Automatic focusing device

Abstract: An automatic focusing device includes a plurality of extraction means (13,14) for extracting different frequency components from among image pickup signals outputted by image pickup means (2), driving direction decision means (11) for deciding the driving direction of a focusing lens (1) based on the signal extracted by the extraction means, detection means for detecting a focusing signal in correspondence with the degree (A) of focusing by dividing (21) a signal at the same point which is extracted by the extraction means, driving speed decision means (12) for deciding the driving speed of the focusing lens based on the detection means, and driving means (7) for driving the focusing lens (1) of optical system to the focusing point based on the driving direction decision means (11) and the driving speed decision means (12).

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an automatic focusing device suitable for use with a video camera and the-like. Related Background Art As an automatic focusing device used in a movie camera such as video camera or the like, there has been known an automatic focusing device called mountain-climbing type in which high frequency components are extracted from image signals obtained from an image pickup element such as CCD and others and a photographing lens is driven so as to make the extracted high frequency components maximum to perform optimum focusing. Such an automatic focusing system is generally recognized as having advantages such that no specific optical focusing member is required and precise focusing can be achieved independently of a distance up to an object, and will be explained in detail by referring to Fig. 1. In Fig. 1, a focusing lens 1 is moved in the direction of an optical axis by a focusing motor 6 to perform a focusing operation. Light passed through the lens 1 is imaged on an image picking-up plane of an image pickup element 2, photo-electrically converted into an electric signal and outputted as a video signal. This video signal is amplified up to the required level by an amplifier 3, converted into a standard television signal by a process circuit 4 for a camera. Simultaneously with this, the output of the amplifier 3 is applied to a band-pass filter (hereinafter referred to as BPF) 8. In the BPF 8, the high frequency components among the video signal are extracted, and then only the video signal corresponding to a focusing detection region set in a portion of a screen is extracted at a gate circuit 9. A peak-hold circuit 10 implements the peak-hold operation at an interval synchronized with an integral multiple of a vertical sync signal. Hereinafter, this peak-holded value will be referred to as focal point voltage value B. A motor speed decision circuit 12 sets a speed of the focusing motor 6 in correspondence with the degree of focusing based on the focal point voltage value B. In other words, the speed of the motor 6 is varied through a motor driver 7 in such a manner that it becomes fast when the lens is excessively out of focus or becomes slow when the out-of-focus condition is small. A motor direction decision circuit 11 sets the motor 6 running in that direction in which the focal point voltage value B increases, according to a method well known in the art as mountain-climbing control. However, the motor direction decision circuit 11 at the start of operation drives the motor 6, for the timing being,in an appropriate direction since the direction in which the focal point voltage value B goes to a maximum is not known at the beginning, and continues to drive it until the direction thereof can be judged. If the lens 1 reaches its extreme end during the period of judgement, a focus encoder 5 detects it and instructs to reverse the direction of the motor 6. However, as is clearly estimated from the conventional example mentioned above, there is a problem in that the degree of focusing may not be accurately judged since the focal point voltage varies not only due to the out-of-focusing or defocusing condition of an object, but also due to the type or contrast thereof. In short, in the case of ordinary objects, no hunting occurs and high-speed automatic focusing operation is achieved by making the speed of the motor 6 fast when the lens is out of focus excessively, and slow when the defocusing condition is small according to the instructions from the motor speed decision circuit 12. However, in the case of an object with low contrast, it is not possible to retard the motor speed even when the out-of-focus condition is small because the focal point voltage is low, thus leading to repetitive hunting in the vicinity of the focal point. On the other hand, in the case of high contrast, the speed of the motor 6 at the time of an excessive defocusing condition is reduced due to high focal point voltage, so that it takes a long time until a focal point is obtained. In addition, document EP-A-0 297 587 discloses an automatic focusing device in which a focusing area is set up in response to input vertical and horizontal synchronizing signals and a fixed oscillator output signal. Within this focus detection area, an image pickup signal is extracted from an output of an image sensing circuit via a gate circuit and supplied to two high pass filters whose characteristics are different from each other. The output signals of these high pass filters are switched corresponding to field intervals in order to integrate the output of each filter. The result of the integration operation is analog-to-digital converted and used to drive a focusing lens in accordance with the hill climbing method so as to increase the value of the integration result. SUMMARY OF THE INVENTION This invention has been made to eliminate the above-mentioned drawbacks, and a primary object thereof is to provide an automatic focusing device which is capable of rapid focusing without hunting, regardless of the type of object to be photographed. It is another object of the present invention to provide an automatic focusing device which can be precisely focused without being effected by the type of object and contrast. These objects are accomplished by an automatic focusing device as defined in claim 1 or claim 2. According to a preferred embodiment, there is provided an automatic focusing device comprising a plurality of extraction means for extracting different frequency components at the same point within an image pickup signal outputted by image pickup means, detection means for detecting a focusing signal in correspondence with the degree of focusing through division of the signal extracted by the extraction means, and driving means for driving a focusing lens to a focusing point based on the output of the detection means. Similarly, according to another preferred embodiment, there is provided an automatic focusing device comprising a plurality of extraction means for extracting different frequency components from among the image pickup signal outputted by the image pickup means, driving direction decision means for determining the driving direction of a focusing motor based on the signal extracted by the extraction means, detection means for detecting a focusing signal in correspondence with the degree of focusing through arithmetic operation on a signal at the same point which is extracted by the extraction means, driving speed decision means for deciding the speed of focusing motor based on the detection means, and driving means for driving the focusing lens of an optical system to the focusing point based on the driving speed decision means and the driving direction decision means. In addition, according to yet another preferred embodiment, there is provided an automatic focusing device capable of driving focusing lens to the focusing point by first deciding the driving direction of the focusing lens based on the signal extracted from among the image pickup signals by the plurality of extraction means, then detecting the focusing signal corresponding to the degree of focusing through division of the extraction signal, and finally deciding the driving speed of focusing lens based on the above-mentioned signal. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the present invention will be apparent after considering several illustrative preferred embodiments, taken in conjunction with the drawings, in which: Fig. 1 shows a block diagram of an ordinary AF system; Fig. 2 is a block diagram illustrating a first embodiment for an AF system according to the present invention; Fig. 3 is a flow chart showing the operation of a motor direction decision circuit in the AF system of Fig. 2; Fig. 4 is a diagram showing characteristics of an evaluation value A used in the AF system of Fig. 2; Fig. 5 is a diagram showing characteristics of a focal point voltage B used in the AF system of Fig. 1; Fig. 6 is a block diagram illustrating a second embodiment of the present invention; Fig. 7 is a block diagram illustrating a third embodiment of the present invention; Fig. 8 is a block diagram illustrating a fourth embodiment of the present invention; Fig. 9 is a flow chart showing the operation of a motor direction decision circuit in the system of Fig. 8; Fig. 10 is a diagram showing characteristics of an evaluation value A and data E in the system of Fig. 8; Fig. 11 is a block diagram demonstrating a fifth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 2 in which the construction of a first embodiment is diagrammatically illustrated in a block form, the same reference numerals are provided to the same or similar constituents as used in Fig. 1, and the explanation thereof is omitted. The video signal amplified up to the desired level at the amplifier 3 is supplied to a first BPF 13 and a second BPF 14. It should be noted that the transit center frequency of the second BPF 14 is lower than that of the first BPF 13 and that only a specific frequency is extracted at the first BPF 13. In the gate circuit 9, only the signal corresponding to the portion set in a focal point detection region within a scene is extracted and applied to a peak-hold circuit 17. Similarly, the output of the second BPF 14 which is lower in level than that of the first BPF 13 is inputted into a sample-hold circuit 18. The peak-hold circuit 17 holds a value that becomes maximum during one vertical operation period and provides to the sample-hold circuit 18 a timing signal H used for holding its value. The sample-hold circuit 18 holds data in accordance with a peak-hold timing signal H of the peak-hold circuit 17. In short, the holding value of the peak-hold-circuit 17 and the holding value of the sample-hold circuit 18 correspond to different frequency components extracted at the same point within the video signal. The outputs of the peak-hold circuit 17 and the sample-hold circuit 18 are analog-digital converted at A/D converters 19 and 20, respectively, and then inputted into a divider 21. The divider 21 implements digital operation simultaneously with the end of a vertical scanning period, provided that the output C (the value from which more higher frequency is extracted) of the A/D converter 19 is used as numerator and the output D (the value from which more lower frequency is extracted) of the A/D converter 20 is used as denominator, and outputs the result of the digital operation as an evaluation value A for the degree of focusing every vertical scanning period. This evaluation value A is inputted into the motor direction decision circuit 11 and the motor speed decision circuit 12. The motor direction decision circuit 11 discriminates the motor direction using the evaluation yalue A, and transmits the result of its discrimination to the motor driver 7. The motor speed decision circuit 12 determines the speed of motor 6 so that it is proportional to the reciprocal of the evaluation value A and transmits it to the motor driver 7. In short, when the evaluation value A is low and far apart from the focusing point, the speed of motor 6 is made fast. And, when the evaluation value A is high and is close to the focusing point, the speed of motor 6 is made slow. The motor driver 7 drives the focusing motor 6 in conformity with the prescribed speed and direction of the motor 6 to control the focusing lens 1, thereby attaining the automatic focusing operation. The operation of the motor direction decision circuit 11 will be explained herein by referring to a flow chart shown in Fig. 3. First of all, it is checked, at the motor direction decision circuit 11 and based on the input from the focus encoder 5, whether or not the motor 6 is positioned at its extreme end (Step 1). If it is, the driving direction of the motor 6 is reversed (Step 2). If it is not, it is judged by comparison with the data A' of the preceding field whether the data A is increasing (Step 3). As a result, if it is decreasing, the direction of motor 6 is reversed, and if it is increasing, the same direction is maintained. Next, the difference between the evaluation value (Fig. 4) in the first embodiment and the focal point voltage B (Fig. 5) in the conventional example of Fig. 1 will be explained by referring to Figs. 4 and 5. As will be seen from Fig. 5, the focal point voltage B is comparatively low even in the case of an object with low contrast, but becomes high in the case of an object with high contrast. However, as indicated in Fig. 4, the evaluation value A in the first embodiment becomes approximately constant independently of the contrast of the object. Fig. 6 is a block diagram showing the construction of a second embodiment. In the same manner as described in the first embodiment, the video signal amplified up to the desired level at the amplifier 3 is first converted into digital data at an A/D converter 24. This digital data is filtered by a next third BPF 22 and a fourth BPF 23, respectively, to extract the specific frequency components. At this juncture, the frequency characteristics attained by the third BPF 22 and the fourth BPF 23 are almost the same as those attained by the first BPF 13 and the second BPF 14 in the case of the first embodiment. Then, a gate circuit 25 extracts digital data only in the focal point detection region, and a peak-hold circuit 26 holds the maximum value from the third BPF 22 during the vertical scanning period. Simultaneously with this, a latch register 27 holds the value from the fourth BPF 23 according to a peak-hold pulse H provided by the peak-hold circuit 26. In this way, an automatic focusing operation similar to that in the first embodiment is performed. Referring to Fig. 7, there is illustrated a third embodiment of the present invention, in which an analog filter is used as a BPF. For this purpose, this embodiment has been constituted by disposing the A/D converters 19 and 20 of the first embodiment in the preceding stages of the peak-hold circuit 17 and the sample-hold circuit 18. As a result, a peak-hold circuit 26 and-a data latch register 27 are constituted as a digital type in order to process digital data. The remaining operation is similar to that in the respective first and second embodiments described above. As explained above, according to the present invention, rapid focusing operation without hunting is made possible, regardless of the type of object, by deciding the driving direction of the focusing lens on the basis of the signal extracted from among the image pickup signals by means of the plurality of the extraction means, detecting the focusing signal in correspondence with the degree of focusing through division in respect of the extracted signals, determining the driving speed of the focusing lens based on the detected focusing signal, and driving the focusing lens toward the focusing point. In the following, there will be explained a fourth embodiment of the present invention in which the performance of the judgement of the driving direction and the speed of the focusing motor is improved over the systems in the above-mentioned first, second and third embodiments. In Fig. 8, there is shown a system block diagram of a fourth embodiment, in which the same constituents as that illustrated in the first embodiment of Fig. 2 are indicated by the same reference numerals, so that there is no detailed explanation therefor. According to this fourth embodiment, the divider 21 implements a digital operation upon the end of the vertical scanning period on the condition that the output C (the value from which higher frequencies are extracted) of the AID converter 19 is used as a numerator and the output D (the value from which lower frequencies are extracted) of the A/D converter 20 is used as a denominator, and outputs it as evaluation value A for the degree of focusing every vertical scanning period, all the operation described above being just the same as that shown in Fig. 2. This evaluation value A is inputted into the motor speed decision circuit 12. Then, the motor speed decision circuit 12 determines the speed of motor 6 so that it is proportional to the reciprocal of the evaluation value A and transmits it to the motor driver 7. In short, when the evaluation value A is low and is far apart from the focusing point, the speed of motor 6 is made fast, and when the evaluation value A is high and is close to the focusing point, the speed of motor 6 is made slow. In addition, the output C of the A/D. converter 19 and the output D of the A/D converter 20 are compared at a comparator 29 to obtain a signal to be used for controlling a switch 30. As a result, the switch 30 selects the greater one of the two outputs C and D, which is entered, as data E, into the motor direction decision circuit 11. The motor direction decision circuit 11 determines the running direction of the motor 6 using the data E and the input from the focus encoder 5, and transmits it to the motor driver 7. The motor driver 7 drives the focusing motor 6 to the specified speed and direction to adjust the focusing lens 1, thereby attaining the automatic focusing operation. The operation of the motor direction decision circuit 11 will be explained by reference to a flow chart of Fig. 9. To begin with, the motor direction decision circuit 11 checks whether the motor is positioned at its extreme end, based on the input from the focus encoder 5 (Step 1'). If it is, the direction of motor 6 is reversed (Step 2'). If it is not, Step 3' checks whether the data E is increasing by comparison with the data E' of the preceding field. If it is decreasing, the direction of the motor 6 is reversed (Step 2'), and if it is increasing, the same direction is maintained. The difference between the estimation value in the instant embodiment and the focal point voltage B of the prior art shown in Fig. 1 is just the same as described previously with reference to Figs. 4 and 5. As recalled, the focal point voltage B in the case of an object with low contrast remains low, but it becomes larger in the case of an object with high contrast. On the other hand, the evaluation value A of the fourth embodiment is kept almost constant independently of a wide range of contrast of the object. Referring to Fig. 10, there is shown the difference between the evaluation value A and the data E. Assume herein that the same object is to be photographed, the degree of focusing may be distinguished from the evaluation value A, but the output does not vary when the object is out of focus excessively. Contrary to this, in the case of data E, the value at the focusing point varies largely depending on the object, but a slope of a mountainous curve may be definitely detected even when the object is out of focus excessively. Accordingly, it will be understood that the evaluation value A is suited for speed control of the motor 6 and the data E is suited for direction control of the motor 6. Fig. 11 is a block diagram showing the construction of a fifth embodiment. In the same manner as described in connection with the fourth embodiment, the video signal amplified up to the desired level at the stage of the amplifier 3 is converted into digital data at the A/D converter 24. This digital data is filtered by the third BPF 22 and the fourth BPF 23 to extract only the specific frequency components. In this process, frequency characteristics attained by the third BPF 22 and the fourth BPF 23 are almost the same as that attained by the first BPF 13 and the second BPF 14 used in the first embodiment. And then, the gate circuit 25 extracts only the digital data in the focal point detection region, and the peak-hold circuit 26 holds the maximum value of the third BPF 22 during one vertical scanning period. Simultaneously with this, the latch register 27 holds the value from the fourth BPF 23 according to the peak-hold pulse H supplied from the peak-hold circuit 26. The output C of the peak-hold circuit 26 and the output D of the latch register 27 are added to each other at an adder 28, and the sum is supplied to the motor direction decision circuit 11. The direction decision capability of the output E of the adder 28 can be further improved by adding a plurality of filter outputs thereto, even in the case of excessive defocusing. The motor speed decision circuit 12 receives the evaluation value A that is obtained by dividing the output C of peak-hold circuit 26 by the output D of latch register 27 at the divider 21. Finally, the automatic focusing operation will be performed in the same manner as described relative to the first embodiment. As explained above, according to the present invention, rapid focusing operation without hunting is made possible, regardless of the type of object, by deciding the driving direction of the focusing lens on the basis of the signal extracted from among the image pickup signals by means of the plurality of the extraction means, detecting the focusing signal in correspondence with the degree of focusing through division in respect of the extracted signals, determining the driving speed of the focusing lens based on the detected focusing signal, and driving the focusing lens toward the focusing point.

Claims: An automatic focusing device comprising: image pickup means (2, 3); extracting means (9, 13, 14) for extracting different frequency components from the image pickup signal output from said image pickup means; detecting means (11, 12) for detecting a focusing condition using the signals extracted by said extracting means; focusing means (1, 6, 7) for performing a focusing operation based on the output of said detecting means; characterized in that said extracting means (9, 13, 14) is adapted to extract said different frequency components at an identical point in a focus detecting area from the image pickup signal output from said image pickup means (2, 3), wherein said identical point is a point at which one of the different frequency components is a peak value. An automatic focusing device comprising: image pickup means (2, 3); extracting means (9, 13, 14) for extracting different frequency components from the image pickup signal output from said image pickup means; focusing control means (11, 12, 29, 30) for detecting a focusing condition using the signals extracted by said extracting means and for performing a focusing operation based on the output of said detecting means; characterized in that said extracting means (9, 13, 14) is adapted to extract said different frequency components at an identical point in a focus detecting area from the image pickup signal output from said image pickup means (2, 3) said identical point is a point at which one of the different frequency components is a peak value, said focusing control means (11, 12, 29, 30) is adapted to control a focus adjusting direction on the basis of a level of the greatest one of the frequency components extracted by said extracting means. An automatic focusing device according to claim 1 or 2, wherein said extracting means includes a plurality of band-pass filters (13, 14) for extracting a plurality of signal components having transit center frequencies different from each other within the image pickup signals, and filters therefor. An automatic focusing device according to Claim 3, wherein said extracting means includes a timing generation means for detecting the level of frequency components at said identical point in said focus detection area by use of said respective band-pass filters. An automatic focusing device according to Claim 4, wherein said extracting means includes a peak-hold circuit (17) for detecting a peak value in the output of one of said plurality of band-pass filters, and wherein said timing generation means includes a sample and hold circuit (18) for sampling and latching the output of the another one of said plurality of band-pass filters at the timing that has been used by said peak-hold circuit for detecting the peak value. An automatic focusing device according to claim 1 or one of claims 3 to 5 based on claim 1, wherein said focusing means includes a focusing lens (1) and a focusing motor (6) for driving said focusing lens. An automatic focusing device according to claim 6, wherein said focusing means includes a direction decision circuit (11) for determining the driving direction of said focusing lens and a speed decision circuit (12) for deciding the driving speed of said focusing lens, and wherein said focusing means is adapted to perform a focusing operation based on the outputs of said direction decision circuit and said speed decision circuit. An automatic focusing device according to claim 2 or claims 3 to 5 based on claim 2, wherein said focusing control means includes a focusing lens (1) and a focusing motor (6) for driving said focusing lens. An automatic focusing device according to claim 8, wherein said focusing control means includes a direction decision circuit (11) for determining the driving direction of said focusing lens and a speed decision circuit (12) for deciding the driving speed of said focusing lens, and wherein said focusing control means is adapted to perform a focusing operation based on the outputs of said direction decision circuit and said speed decision circuit. An automatic focusing device according to claim 7 or 9, wherein said speed decision circuit is adapted to determine a focusing speed by performing arithmetic operation on the signal having said respective frequency components extracted by said extracting means. An automatic focusing device according to Claim 10, wherein said speed decision means performs judgement of speed on the basis of the information obtained from a division performed on the outputs of said respective band-pass filters. An automatic focusing device according to claim 10 based on claim 7, wherein said focusing means includes dividing means (21) adapted to perform a division on said different frequency components at said identical point. An automatic focusing device according to claim 10 based on claim 9, wherein said focus control means includes dividing means (21) adapted to perform a division on said different frequency components at said identical point. An automatic focusing device according to Claim 12 or 13, wherein said dividing means is adapted to divide the higher signal components of said different frequency components by the lower signal components thereof and to normalize the higher signal components by use of the lower signal components. An automatic focusing device according to Claim 14 based on claim 13, wherein said direction decision means performs judgement of direction on the basis of the output signal with higher level of said respective band-pass filter. An automatic focusing device according to one of the preceding claims, wherein said image pickup means is adapted to photo-electrically convert an image of an object formed on an image pickup plane and to output the photo-electrically converted object image as image pickup signals.

Inventors: ARAI HIDEYUKI, SUDA HIROFUMI, ARAI, HIDEYUKI, SUDA, HIROFUMI, Arai, Hideyuki, c/o Canon Kabushiki Kaisha, SUDA, HIROFUMI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N5
IPC codes: G02B7, H04N5