detection areas is constructed as a unit independent from others, there can be designed, for example, an optical system independent for each detection area, and, among the paths of the light beams coming from different areas of the pupil of the objective lens and leading to the respectively corresponding photosensor means, at least one may be deflected, for example by a mirror or a prism, in a direction different from those of other paths. It is therefore rendered possible to select the optical path length from the objective lens to the photosensor means or the performance of the re-imaging lens provided in said optical path, independently for each detection area, and there can be achieved optimum designing in consideration of the space utilization, such as the positioning of the photosensor means constructed as the independent unit.

FIG. 1 is a schematic perspective view of the principal part of a focus state detecting device, constituting an embodiment of the present invention;

FIG. 2 is a schematic perspective view of another embodiment of the focus state detecting device of the present invention, applied to a single-lens reflex camera;

FIG. 3 is a schematic perspective view of still another embodiment of the focus state detecting device of the present invention, applied to a single-lens reflex camera;

FIG. 4 is a schematic perspective view of still another embodiment of the focus state detecting device of the present invention, applied to a single-lens reflex camera;

FIG. 5 is a schematic perspective view of still another embodiment of the focus state detecting device of the present invention, applied to a single-lens reflex camera;

FIG. 6 is a schematic perspective view of still another embodiment of the focus state detecting device;

FIGS. 7A, 7B and 7C are schematic cross-sectional views showing the structure and function of the field lens;

FIG. 8 is a schematic perspective view showing the structure of a conventional focus state detecting device; and

FIGS. 9 to 12 are views showing the positional relationship between the photosensor and the images of the field mask formed by re-imaging lenses.

Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. FIG. 1 schematically illustrates the principal part of the optical system of a focus state detecting device, constituting a first embodiment of the present invention, wherein the components correspondingly numbered as in FIG. 1 have substantially the same functions, and include a field mask 31, a field lens 32, diaphragms 33-1, 33-2, 33-3, re-imaging lenses 34-1, 34-2, 34-3, photosensors 35-1, 35-2, 35-3, etc. In the present embodiment, however, three apertures 31-1, 31-2, 31-3 provided at the approximate center and at the lateral parts of the field mask 31 are all cross-shaped, for achieving cross-type focus state detection.

Consequently the re-imaging lenses 34 and the sensor arrays of the photosensor 35 are provided in six pairs in total. More specifically, corresponding to the central aperture 31-1 of the field mask 31 there are provided two pairs of re-imaging lenses 34-1 and two pairs of sensor arrays 35-1a, 35-1b, 35-1c, 35-1d. Also corresponding to the aperture 31-2 of the field mask 31, there are provided two pairs of re-imaging lenses 34-2 and a photosensor 35-2 provided with two pairs of sensor arrays, and, corresponding to an aperture 31-3 there are provided two pairs of re-imaging lenses 34-3 and a photosensor 35-3 provided with two pairs of sensor arrays. The optical paths for the distance measuring areas at right and at left are substantially perpendicularly deflected, in mutually opposite directions with respect to the central optical path, by means of mirrors 38-1, 38-2 positioned between the field lens 32 and the diaphragm members 33 positioned in front of the re-imaging lenses 34, so that the re-imaging lenses 34-2, 34-3 and the photosensors 35-2, 35-3 are positioned in mutually opposite manner, across the central optical path. For obtaining such configuration, the re-imaging lenses 34-1, 34-2, 34-3 are constructed as independent units, in contrast to the structure shown in FIG. 8. Also the photosensors 35-1, 35-2, 35-3 are likewise constructed as independent units.

Consequently, in such focus state detecting device capable of distance measurement in three areas, the optical systems from the field lens 32 to the photosensors 35-1, 35-2, 35-3 are rendered mutually independent, and the photosensors 35-1, 35-2, 35-3 are contained in separate packages whereby the aforementioned positional adjustment of the photosensors can be independently effected for each distance measuring area, in optimum manner. Also the arrangement of the re-imaging lens and the photosensor can be selected for each optical path. Therefore, if the width L of the focus state detecting device is to be minimized for the purpose of space saving, the optical path length from the field lens 32 to the photosensor 35 may be selected shorter in the lateral optical paths to the photosensors 35-2, 35-3 than in the central optical path.

FIGS. 2 to 6 illustrate other embodiments of the focus state detecting device of the present invention, applied to a single-lens reflex camera. In these drawings, the focus state detecting light beams, coming from an unrepresented phototaking lens, are transmitted by a half-transparent portion of a quick-return mirror 21 and are guided by a sub mirror 22 to the focus state detecting device. In said device, components the same as or equivalent to those in FIG. 1 are represented by same reference numbers. The focus state detecting device is further provided with a mirror 37 for deflecting the detecting light beam, introduced by the sub mirror 22 and transmitted by the field lens 32, toward the front (toward the phototaking lens) thereby limiting the space required below.

Among the light beams deflected by the mirror 37, the central one is transmitted by the diaphragm 33-1 and the re-imaging lens 34-1 to the photosensor 35-1 without deflection, but the outer light beams are respectively deflected by the mirrors 38-1, 38-2. In the embodiment shown in FIG. 2, the lateral light beams are laterally deflected toward the photosensors 35-2, 35-3. Also in the embodiments shown in FIGS. 3 and 4, said lateral light beams are respectively deflected upwards or downwards, to the photosensors 35-2, 35-3. In these embodiments, the photosensors 35-1, 35-2, 35-3 are contained, as independent units, in unrepresented packages.

FIG. 6 shows still another embodiment of the focus state detecting device. The central light beam is deflected by the mirror 37, then transmitted by the diaphragm 33-1 and the re-imaging lens 34-1 and guided to the photosensor 35-1 as in the foregoing embodiments, but said mirror 37 only has a width corresponding to that of the central light beam, so that the lateral light beams are guided, after being transmitted by the field lens 32, to the corresponding photosensors 35-2, 35-3 without deflection. Also in this embodiment, the photosensors 35-1, 35-2, 35-3 are constructed as independent units. This configuration allows simplifying the optical system, dispensing with the mirrors 38-1, 38-2 shown in FIGS. 2, 3, 4 for guiding the lateral light beams to the photosensors 35-2, 35-3. Also the photosensors can be divided into two groups, namely the central one 35-1 and the lateral ones 35-2, 35-3, and placed in mutually separate positions, so that the device can be made compact through effective space utilization.

Also if spaces are available on both sides of the central photosensor 35-1, three photosensors 35-2, 35-1, 35-3 may be arranged linearly, without deflection of the lateral optical paths, as shown in FIG. 5. In order to arrange the packages of the photosensors as in the embodiments shown in FIGS. 2 to 5, the selection of the arrangement of the lateral optical paths is important.

In the focus state detecting device employing the photosensors constructed as a single unit, as shown in FIG. 8, the optical axes 51-2, 51-3 of the lenses for the lateral distance measuring areas are shifted from the centers of said lenses in the cross-sectional shape of the field lens 32, as shown in FIG. 7A, in order to minimize the photosensors. In such configuration, the light beams of the lateral distance measuring areas are deflected toward the central light beam, by passing through the peripheral areas of the lenses 32-2, 32-3, whereby the photosensor 35 can be reduced in dimension.

The field lens of such configuration is effective for reducing the width of the focus state detecting device also in case the lateral optical paths are deflected laterally as shown in FIG. 2.

On the other hand, in case of the arrangements of the photosensors shown in FIGS. 3 to 6, there should be secured a certain distance between the optical paths at the position of the photosensors. It is therefore preferable, as shown in FIG. 7B, to bring the lateral optical paths closer to the optical axes of the lenses 32-2, 32-3 thereby reducing the deflection of optical paths toward the central optical path, and to arrange the three optical paths in substantially parallel manner behind the field lens.

It is also possible, as shown in FIG. 7C, to arrange the light beams to pass through the vicinities of the optical axes of the component lenses 32-1, 32-2, 32-3 of the field lens 32, thereby totally eliminating the deflection of the lateral optical paths and thus securing the space for the photosensors.

According to the present invention explained in the foregoing, .in the focus state detecting device of divided-pupil phase difference system, capable of distance measurement in plural areas, the photosensors are constructed as separate units respectively corresponding to the distance measuring areas, so that the adjustment for aligning the sensor arrays of the photosensor with the image of the field mask formed by the re-imaging lens can be independently executed for each distance measuring area. Consequently optimum and precise adjustment can be achieved for each distance measuring area, even when said area requires rigorous positional adjustment, as in the case of cross shaped distance measuring area. It is therefore rendered possible to significantly improve the accuracy of focus state detection, in all the distance measuring areas of the focus state detecting device having plural distance measuring areas.