1. Field of the Invention
This invention relates to an improvement on an automatic focusing device which converts the image of an object into a video signal by means of a two-dimensional image sensor, etc. and adjusts the focal point of a photo taking lens on the basis of this video signal.
2. Description of the Related Art
There has been known a method for automatically controlling a video camera into an in-focus state by detecting the degree of definition of an image formed on the image sensing plane by detecting the high frequency component of a video signal and by controlling the position of the lens of the camera in such a way as to obtain a maximum degree of the definition (high frequency component). The method is based on the concept: The video signal conspicuously changes at the edge part of the image of an object thus resulting in an increase in the high frequency component of the video signal. This high frequency component increases accordingly as the object's image is closer to an in-focus state.
FIG. 12 of the accompanying drawings shows in outline the arrangement of the conventional automatic focusing device of this kind. Referring to FIG. 12, a photo taking lens 1 forms an object's image on the image sensing plane of an image sensor 2. The image is converted into an electrical (video) signal by the image sensor 2. A preamplifier 3 which operates at the ensuing stage amplifies the video signal coming from the image sensor 2. The amplified video signal is supplied to a processing circuit 4 and a band-pass filter (BPF) 5. The processing circuit 4 perform s a signal processing operation in a predetermined process and produces the received signal as a standard TV signal (video signal). Meanwhile, the band-pass filter 5 extracts a high frequency component from the output of the preamplier 3 and supplies it to a gate circuit 6. By this, only a signal in an area that is to be used for focus detection within one picture (consisting of one field or one frame) of the video signal is selected and allowed to pass the gate circuit 6. A detection circuit 7 performs a detecting operation on the output of the gate circuit 6. The circuit 7 then forms and produces a signal indicative of a maximum amplitude value, i.e. a peak value, at a point of the signal area abounding with the high frequency component. The signal produced from the detection circuit 7 represents the focused degree of the photo taking lens 1. The greater the value of this signal, the nearer the photo taking lens 1 to an in-focus state. A motor driving circuit 8 is arranged to drive a motor 9 to shift the photo taking lens 1 according to the output value of the signal from the detection circuit 7. The lens 1 is thus automatically brought into an in-focus state.
This method has long been known by the name of "Hill Climbing Control Method," which is described in "NHK Gijutsu Kenkyu Hokoku", 1965, Vol. 17, No. 1 (Whole No. 86), p 21. However, the area of video signal to be allowed to pass by the gate circuit 6, that is, a focus detecting area is fixedly set in the middle part of the image plane. Therefore, there has been a fear that, in cases where the position of a main object changes within the image plane as a result of some movement of either the object or the camera, the lens might be focused on a wrong object the image of which comes to the middle of the image plane under such a condition.
To solve this problem by allowing the lens to be continuously focused on the main object even in the event of occurrence of a change in the position of the main object within the image plane, a patent application has been filed for an invention whereby a feature of a main object appearing on the image sensing plane is detected; any movement of the main object is detected on the basis of the feature detected; and the focusing area is arranged to trace the moving main object.
U.S. patent application Ser. No. 737,163, filed May 23, 1985, now abandoned, discloses an example of a device of the above stated kind. In this case, a distance measuring frame, i.e. the focus detecting area, is moved according to any change taking place in the feature of the object's image with the feature of the object's image extracted from a chrominance signal. U.S. patent application Ser. No. 106,427 filed Oct. 8, 1987 discloses another example of such a device, wherein: The image measuring frame is arranged to be shifted in such a way as to adjust the center of the frame to a point on the image sensing plane where the peak value of the high frequency component is detected.
The focus detecting area (or a distance measuring frame) on the image sensing plane is arranged to be set by the area of the video signal allowed to pass through the gate circuit 6. Meanwhile, the conventional device may be divided into two types. One has the focus detecting area arranged to be a relatively wide area within the image (sensing) plane. The other has the focus detecting area arranged to be a relatively narrow area in the middle of the image plane. However, these types have presented the following problems: In the former, some objects other than a main object might be brought into the focus detecting area either by its movement or by the movement of the camera. In that event, the lens might be focused on a wrong object instead of the main object. In the case of the latter, while no wrong object is likely to come into the focus detecting area, a main object of focusing tends to come out of the focus detecting area thus frequently necessitating the restart of a focusing action. This appears for the photographer to be an inadequate focusing action. The possibility of having such a restart, therefore, must be eliminated.
In respect of this, a device for making the size of the focus detecting area variable has been disclosed in U.S. patent application Ser. No. 154,078 filed Feb. 9, 1988. According to the disclosure, the moving extent of the image is detected and a period of time during which the automatic focusing action is to be inhibited is controlled according to the value of the moving extent; the use of a filter which determines a focus detecting characteristic is changed over to another filter according to the moving extent of the image; and the distance measuring area (focus detecting area) is changed over to another area according to the moving extent of the image.
In addition to the above stated device, devices arranged to make the size of the distance measuring area variable have been disclosed also in Japanese Patent Applications Laid-Open No. SHO 60-126976, SHO 60-120675 and SHO 61-18271, etc. However, after the disclosure of the prior art devices, research and development work in this connection has been furthered by the inventors. As a result, a method for improvement over these devices has been developed. In accordance with this method, changes in the position and size of the object can be detected at a higher degree of accuracy without being affected by changes in the position and size of the object and the influence of the background; and the changes detected can be reliably traced.