1. Field of the Invention
This invention relates to systems for detecting the condition of sharp focus when an image-forming optical system is focused for differing object positions, and, more particcularly, to a sharp focus detecting system of the type in which a real image formed by the image-forming optical system, or an image equivalent thereto is scanned by an ever-oscillating photoconductive cell to measure the degree of sharpness of the image.
2. Description of the Prior Art
For detection of the condition of sharp focus when an image-forming optical system is focused for differing object positions by utilizing nonlinear light response characteristics (called photoelectric effect) of the kind which is unique in the case of photoconductive materials such as cadmium sulfide and cadmium selenide, there is known an arrangement in which a photoconductive element having a light sensitive image receiving surface positioned in a plane at which a real image of an object is formed in sharp focus by the image-forming optical system cooperates with a scanning device such as oscillatory motion imparting device to scan the real image across the image plane. In this manner, the degree of sharpness of the image can be measured with high accuracy. Various methods of, and apparatus for detecting the condition of sharp focus for an image-forming optical system based on the principle of this arrangement have been proposed, and some of them have already put into practice, as disclosed, for example, in Japanese patent publication No. 40-4788 assigned to the assignee of the present invention, U.S. Pat. No. 3,511,155 granted to Yamada, and U.S. Pat. No. 3,720,151 granted to Katsuyama.
Japanese patent publication No. 40-4788 provides a method of detecting whether or not an image-forming optical system is in sharpest focus for a given object comprising projecting a real image of the object through the image-forming optical system onto the light sensitive surface of a photoconductive element, oscillating the photoconductive element as passing through a critical focal plane to vary the length of the projection path between the optical system and the photoconductive element. In this manner, pulsating current is introduced into the fundamental output of the photoconductive element, and sensing the direct-current component of the resultant output of the photoconductive element.
U.S. Pat. No. 3,511,155 concerns an automatic focusing apparatus for use in an optical instrument such as a camera employing a sharp focus detecting method which comprises forming a real image of an object with projection light passing through an optical system on a light sensitive surface of a cadmium sulfide element positioned at a plane conjugate to the focal plane of the camera and fixedly mounted on a pivotal support plate, imparting periodic pivoting movement to the support plate to oscillate the CdS element at a frequency of several tens of cycles along the optical axis, and sensing a pulse produced each time the CdS element surface passes through the plane of focus to control operation of a servo motor. The optical system is therefore automatically focused to the object.
U.S. Pat. No 3,720,151 concerns a modification of the automatic focusing apparatus disclosing in the above-identified U.S. Pat. No. 3,511,155. In this arrangement the CdS element is fixedly secured in position being conjugate to the focal plane, an optical path length varying lens assembly comprising concave and convex lenses in employed as arranged in front of the CdS element. With this arrangement, the image plane is caused to oscillate with respect to the CdS element by imparting symmetrical oscillation of predetermined period into the concave and convex lenses along the optical axis thereof.
The apparatus employing such methods are characterized by an interpolation between two bracketing conditions of focus which is effected by imparting periodic variation to the spatial relationship between the photoelectric element capable of sensing the degree of sharpness of an image and the focal plane at which a real image of an object is formed by an image-forming optical system. Such apparatus has not only an advantage of realizing condition of optimum focus condition with a relative high degree of dependability even from the theoretical point of view, but also an advantage of facilitating the measurement of the degree of focus of an image in association with the direction in which an adjustment must be made to bring the image into focus.
In application to optical instruments such as photographic cameras at the automatic sharp focus detecting apparatus or automatic focusing apparatus thereof, however, various problems are encountered wherein the apparatus is incompatible with relatively simple camera structures. Such apparatus also takes up a large amount of space, and is not readily adapted to relatively low unit cost production.
On the other hand, at present, in the case of high performance single lens reflex cameras, state-of-the-art systematization is proficing an ever-increasing number of attachment which serve as the exchangeable components of the camera. By suitable selection of these units for association with the camera, it is made possible to impart desired capabilities to the camera. With respect to such optical instruments having available a number of exchangeable components, it is preferable to provide sharp focus detecting apparatus of the type described, or automatic focusing apparatus having an identical function thereto in the form of an exchangeable component constituting one of these component to overcome the above mentioned drawbacks.