Television – Camera – system and detail – Optics
Reexamination Certificate
1998-10-13
2002-04-23
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Optics
C348S350000
Reexamination Certificate
active
06377305
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an image sensing apparatus having an automatic focusing function for focusing the image of a subject.
So-called digital still cameras photoelectrically convert the image of an object, which has been formed by an image sensing optical system, as a still image using an image sensing device, and record the converted image in a memory or the like. Focus detection devices for automatic focusing used in such digital still cameras usually rely upon one of the following four methods:
(1) TTL (Through The Lens) secondary image forming phase-difference detection: Optical images that have been formed by passage through different pupil areas of an image sensing optical system are formed again as a pair of secondary images on a focus detection lens via a secondary image forming optical system and the state of focus of the image sensing optical system is detected from the spacing between the two secondary images.
(2) Passive triangulation: Two images of an object are formed on a focus detection sensor by two optical systems spaced apart by a predetermined baselength, and absolute distance to the object is sensed from the spacing between the two images formed.
(3) Active triangulation: A rangefinding pattern formed on an object by a light projection system is received by light-receiving systems spaced apart by a predetermined baselength, and the absolute distance to the object is sensed based upon outputs from the light-receiving systems.
(4) Hill-climbing sharpness detection: Part of an image sensing optical system, or the image sensing device, is oscillated minutely along the direction of the optic axis and the state of focus of the image sensing optical system is detected from the degree of fluctuation of high-frequency components (which are in synchronization with the oscillation) of the object image formed on the image sensing device.
The following finders are used as monitors for verifying the photographic area of the above-mentioned digital still cameras:
(a) an optical TTL finder, which allows the photographer to view the image of the object formed by the image sensing optical system;
(b) an optical rangefinder, which allows the photographer to view an image formed by a finder optical system that is different from the image sensing optical system; and
(c) a photoelectric finder whereby an output obtained by photoelectrically converting the image of an object is displayed on a monitor, such as an a liquid crystal display.
The prior art described has a number of shortcomings, which will now be set forth.
The secondary image forming phase-difference detection method and the passive triangulation method require the use of a photoelectric converting sensor for focus detection in addition to the image sensing device for acquisition of the photographic image. This raises the cost of the focus detection mechanism and increases the size of the image sensing device.
With the passive triangulation method and the active triangulation method, the focal length and baselength of the rangefinding optical system cannot be made very large. As a result, it is required that the dimensional precision of the component parts be very high in order to assure measurement accuracy.
The active triangulation method, besides having the drawback set forth above, requires a special-purpose light receiving device for receiving projected light. This raises the cost of the focus detection mechanism.
With the hill-climbing sharpness detection method, the in-focus position cannot be detected instantaneously when the object is greatly out of focus. Though this is not a major obstacle in a movie camera, it does make a digital still camera difficult to use and can result in lost photo opportunities.
The conventional finders have the following drawbacks:
The optical TTL finder requires a, such as quick-return mirror or half-mirror, for separating and switching between a photographic light flux and a finder light flux. This results in an apparatus of large size.
The optical rangefinder uses a double-image coincidence mechanism in order to display the state of focusing. The result is a complex, costly structure.
The photoelectric finder displays images at a low resolution and makes it difficult to confirm the state of focusing accurately.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an image sensing apparatus that is capable of performing autofocusing highly accurately through a simple structure.
According to the present invention, the foregoing object is attained by providing an image sensing apparatus comprising photoelectric conversion means for photoelectrically converting an image of an object obtained through image forming optics; optical path changeover means provided on an optical path between the image forming optics and the photoelectric conversion means and movable between first and second positions for changing over the optical path in such a manner that a first image of the object is formed on the photoelectric conversion means when the optical path changeover means is at the first position and a second image of the object is formed on the photoelectric conversion means when the optical path changeover means is at the second position; focus detection means for detecting the state of focus of the image forming optics using the first image when the optical path changeover means is at the first position; and image sensing means for picking up the second image using the photoelectric conversion means when the optical path changeover means is at the second position.
In accordance with this image sensing apparatus, focus detection and image pick-up can be performed by a single photoelectric conversion means. As a result, it is unnecessary to separately provide costly photoelectric conversion means for focus detection, thus making it possible to provide a small-size, inexpensive image sensing apparatus. In addition, a low-resolution image for focusing and a high-quality image for photography can be obtained using the same image forming optical system. Specifically, the apparatus utilizes an image forming optical system for image pick-up and the photoelectric conversion means thereof effectively to make possible rangefinding by TTL secondary image phase-difference detection or passive triangulation. As a result, it is unnecessary to separately provide costly photoelectric conversion means for focus detection, thus making it possible to provide a small-size, inexpensive image sensing apparatus capable of highly accurate rangefinding through a simple structure.
According to a preferred aspect of the present invention, the optical path changeover means of the image sensing apparatus has focus detection optics for forming two secondary images as the second image of the object from a pair of light fluxes obtained by passing the image of the object through different pupil areas; the apparatus further comprising means for detecting a phase difference between the two secondary images, which have been formed on the photoelectric conversion means, when the optical path changeover means is at the first position.
As a result, the state of focus of the image forming optical system is detected by TTL secondary image forming phase-difference detection. This makes it possible to perform accurate detection of focusing in a short period of time even in a case where the object is greatly out of focus.
According to a preferred aspect of the present invention, the optical path changeover means of the image sensing apparatus includes: a first mirror for deflecting an image forming light flux in a direction different from an image forming optic axis connecting the image forming optics and the photoelectric conversion means; and a second mirror for returning the deflected light flux to the image forming optic axis.
As a result, a focus detection optical system having a prescribed optical path can be accommodated in a small space, thus making it possible to reduce the size of the image sensing apparatus.
According to a preferred aspect of the present
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ho Tuan
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