GB2068193A

GB2068193A - Focus detecting device for a camera

Info

Publication number: GB2068193A
Application number: GB8035910A
Authority: GB
Original assignee: Seiko Koki KK
Current assignee: Seiko Koki KK
Priority date: 1979-11-09
Filing date: 1980-11-07
Publication date: 1981-08-05
Also published as: GB2068193B; DE3041768A1; JPS5669606A

Abstract

In a focus detecting device a light source 10 illuminates the object being viewed by transmitting light along paths co-axial with the focus detecting optical system 1,2. <IMAGE>

Description

SPECIFICATION Focus detecting device for a camera This invention relates to a focus detecting device for a camera, e.g. to an automatic focusing system capable of performing focus detection without the focus detecting efficiency being degraded even when the intensity of the light reflected from the object is low.

According to a well known automatic focus detecting device an in-focus signal is produced by providing photosensitive elements arranged to receive light through two separate light paths and by electrically processing the photoelectric outputs of the photosensitive elements, the taking lens of the camera then being adjusted to a position corresponding to the in-focus signal.

A conventional focus detecting device of this type has the disadvantage that the photoelectric output of the photosensitive elements is insufficient for focus detection when the object is dark. Accordingly, it has been proposed to solve this problem by an arrangement wherein the object is illuminated.

For instance, in the published specification of Japanese Patent Application No.

39543/75 and in the published specification of Japanese Patent Application No.

83524/76, a lamp is provided for the camera for illuminating the object when the brightness of the object is insufficient for focus detection.

Although it is desirable that the illuminating light is intensely directionai to obtain efficient illumination, it is useless unless the scan for illumination of the object is coincident with the scan for ranging. Thus, the provision of the illuminating system requires complex adjustment to obtain this coincidence, and such adjustment is difficult to provide in a small camera where there is limited space for a separate light path for illumination.

An object of the invention is therefore to provide a focus detecting device for a camera in which adjustment of the scan of the illuminating light source is not necessary.

According to the present invention there is provided a focus detecting device for a camera comprising a photo-sensitive element array for receiving light which is reflected by an object to be photographed and which is transmitted along two separate light paths, a focus detecting circuit controlled by electrical output signals from said array, and a light source arranged to illuminate the object by transmitting light along the optical axis of at least one of said light paths.

Preferably, the light source is arranged to become operable when the brightness of the object is below a predetermined level or when changing the camera to a flash photography mode or upon switching-on the power switch for internal electronic flash equipment of the camera.

Preferably, a fixed mirror is positioned in one of the light paths and a movable mirror is positioned in the other light path, and the movable mirror may scan over a range including the near and far focusing positions of the camera.

The fixed mirror is preferably a semi-transparent mirror, the arrangement being such that in the focus position the object is illuminated by the transmission of light from the light source along the optical axes of both of said light paths.

In one embodiment, there are two photosensitive element arrays associated, respectively, with the two light paths, and two semitransparent mirrors are provided, one semitransparent mirror being situated in one light path between the array associated with that path and the fixed mirror and the other semitransparent mirror being situated in the other light path between the array associated with that path and the movable mirror, and the light source is positioned between the two semi-transparent mirrors so that in the focus position the object is illuminated by the transmission of light from the light source along the optical axes of both of said light paths.

The light from the light source may be modulated and the light source may be an infra-red light source.

A shutter may be provided to obstruct, alternately, the two light paths, the optical information transmitted along the light path which is open being stored in a memory and then compared with the optical information transmitted later along the other light path.

Alternatively, the angular position of the movable mirror is converted into a pulse count and digitally processed.

The invention also comprises a camera provided with a device as set forth above.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a schematic illustration of one embodiment of a focus detecting device according to the present invention, and Figure 2 is a schematic illustration of another embodiment of a focus detecting device according to the present invention.

Referring to Fig. 1, a focus detecting device according to the present invention comprises a semi-transparent or half mirror 1, a movable mirror 2 rotatable in the direction of the arrow, a stationary light ray path 3, a moving light ray path 4, a plano-convex lens 5 dis posed.coaxially with the stationary path 3, a photosensitive element array 6 consisting of a plurality of photosensitive elements a, b, c, d, e, a focus detecting circuit 7 for producing an in-focus signal from the electrical outputs of the photo-sensitive elements, and a mask 8 for shielding the photo-sensitive elements from light other than the rays of light traver sing the stationary path 3 and the moving path 4.The mirror 2, instead of rotating, may be movable backwards and forwards between limits corresponding to a minimum focus range and a maximum focus range, for exam ple infinity.

A focus detecting device as so far described has been formerly proposed and a detailed description of the device is provided in U.K.

Patent Specification No. 2,021,348; therefore, only a brief description will be provided here of the manner of operation of the system. The light reflected from the object and transmitted along the stationary light ray path 3 passes through the semi-transparent or half mirror 1 and is focused on the photosensitive element array 6 by the lens 5. The light reflected from the object and transmitted along the moving light ray path 4 is reflected by the movable mirror 2 and the semi-transparent mirror 1, condensed by the lens 5 and focused on the photosensitive element array 6.

Thus, the optical information transmitted along both the stationary and moving light paths is applied as composite information to the photosensitive element array 6. The distance between the object and the camera is scanned, in order to detect the position of the object, as the movable mirror 2 rotates in the direction of the arrow to scan between the near and far points of its scanning range.

The output produced by the photosensitive elements a, b, c, d, e, varies during the scanning operation. When the images formed by the light rays transmitted along the stationary and moving light paths coincide, a difference signal having the largest value is produced, this difference signal being formed by the difference between the largest composite photoelectric output provided at one photosensitive element and the smallest composite photoelectric output provided at another photosensitive element. An in-focus signal is thereby produced to detect the position (the angle of turning) of the movable mirror 2 corresponding to the in-focus position, enabling the distance between the camera and the object to be determined.

In accordance with the invention, as shown in Fig. 1, a light source 10 for illuminating the object is provided. The light source is controlled by a control unit 11. The light source may be modulated if required. The light rays shown by dash lines, emitted from the source 10 are condensed by a condenser lens 9, and some are reflected by the semitransparent mirror 1 and projected onto the object, while others of these rays are transmitted through the semi-transparent mirror 1 and reflected by the movable mirror 2. Thus, both the light reflected by the semi-transparent mirror 1 and the light transmitted through the mirror 1 are projected onto the object when the movable mirror 2 is at the in-focus position.

In use of the camera when the object is dark so that the output of the photosensitive elements is insufficient for focusing, the object is illuminated by the light emitted from the illuminating light source 10, to thereby make possible focusing of the camera as under normal light conditions.

Referring now to Fig. 2 which illustrates another embodiment according to the present invention, there is shown a fixed mirror 21, a movable mirror 22, rotatable or movable backwards and forwards, the mirrors being disposed at positions to receive light transmitted along a stationary light path 23 and a moving light path 24, respectively semi-transparent or half mirrors 40a and 40b, lenses 25a and 25b, photosensitive element arrays 26a and 26b each constituted similarly to the photosensitive element array 6 of the embodiment of Fig. 1, a circuit 27 for producing an in-focus signal from the electrical outputs of the photo-sensitive elements, a mask 28, a light source 30 for illuminating the object, and a light source control unit 31.

Detailed description of the manner of operation of the system as described above will be omitted as it is described in the published specification of Japanese Patent Application No. 83524/76.

Basically, the device of Fig. 2 detects the coincidence of images of the object formed, respectively, by light rays transmitted along the stationary light path 23 and the moving light path 24 and gives an in-focus signal when coincidence is detected. Light reflected along the fixed path 23 is received by array 26a and light reflected along the moving path 24 is received by array 26b. At the in-focus position of the mirror 22, the outputs of the array 26a and 26b are the same and the circuit 27 gives an in-focus signal. The illuminating light source 30 is disposed between the semi-transparent mirrors 40a and 40b and the light rays are projected on to the object by transmission along paths co-axial with the paths 23 and 24, the light emitted from.the light source 30 being reflected by the fixed mirror 21 and by the moving mirror 22 and projected on to the object.

In a modification, the device may be arranged to illuminate the object only along a path corresponding to the stationary light path 23 by disposing the illuminating light source at the alternative position 30' as shown in Fig. 2.

The focus detecting device of the present invention is applicable to the device disclosed in U.K. Patent Specifications Nos. 2,016,852 and 2,049,231.

In the application of the embodiment of the present invention shown in Fig. 1 to the device disclosed in Specification No.

2,016,852, the light rays transmitted along the stationary light path 3 and the light rays transmitted along the moving light path 4 are shielded alternately by a shutter, not shown, to transmit the light rays of the two paths to the photosensitive element array 6 alternately.

The optical information transmitted through the stationary light path is stored in a memory and is then compared with the optical information transmitted through the moving light path 4 inputted afterwards. An in-focus signal is produced when the two optical informations coincide.

In the application of the embodiment of the present invention shown in Fig. 1 to the system disclosed in U.K. Patent Specification No. 2,049,231, the angle of rotation of the movable mirror 2 is converted into a pulse count and digitally processed. The optical system for focus detection has a shutter, not shown, between the movable mirror 2 and the semi-transparent mirror 1. In this system, first, the optical information transmitted through the stationary light path is stored in a memory; next the light received through both the stationary light path and the light received through the moving light path is used to provide composite optical information. This composite optical information varies with the angle of rotation of the movable mirror 2.

When the composite optical information coincides with the previously memorized information, an in-focus signal is produced.

The above described focus detecting device of the present invention is capable of detecting the focus of an object under conditions wherein automatic focus detection was previously impossible due to low brightness of the object, by the provision of the light source for illuminating the object.

It will be obvious to those skilled in the art that the arrangement and form of the illuminating light source of the focus detecting device of the present invention are not limited to the arrangement and form as hereinbefore deScribed, but that the light source may read ily--have various manners of arrangement and be of diverse forms. Thus, the light source is not limited to visible light. Any kind of light, such as infra-red light, may be used, provided that the wave length of the light is suitable for detection by the photosensitive elements.

It is preferable to use modulated light to discriminate from natural light.

It will be well understood that with the focus detecting device according to the present invention accurate focus detection is possible even when the object is dark or the object contrast is low. Further, the device of the present invention is advantageous for mounting on a highly compact camera since the same light path is used both for focus detection and for illumination.

Furthermore, no adjustment is required to make the scanning of the ranging operation for focus detection and the scanning of the illuminating light source coincide with each other. Still further, the light spot illuminating the object is positioned at the centre of the field of view since the scanning for focus detection and the scanning for illumination are in precise coincidence; therefore, a photographing operation can be performed without observing the object through the view finder when visible light is used for illumination.

The light source may be adapted to become operable automatically when the intensity of the light reflected from the object is below a predetermined level or when the camera is changed to flash photography mode or when the power switch for an internal electronic flash equipment of the camera is switched on.

Claims

1. A focus detecting device for a camera comprising a photo-sensitive element array for receiving light which is reflected by an object to be photographed and which is transmitted along two separate light paths, a focus detecting circuit controlled by electrical output signals from said array, and a light source arranged to illuminate the object by transmitting light along the optical axis of at least one of said light paths.

2. A device according to claim 1 in which said light source is arranged to become operable when the brightness of the object is below a predetermined level.

3. A device according to claim 1 in which said light source is arranged to become operable upon changing the camera to a flash photographing mode or upon switching-on a power switch for internal electronic flash equipment of the camera.

4. A device according to any one of claims 1 to 3 comprising an optical system having a fixed mirror positioned in one of said light paths and a movable mirror positioned in the other one of said light paths.

5. A device according to claim 4 in which the movable mirror is movable to scan over a range including the near and far focusing positions of the camera.

6. A device according to claim 4 or claim 5 in which the fixed mirror is a semi-transparent mirror, the arrangement being such that in the focus position the object is illuminated by the transmission of light from the light source along the optical axes of both of said light paths.

7. A device according to claim 4 or claim 5 in which there are two photosensitive element arrays associated, respectively, with the two light paths, and two semi-transparent mirrors are provided, one semi-transparent mirror being situated in one light path between the array associated with that path and the fixed mirror and the other semi-transparent mirror being situated in the other light path between the array associated with that path and the movable mirror, and the light source is positioned between the two semi transparent mirrors so that in the focus position the object is illuminated by the transmission of light from the light source along the optical axes of both of said light paths.

8. A device according to any preceding claim in which the light from the light source is modulated.

9. A device according to any preceding claim in which the light source is an infra-red light source.

10. A device according to any one of claims 1 to 6 comprising a shutter arranged to obstruct, alternately, the two light paths, the optical information transmitted along the light path which is open being stored in a memory and then compared with the optical information transmitted later along the other light path.

11. A device according to claim 5 or claim 6 in which the angular position of the movable mirror is converted into a pulse count and digitally processed.

1 2. A focus detecting device for a camera substantially as hereinbefore described with reference to and as shown in Fig. 1 or Fig. 2 of the accompanying drawings.

1 3. A camera including a focus detecting device according to any preceding claim.

14. A focus detecting device for the camera provided with a photosensitive element array for receiving the light transmitted through two separate light passages and capable of electrically detecting the focus by the image coincidence method using the output of the photosensitive elements of said photosensitive element array wherein an illuminating light source being disposed at a position coaxial with the optical axis of at least one of said two separate light passages and suitable to illuminating the objects.

1 5. A focus detecting device for the camera comprising an optical system attached to the camera body for transmitting the light from objects through a first and second light passages, a photosensitive element array consisting of a plurality of photosensitive elements for receiving the light transmitted through said first and second light passages, a focus detecting circuit for detecting the focusing of the taking lens by using the electric output signals of said photosensitive element array, and an illuminating light source disposed at a position coaxial with at least one of said first and second light passages.